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Physics Laboratory Manual PHYC 10190 Aspects of Physics for Ag. Science 2014-2015 Name................................................................................. Partner’s Name ................................................................ Demonstrator ................................................................... Group ............................................................................... Laboratory Time ............................................................... 1 2 Contents 4 Introduction Laboratory Schedule 5 Experiments: Current 7 Resistance 21 Voltage 31 Voltage, Current and Resistance 39 Conservation of Momentum 47 Conservation of Momentum II 53 3 Introduction Physics is an experimental science. The theory that is presented in lectures has its origins in, and is validated by, experimental measurement. The practical aspect of Physics is an integral part of the subject. The laboratory practicals take place throughout the semester in parallel to the lectures. They serve a number of purposes: • • • an opportunity, as a scientist, to test theories; a means to enrich and deepen understanding of physical concepts presented in lectures; an opportunity to develop experimental techniques, in particular skills of data analysis, the understanding of experimental uncertainty, and the development of graphical visualisation of data. Some of the experiments in the manual may appear similar to those at school, but the emphasis and expectations are likely to be different. Do not treat this manual as a ‘cooking recipe’ where you follow a prescription. Instead, understand what it is you are doing, why you are asked to plot certain quantities, and how experimental uncertainties affect your results. It is more important to understand and show your understanding in the write-ups than it is to rush through each experiment ticking the boxes. This manual includes blanks for entering most of your observations. Additional space is included at the end of each experiment for other relevant information. All data, observations and conclusions should be entered in this manual. Graphs may be produced by hand or electronically (details of a simple computer package are provided) and should be secured to this manual. There will be six 2-hour practical laboratories in this module evaluated by continual assessment. Note that each laboratory is worth 5% so each laboratory session makes a significant contribution to your final mark for the module. Consequently, attendance and application during the laboratories are of the utmost importance. At the end of each laboratory session, your demonstrator will collect your work and mark it. Remember, If you do not turn up, you will get zero for that laboratory. You are encouraged to prepare for your lab in advance and bring completed pre-lab assignments to the lab 4 Laboratory Schedule Depending on your timetable, you will, attend as detailed below, on either a Friday (15:00-17:00) or Wednesday (14:00-16:00) afternoon. The class is divided into groups, numbered 1-4 in the table below. Please consult the notice boards, Blackboard, or contact the lab manager, Thomas O’Reilly (Science East 141), to see which of the experiments you will be performing each week. This information is also summarized below. Dates 15-19 Sept Semester Week 2 22-26 Sept- 3 29 Sept - 3 Oct 4 27 - 31 Oct 8 3 - 7 Nov 9 10-14 Nov 10 Lab 143 Current: 1,3 Resistance: 2,4 Voltage: 1,3 Voltage, Current: 2,4 5 Room SCIENCE EAST Lab 144 Momentum: 2,4 Momentum 1,3 Momentum II: 2,4 Momentum II: 1,3 Lab 145 Current: 2,4 Resistance: 1,3 Voltage: 2,4 Voltage, Current: 1,3 6 Name:___________________________ Student No: ________________ Partner________________ Date:________ Demonstrator:____________________ Current What should I expect in this experiment? In this experiment you will carry out a number of investigations on simple electric circuits in order to determine some of their characteristics. Through these investigations you will develop rules of your electric circuit model which allow you to make predictions on other circuits. Pre-lab assignment Shown below are three arrangements of a bulb, battery one copper wire. Choose the arrangements in which the bulb will light. You may choose more than one arrangement. Explain your answer. Introduction Circuit Diagrams In this experiment you will encounter circuit diagrams which represent different electric circuits. Circuit diagrams consist of lines and symbols, the ones which you will encounter in this lab are shown below, Please note the following carefully: 1. Direct contact and connection by a wire are represented by a line or a group of lines. It is not possible to tell whether two circuit elements are connected directly together or if they are connected by a wire. Circuit diagrams only show electrical connections, they do not represent a physical layout. 7 The diagram above illustrates two different physical layouts; both are represented by the same circuit diagram, shown to the right. Experiment 1: Current In this and the orther electronics experiments, you will develop a model for electric circuits. The development of your model will always be guided by the observations you make. All reasoning you do to make predictions should be based on your model only, not on prior knowledge of electric circuits. In today’s experiment you begin your investigation of electric circuits and some of their properties. Starting from the basics of electric circuits, you will develop a consistent model of simple resistive circuits. Make sure you check your answers with your demonstrator when asked to do so. Equipment/Apparatus check Check that you have at your disposal: a battery, a bulb and a single wire. Section 1: Complete Circuits i. Consider the three arrangements of a battery, a bulb and a single wire in the pre-lab assignment. Use your pre-lab answers to complete the “Idea” column of the table below. Arrangement 1 2 3 Idea (on/off) Observation (on/off) ii. Connect the circuits and verify your ideas, enter your observations in the final column of the table. 8 9 10 11 12 13 14 15 16 17 18 19 20 Name:___________________________ Student No: ________________ Partner________________ Date:________ Demonstrator:____________________ Resistance What should I expect in this experiment? In this experiment you will carry more investigations on simple electric circuits in order to determine some more of their characteristics. Through these investigations you will develop more rules of your electric circuit model which allow you to make predictions for other circuits. Pre-lab assignment In your own words describe what is meant by electrical current and electrical resistance. In the circuit below all the bulbs are identical. How does the brightness of bulb A compare to the brightness of bulb B? Choose one answer. o Bulb A is brighter than bulb B o Bulb A is as bright as bulb B o Bulb A is dimmer than bulb B o I do n’t know Explain your answer to the previous question. How does the brightness of bulb A compare to the brightness of bulb C? Introduction In the Current laboratory we discovered some properties of basic electric circuits. These laid the foundations for our model of electric circuits. We will treat the two assumptions we made: that there is a flow called current in a complete circuit, and that the brightness of a bulb indicates the magnitude of the current through it, as the first two rules of our model. In today’s experiment we expand our model and broaden our understanding of electric circuits by investigating other types of circuits. Equipment/Apparatus Check Check to make sure you have a power supply, an ammeter, six wires, three bulb holders, three bulbs and two crocodile clips. Section 1: Power Supply In the Current laboratory you worked solely with batteries, bulbs and wires. In this experiment you will use a new component called a power supply. 21 i. Attach two crocodile clips to the leads of the power supply and set up a single bulb circuit as shown in the diagram at right. The switch on the power supply should be set to 9 V. Figure 1.1 A single bulb connected to a power supply ii. Do you think the brightness of the bulb would change if another bulb were added in series to the circuit as shown in Figure 1.2? Explain. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Set up the circuit shown at right and verify your ideas. What did you observe?. ____________________________________ ___________________________________ ____________________________________ ____________________________________ Figure 1.2 Two bulbs in series connected to a power supply _____________________________________ iii. How do your observations compare to your observations from the Current laboratory? Do the circuits containing a battery behave differently to circuits containing a power supply or do they behave the same? ______________________________________________________________________ ______________________________________________________________________ Section 2: Resistance in Series Circuits In what follows, you may assume that a power supply and a battery behave identically. In the last electronics experiment we observed that when a bulb was added in series, the bulbs in the circuit dimmed. We took this as evidence that the current through the bulbs and the battery decreased. We now try to incorporate this finding into our model for electric circuits. 22 i. Consider the two bulb series circuit which you have set up in Section 1. We label the bulbs A and B. Predict what would happen to the brightness of bulb A if a third bulb were added in series to the circuit as shown in Figure 2.1 above. _____________________________________________________________________ ____________________________________________________________________ ii. Set up the circuit and verify your prediction. _____________________________________________________________________ ____________________________________________________________________ To explain why the brightness of bulbs and the magnitude of current through the battery is different in different circuits, it is helpful to think of bulbs as providing an obstacle or a resistance to the current. In this picture, an increase in the circuit’s resistance causes a decrease in current through the battery and vice versa. iii. What can you infer about the total resistance of a circuit as more bulbs are added in series? _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ iv. Formulate a rule which allows you to predict how the current through the battery would be affected as the number of bulbs added in series were increased or decreased. Include the concept of resistance in your rule. This is the third rule of our model, to go with the two rules you made in the first electronics laboratory. _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ Discuss your answers with your demonstrator. Section 3: Parallel Circuits We have seen how adding bulbs in series affects the current through the battery and how it affects the brightness of the bulbs in the circuit. We now look at a different circuit and compare its properties to that of the circuits we have seen previously. 23 The circuit shown in Figure 3.1 is called a parallel circuit. Figure 3.2: Single bulb circuit. Figure 3.1: Two bulbs in parallel. i. Compare the circuit diagram of the parallel circuit above to that of a single bulb circuit shown in Figure 3.2 to the right. What are the similarities and differences between the two circuits represented by the diagrams? How many complete conducting routes/pathways exist in each circuit? _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ ii. How do you think the brightness of each bulb in a parallel circuit will compare to that of a bulb in a single bulb circuit? _____________________________________________________________________ Set up the parallel circuit as shown below in Figure 3.3. Compare the brightness of the bulbs in the parallel circuit to that of a bulb in a single bulb circuit. Write down your observations below. _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ Figure 3.3 Parallel circuit set-up Compare the brightness of each of the bulbs in parallel. through the bulbs compare to each other? How do the currents _____________________________________________________________________ In a parallel circuit we often think the current splitting and recombining junctions or nodes in the circuit. junction or node can be defined as electrical connection between three more components. of at A an or Draw nodes on figure 3.5, where appropriate. 24 Figure 3.4 A node iv. Consider the following student statements about the circuits above. Student 1: “When the current reaches the first node in the parallel circuit, it splits evenly between bulbs D and E. I know that the bulbs are equal in brightness to a single bulb and they are also equal in brightness to each other. Therefore the current through battery 2 must be double the amount of current through battery 1.” Student 2: “I disagree. I know that all batteries have the same current and all bulbs are the same brightness, so the same current flows through each bulb.” Which of the students, if any do you agree with? Explain. _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ Discuss your answer to part iv with your demonstrator Section 4: Measurement of current In this section, we make a series of measurements that will add some quantitative evidence to the student statements shown in the previous section. i. Use an ammeter to measure the current in a single bulb circuit as shown in Figure 4.1. Write down your measurement below. _____________________________________________________________________ 25 How does the current through the battery compare to the current through the bulb. Explain briefly _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ Figure 4.1 ii. In the circuit of Figure 4.2, the ammeter is measuring the current through one of the bulbs in a parallel circuit. On the basis of your observations and measurements, predict the reading on the ammeter. _____________________________________________________________________ ____________________________________________________________________ Set up the circuit and verify your prediction. iii. Predict the reading on the ammeter when it is connected to read the current through the battery in a parallel circuit as shown below. Explain briefly _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ Figure 4.3 Parallel circuit with ammeter 26 Set up the circuit shown of Figure 4.4, which measures the current through the battery in a parallel circuit. How does your observation compare to your prediction? _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ _________________________________ iv. Compare the current through the battery in the parallel circuit to the current through the battery in the single bulb circuit. Do your measurements support the idea that the current through a battery can be different in different circuits? _____________________________________________________________________ v. Which of the student statements in part iv of Section 3 is consistent with your measurements? _____________________________________________________________________ What can you infer about the resistance of a parallel circuit as compared to that of a single bulb circuit? _____________________________________________________________________ Discuss your answers with your demonstrator Section 5: Resistance in parallel circuits In the previous section we have seen that when we add a bulb in parallel to a single bulb circuit the current through the battery increases. We now carry out more experiments to investigate the properties of parallel circuits. Figure 5.2 A two bulb series circuit i. Consider the circuit shown at in figure 5.1. The black box represents an arrangement of circuit elements. A change is made within the black box and as a result the brightness of the indicator bulb A increases. What can you infer about the change in resistance of the circuit after the connections in the box have been changed? 27 _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ ii. Set up a two bulb series circuit. We call bulb A an indicator bulb as, throughout this experiment, it indicates the current through the battery. Suppose that you added a third bulb, C, in parallel with bulb B as shown in figure 5.3. Do you expect the brightness of bulb A to change when bulb C is added? Explain briefly _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ Predict how the brightness of the bulbs in the diagram of Figure 5.3 would rank from greatest to least. Carefully explain how you used your model to make your prediction. _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ ____________________________________________________________________ iii. Now add bulb C in parallel to bulb B as shown in Figure 5.3. brightness of the indicator bulb A change? How does the _____________________________________________________________________ iv. Considering your answer to question i, what can you infer about the change in the resistance of the circuit as bulb C is added in parallel to bulb B? _____________________________________________________________________ Is your answer consistent with what you found in Section 3 when you added a bulb in parallel with a single bulb? Explain briefly. _____________________________________________________________________ ____________________________________________________________________ 28 v. Revise the third rule so that it describes how the current through the battery changes when a bulb is added in parallel or in series to a circuit. Include the concept of resistance in your rule. Make sure you rule incorporates the results from Sections 4 and 5. _____________________________________________________________________ ____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ ____________________________________________________________________ Check your rule with your demonstrator vi. Through the investigations carried out in the previous sections and in the Current laboratory, you have developed a large portion of your model on electric circuits. To help you summarise some of the major concepts, complete the table below. 29 30 Name:___________________________ Student No: ________________ Partner________________ Date:________ Demonstrator:____________________ Voltage What should I expect in this experiment? In the Current and Resistance laboratories we started to build a model for electric circuits. In the next experiment we will investigate more complicated circuits and introduce the third and final concept to our model. Pre-lab assignment 1. A bulb is added in series to a given circuit. What happens to the resistance of the circuit? What happens to the current through the battery? 2. A bulb is added in parallel to a given circuit. What happens to the resistance of the circuit? What happens to the current through the battery? Equipment/Apparatus Check Check to make sure you have a power supply, two crocodile clips, five pieces of wire, four bulb holders, four bulbs and a voltmeter. Section 1: Voltage i. Set up a single bulb circuit with the power supply switch at 3 V, 6 V and 9 V, and describe any changes in bulb brightness. _________________________________ __________________________________ _________________________________ In the Current and Resistance laboratories, you changed the brightness of bulbs by adding other bulbs to the circuit. Is there a difference in the way you changed the brightness in this experiment? Explain briefly. _____________________________________________________________________ ____________________________________________________________________ Section 2: Power Supply In the previous section we saw that a change in the power supply affected bulb brightness. In the following experiments you will investigate this further. We introduce another circuit element, called a voltmeter. A voltmeter measures a quantity known as voltage which is related to the ability of a battery or power supply to push/drive current around the circuit. The voltmeter must be connected in parallel to the element which you are measuring, as shown in Figure 2.1. 31 32 33 34 35 36 37 38 Name:___________________________ Student No: ________________ Partner________________ Date:________ Demonstrator:____________________ Voltage, Current and Resistance What should I expect in this experiment? In this experiment we will complete the model of electric circuits. Pre-lab assignment Consider the circuit to the right in which all the bulbs are the same: (a) Based on current, rank the bulbs by their brightness (b) Based on voltage, rank the bulbs by their brightness. Introduction In the voltage experiment, we investigated a number of complex circuits and added the concept of voltage to our model. The model thus far consists of the following parts: 1. There is a flow in a complete circuit, which we call current. 2. The brightness of a bulb is an indicator for the current through the bulb. An increase in brightness indicates an increase in current through a bulb. Conversely, if the current through a bulb increases, the brightness of the bulb increases. 3. When the resistance of a circuit increases, the current through the battery or power supply decreases, and vice versa. 4. At a node, current splits according to the resistance in each branch. The branch with greater resistance gets less current. 5. The brightness of a bulb is an indicator for the voltage across the bulb. 6. In a series circuit consisting of a power supply, a bulb and a box, the voltage across the power supply is constant. The greater the resistance of the box, the greater the voltage across it, and the smaller the voltage across the bulb. 7. Kirchhoff’s Loop Rule: along any loop that contains a power supply, the sum of the voltages across all elements other than the power supply is equal to the voltage across the power supply. Section 1: Voltage in parallel circuits 39 Connect the voltmeter across bulb A as shown in the diagram above. Note the reading on the voltmeter. _________________________________________ 40 41 42 Section 3: Ohm’s Law In this section, you will further investigate the relationship between voltage, current, and resistance. i. Set up the circuit shown at right with the power supply switch at 3 V. In the space below, draw the corresponding circuit diagram. 43 Connect the voltmeter at the 15 V terminal. Note the reading on both the voltmeter and ammeter while the switch on the power supply is at 3 V and enter the values in the table below. Move the switch through 6 V, 9 V and 12 V, each time writing down the corresponding reading on the voltmeter and ammeter. Enter your values in the table below. If your power supply allows you to make more measurements, do so. Voltmeter Reading (V) Ammeter Reading (mA) ii. Replace the bulb with a resistor as shown to the right. Measure current and voltage for each of the four settings of the power supply. Enter values in the table below. Caution: Do not delay in making measurements and turn off power supply when finished. Voltmeter Reading (V) Ammeter Reading (mA) iii. Using the values in the tables, plot the data points for bulb and resistor in the graph below. Plot current on the horizontal axis and voltage on the vertical axis. Clearly label each data set. Do not draw any lines on the graph yet. 44 iv. The data points for the resistor should lie on straight line that passes the axes close to the origin. Draw this line, and find a numerical value for its slope. What units does the slope have? _____________________________________________________________________ ____________________________________________________________________ The slope of your graph is equal to the resistance (R) of the resistor. Show that Ohm’s Law holds for a resistor: V = IR. _____________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Determine the value for R. Show you work. _____________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ 45 46 Name:___________________________ Student No: ________________ Partner________________ Date:________ Demonstrator:____________________ Conservation of Momentum What should I expect in this experiment? You will investigate whether or not momentum is conserved in collisions. Pre-lab assignment In your own words, give definitions of conservation and momentum. Two cars have the same mass but one is travelling twice as fast as the other, which has the larger momentum? Introduction Momentum is a vector and is defined as mass multiplied by velocity: p = m v The principle of the conservation of momentum states that in any interaction the total momentum before is equal to the total momentum after. Momentum is conserved in both collisions (think of two snooker balls colliding) and explosions (the vector sum of the momenta of all the debris will sum to zero). In this experiment you will take data before and after collisions, and see whether or not the experimental data confirms or rejects the theory of conservation of momentum. First though, let’s try an example of the principle in action. Suppose your 50 kg friend faces you, at rest, wearing roller-blades and you throw a 5 kg bowling ball at 1 ms-1 to them which they catch. How fast will they move backwards? What is the momentum of the bowling ball before the collision? What is the momentum of the skater before the collision? What is the total momentum before the collision? What is the total momentum after the collision? What is the total mass after the collision? What is the velocity of the combined ball + skater after the collision? 47 Apparatus The apparatus you will use is shown here and consists of two carts that can travel along a low friction track and an Explorer GLX data logger with two photogates to time the motion of the carts. Each cart has a mass of 0.5 kg which can be adjusted by the addition of steel blocks each of mass 0.5 kg. To make sure gravity isn’t assisting the carts moving in one direction, ensure the track is completely level! (Use the height adjustment and a spirit level and don’t move the track once everything is correctly aligned.) Setting up the GLX • • • • • Connect the GLX to its AC adapter and press the power( ) button. Place photogate 1 at the 40 cm mark on the track and Photogate 2 at the 80 cm mark. Connect the photogates to the digital adapter and connect it to the GLX. A screen will appear, select “Photogate timing” and enter the flag length (0.0250 m) and photogate spacing (0.4000 m) by pressing to edit and again to confirm. Then press the home key. Press F2 to navigate to the table screen and then press twice and select “Time between Gates”. Investigation 1: Test the conservation of momentum in a head-on collision between carts of equal mass where one cart is at rest. You are going to fire cart ‘A’ at a stationary cart ‘B’ and observe the collision. Small springs on the edge of cart ‘A’ allow it to be launched from the end of the track. After an initial acceleration due to the spring it travels at constant velocity. First of all you must estimate the velocity of cart ‘A’. Use the wooden block to depress the spring to ‘fire’ the cart. Explain how you can measure the velocity of cart ‘A’ using ruler and stopwatch. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 48 Use the GLX to make three independent estimates of the velocity of cart ‘A’. Fill in the table below using the information you have and fill in the table below. • Attach the flags to the carts, you will only be using cart A for this part of the experiment. • Compress the spring of cart A and place at the 0cm mark on track. • • On the GLX, Press the key to begin recording data and press again once the cart has completed a run. Repeat the above steps until you have 3 competed runs. Reset the GLX once the data is collected. Make an estimate of the uncertainty on your estimate of the velocity. Think about whether distance or time has the most significant uncertainty. Cart ‘A’ Distance (m) Time (s) Velocity (m/s) Trial 1 Trial 2 Trial 3 What is your best estimate of the velocity of cart A before the collision? Why is this the best estimate? How large do you think the uncertainty is? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Place cart ‘B’ at rest on the track and fire cart ‘A’ toward it. After the collision, how could you work out the velocity of each cart? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 49 The GLX must now be configured for collision timing. This allows each photogate to measure velocity using the length of the flag and the time it spent in the gate. Measurements of the velocity of each cart are taken after the collision. If the total momentum of the carts is equal to the momentum of cart A before the collision, momentum is conserved. Fill in the table below for the velocities of each cart after the collision. Repeat the measurements three times. • • • • • • • • • • Move the first photogate to the 50cm mark and the second to the 90cm mark. Place cart B on the track so that the rear of the cart is at the 45cm mark with the flag attached to the front. Place cart A at the start of the track. Switch on the GLX. Make sure that the photogates are connected to the digital adapter and connect the adapter to the GLX. A menu should appear, select ‘Collision Timer’. Ensure that ‘Velocity 1’ and ‘Velocity 2’ are set to visible. Navigate to the home screen and select ‘Tables’. Press the select key and set ‘Velocity 1’ to be displayed in column 1 and ‘Velocity 2’ to be visible in column 2. Press the button to begin recording data. Take great care to ensure that you do not accidentally trigger the photogates as this will affect your results. Distance (m) Time (s) Velocity (m/s) Cart A Trial 1 Cart B Cart A Trial 2 Cart B Cart A Trial 3 Cart B What is your best estimate of the velocity of cart A after the collision? Comment if necessary. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 50 What is your best estimate of the velocity of cart B after the collision? Comment if necessary. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Analysis What is the total momentum of the carts before the collision? What is the total momentum of the carts after the collision? (Don’t forget the units) Do your results confirm or reject the hypothesis that in collisions momentum is conserved? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Investigation 2: Test the conservation of momentum in a head-on collision between carts of unequal mass where one cart is at rest. Change the mass of the stationary cart by adding a series of steel bars each of 0.5 kg. Measure the velocities of each cart after the collision and fill in the table below. For the heavier weights (1.5kg and 2.0kg), move the second photogate closer to the front of cart B. Do not forget to include units in your table. 51 Mass Cart B Velocity / Momentum / Total momentum Cart A 0.5kg Cart B Cart A 1.0kg Cart B Cart A 1.5kg Cart B Cart A 2.0kg Cart B Comment on the numbers in the final column. What should they be if the conservation of momentum holds? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 52 Name:___________________________ Partner________________ Date:________ Student No: ________________ Demonstrator:____________________ Conservation of Momentum II. What should I expect in this experiment? In this experiment you will further investigate the principle of conservation of momentum. Pre-lab assignment A car of mass 1000 kg hits a stationary car of the same mass at a velocity of 30 km/h and they move off together. What is the velocity of the two cars after the collision? Introduction As you have already seen the principle of conservation of momentum can be used to predict what will happen in a collision. In this experiment you will use the principle to make predictions about an explosive collision and make measurements to test these predictions. Apparatus The apparatus you will use is shown here and consists of two carts that can travel along a low friction track and an Explorer GLX data logger with two photogates to time the motion of the carts. Each cart has a mass of 0.5 kg which can be adjusted by the addition of steel blocks each of mass 0.5 kg. To make sure gravity isn’t assisting the carts moving in one direction, ensure the track is completely level! (Use the height adjustment and a spirit level and don’t move the track once everything is correctly aligned.) Setting up the GLX • • Connect the GLX to its AC adapter and press the power( ) button. Connect the photogates to the digital adapter and connect it to the GLX. 53 • • A screen will appear, select “Photogate timing” and enter the flag length (0.0250 m) and photogate spacing (0.4000 m) by pressing to edit and again to confirm. Then press the home key. Press F2 to navigate to the table screen and then press twice and select “Time between Gates”. Procedure: Starting from both carts at rest, an ‘explosion’ will send them apart. (The explosion occurs when the spring in cart ‘A’ recoils against ‘B’.) The total momentum before the explosion is zero. You must check whether the total momentum afterwards is also zero. If the total momentum after the collision is zero then mAv A + mB vB = 0 ⇒ mAv A = −mB vB Remember that momentum and velocity are vector quantites, so that the direction as well as the size are important. Here, a negative velocity or momentum is pointing in the opposite direction along the track to a positive one. To work out velocities you need to measure how far something travels in a given time. As you have been done you can calculate the velocity by measuring the distance a cart travels in a given time interval. The momentum is then the mass times the velocity. The GLX must be configured for collision timing, as for investigation 2 in the first momentum experiment. This allows each photogate to measure velocity using the length of the flag and the time it spent in the gate. Measurements of the velocity of each cart are taken after the collision. Fill in the table below for the velocities of each cart after the collision. • • • • • • • • Experiment to determine the best set up for the photogates for the measurements that you need to take. Switch on the GLX. Make sure that the photogates are connected to the digital adapter and connect the adapter to the GLX. A menu should appear, select ‘Collision Timer’. Ensure that ‘Velocity 1’ and ‘Velocity 2’ are set to visible. Navigate to the home screen and select ‘Tables’. Press the select key and set ‘Velocity 1’ to be displayed in column 1 and ‘Velocity 2’ to be visible in column 2. Press the button to begin recording data. Take great care to ensure that you do not accidentally trigger the photogates as this will affect your results. Repeat the ‘explosion’ four times, changing the mass of cart B and measuring the distances travelled by both carts for a given explosion. You may need to repeat explosions, keeping the masses the same, to gather information on each cart. Keep the 54 mass of cart A fixed at 0.5kg. What do you expect to happen to the distances travelled by the carts as you increase the mass of cart B? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ Distance (m) Trial 1 Trial 2 Trial 3 Trial 4 Time (s) Velocity (m/s) Mass (kg) Momentum (kg m/s) 0.5 Cart A 0.5 Cart B 0.5 Cart A 1.0 Cart B 0.5 Cart A 1.5 Cart B 0.5 Cart A 2.0 Cart B Comment on whether your data confirms or rejects the hypothesis that momentum is conserved in this ‘explosion’. Does your answer depend on how precisely you can measure the distances and the times? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ How would the ratio of the distances travelled by the carts in the same time relate to the ratio of the masses of the carts if momentum is conserved? ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 55 56