Download Physics Laboratory Manual PHYC 10190 2014-2015

Physics Laboratory Manual
PHYC 10190
Aspects of Physics for Ag. Science
2014-2015
Name.................................................................................
Partner’s Name ................................................................
Demonstrator ...................................................................
Group ...............................................................................
Laboratory Time ...............................................................
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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
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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
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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.
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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.
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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.
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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.
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Set up the circuit shown at right and verify
your ideas. What did you observe?.
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Figure 1.2 Two bulbs in series connected to a
power supply
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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?
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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.
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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.
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ii. Set up the circuit and verify your prediction.
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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?
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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.
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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.
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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?
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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?
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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.
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_________________________________
_________________________________
_________________________________
_________________________________
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
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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.
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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.
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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.
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How does the current through the
battery compare to the current
through the bulb. Explain briefly
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_________________________________
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_________________________________
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.
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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
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_________________________________
_________________________________
_________________________________
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Figure 4.3 Parallel circuit with ammeter
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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?
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_________________________________
_________________________________
_________________________________
_________________________________
_________________________________
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?
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v. Which of the student statements in part iv of Section 3 is consistent with your
measurements?
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What can you infer about the resistance of a parallel circuit as compared to that
of a single bulb circuit?
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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?
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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
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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.
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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
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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?
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Is your answer consistent with what you found in Section 3 when you added a bulb in
parallel with a single bulb? Explain briefly.
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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.
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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.
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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.
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__________________________________
_________________________________
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.
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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.
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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
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Connect the voltmeter across bulb A as shown in the diagram above. Note the
reading on the voltmeter.
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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.
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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.
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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?
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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.
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Determine the value for R. Show you work.
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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?
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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.
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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?
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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?
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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.
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What is your best estimate of the velocity of cart B after the collision? Comment if
necessary.
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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?
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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.
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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?
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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
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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.
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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.
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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
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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?
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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?
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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?
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