Download PRELAB: FORCES AND MOTION

Name: ______________________________
Forces and Motion, p. 1/11
PRELAB: FORCES AND MOTION
1. What is the purpose of the rubber bands in Activity 1-1?
2. What is the difference between a linear relationship and a proportional one?
3. Sketch Prediction 2-2 on the set of axes below. Explain the reasons behind your prediction.
PREDICTION 2-2
velocity
+
0
-
acceleration
+
0
force
+
0
-
time (s)
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 2/11
FORCE AND MOTION
Topic: Newton’s Laws in 1D
Objectives:
•
To quantify “pulls” and “pushes”
•
•
To identify the effect that “pulls” have on the motion of an object
To find the mathematical relationship between the force applied to it and its acceleration
• To critically examine the inferences that can (and can’t) be drawn from observations
Overview:
In previous labs, you have used a motion detector to display position, velocity and acceleration
graphs of various objects moving in one dimension. You were not concerned about how you got the
objects to move, i.e. what pushes or pulls acted on the objects. From your own experiences, you
know that amount of pull or push and motion are related in some way. You’ve probably also heard
in lecture that the acceleration an object undergoes is proportional to force acting on the object.
This mathematical statement, often called Newton’s 2nd law, can be taken as Newton’s definition of
force. Because of the similarities between Newton’s mathematical definition of force and our
notions about pushes and pulls, it seems natural to conclude that our simplistic notion of “pull” and
“push” corresponds exactly to the word “force” in Newton’s law. This lab examines whether this
assumption is a reasonable one. This will be done in two steps. You will first develop a way to
measure “pulls.” You will then examine how the motion of a cart is related to the amount of pull
being exerted on it.
There are two educational purposes for this lab. The most obvious purpose is to give you some
intuitive sense about the relationship between pulls/pushes and motion. The second, far less
obvious, educational goal is acquaint you with some of the issues involved in developing/testing
ideas about the world. Throughout this lab, you will critically examine what claims can be
substantiated by the experimental evidence you collect.
Safety/Equipment Notes:
Mounting Hardware: Your TA will demonstrate how to attach the force probe to the cart.
Do not allow the cart crash into the pulley! Before you start each run, make sure someone is
ready to stop the cart at the far end of the track.
Strong pushes and pulls may damage the force probes. The probes are rated for a
maximum of 50 N.
Writing it up:
In this handout, you will be asked to perform calculations, analyze graphs and answer questions. It is
strongly recommended that you do all the calculations and answer all the questions as you go through
the experiment. This record is for your purposes only. You will not be graded on it. You will be graded
on how well you understand the material covered in this lab, so you should do the entire activity,
discuss the questions with your partners and take careful notes.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 3/11
Investigation 1: Measuring Pulls
In this investigation, you will learn how to construct a QUANTITIVE scale for measuring the
amount of pull.
Activity 1-1: How large is a pull?
If you pull on a rubber band attached at one end, you
know it will stretch. The more you pull, the more it
stretches. Try it.
1. Attach one end of the rubber band to something
on the table that can’t move. Also attach a meter
stick to the table. Now stretch the rubber band so
that it is a several centimeters longer than its relaxed length. Does it always seem to exert the
same pull on you each time it is stretched to the same length? (Most people agree that this is
obvious.)
2. Write down the length you have chosen in the space blow. This will be your standard length for
future measurements.
Standard length of rubber band = _________ cm
3. Attach one end of each of two identical rubber bands to some thing that can’t move and stretch
them together side-by-side to the standard length.
Q1-1: How does the combined pull of two rubber bands compare to what you felt when
only one rubber band was used?
4. Repeat this comparison of how strong the pull feels with three, four and five rubber bands
stretched together to the same standard length.
Q1-2: Suppose you stretched a rubber band to your standard length by pulling on it. Now
you want to create a pull six times as large. How could you create such a pull?
Q1-3: Suppose you applied a pull with a stretched rubber band one day, and several days
later you wanted to feel the same pull or apply it to something. How could you
assure that pulls were the same? Explain.
Q1-4: Do side-by-side rubber bands provide a convenient way of accurately reproducing
pulls of many different sizes? Explain.
You have seen that pulling more rubber bands to the same length requires a larger pull. To be more
precise about the pulls and pushes you are applying, you need a device to measure these pulls
accurately.
Activity 1-2: Measuring “Pulls” with an Electronic Probe
In this activity you will explore the capability of an electronic probe as a measuring device for
“pulls.”
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 4/11
1. Plug the probe into the computer and start the software. Use the software to set the probe to
“pull positive” and fastest sampling rate possible. Display force* vs. time axes.
“Zeroing” the probe: The electrical signal the probe generates can drift with time,
temperature and other factors. Pressing the zero button resets the output of the probe to
zero. This process is called “zeroing.”
2. Zero the probe with nothing pulling on it. The probe should lie flat on the table.
3. Attach one end of a rubber band to something that can’t move, as before, and the other end to
the force probe hook. Pull horizontally on the rubber band with the force probe until the rubber
band is stretched to the same standard length you used in Activity 1-1, and begin graphing while
holding the rubber band steady for the whole graph. Use the software to get an accurate reading
of the force.
Record a typical force reading: ____________
4. Repeat the previous step with different number of rubber bands, arranged side-by-side, as in
Activity 1-1. Record the results in the table below:
Number of
rubber bands
0
1
2
3
4
5
Probe reading
Comment: We are interested in the nature of the mathematical relationship between the
reading of the force probe and the force (in rubber band units). This can be determined from a
graph of the data above by drawing a smooth curve that fits the plotted data points. Some
definitions of possible mathematical relationships are shown below. In these examples, y might
be the force probe reading and x the number of rubber bands.
These graphs show the differences between these three types of mathematical relationship. y can
increase as x increases, and the relationship doesn’t have to be linear or proportional. Proportionality
refers to the special linear relationship where the y intercept is zero, as shown in the example
graph at the right.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 5/11
5. Use the software to create a chart of the data above. Use the chart to make a graph of the data.
Then use the fit routine in the software to find the relationship that best fits the plotted data.
Try a linear fit and any other fit you think is appropriate. When you have found a good fit, print
the graph and the fit equation. Write the fit equation in the space below:
Fit Equation: ________________________
Q1-5: Is it fair to say that the probe output is proportional to the amount of pull on the
hook? Explain.
Q1-6:
Suppose the probe output was not proportional to the amount of pull. Could the
probe still be used to measure “pull”? Would it be as convenient? Explain.
Q1-7: Based on your observations so far, is it possible to identify the quantity that the
probe measures as the “force” referred to in Newton’s laws?
è Check your answers with an instructor.
Comment on units and calibration:
You can use your measurements to define a
quantitative “pull” scale. You might call it the “rubber band scale” or name it after
yourself or your lab partner. Whenever the probe has the reading corresponding to the
pull of one rubber band, the “pull” is equivalent to one “RBU” (i.e. one rubber band
unit), or one “Mary” or one “Sam.” Any larger pull can be measured as some number of
these units.
Your graph relates two different ways of measuring pull, one with standard stretches of
different numbers of rubber bands (RBU’s) and the other with a force probe. Such a
graph is called a calibration curve and is used to compare measurements of quantities made
with two different measuring methods.
Physicists have defined a standard unit of pull called the newton, abbreviated N. For the
rest of your work on forces and the motions they cause, it will be more convenient to
have the probe measure directly in newtons. Then the pulls you measure can be compared
to the forces everyone else measures. All that needs to be done is to calibrate the probe to
read forces in newtons. This has been done by the manufacturer. The calibration can be
checked using applying pulls with known strengths to the force probe (such as with a
spring scale or by handing weights from the probe hook).
Investigation 2: Motion and Force
Now you can use the probe to apply pulls with known strengths to an object. You can also use the
motion detector, as in the previous two labs, to examine the motion of the object. In this way you
will explore the relationship between motion and “pull.”
Activity 2-1: Pushing and Pulling a Cart
In this activity you will move a low friction cart by pushing and pulling it with your hand. You will
measure the probe output, velocity and acceleration. Then you will look for mathematical
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 6/11
relationships between the probe output and the velocity and acceleration, to see whether either is
(are) related to probe output.
1. Set up the cart, probe, and motion detector on a smooth level surface as shown below. The
cart should have a mass of about 1 kg with force probe included. The force probe should be
fastened securely to the cart so that the cart’s body and cable do not extend beyond the end of
the cart facing the motion detector.
Prediction 2-1: Suppose you grasp the probe hook and move the cart forward and backward in
front of the motion detector. Do you think either the velocity or the acceleration graph will look like
the “pull” graph? Is either of these motion quantities related to “pull”? (That is to say, if you apply a
changing pull to the cart, will the velocity or acceleration change in the same way as the probe
output? Explain the basis for your prediction.
velocity (m/s)
+1
0
-1
acceleration (m/s2)
force (N)
+2
0
-2
+2
0
-2
1
2
3
4
5
time (s)
2. To test your predictions, plug in the motion detector. Use the software to set the motion
detector up to measure velocity and acceleration at the maximum possible sampling rate. Display
axes for velocity, force and acceleration. Make sure the time axes for the three graphs are aligned
vertically as shown below.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
PREDICTION
+
force
Name: ______________________________
Forces and Motion, p. 7/11
-
3. Zero the probe. Grasp the probe hook and begin graphing. When you hear the clicks, quickly
pull the cart away from the motion detector and quickly stop it near the far end of the track.
Then quickly push it back toward the motion detector and again quickly stop it. Pull and push
the force probe hook along a straight line parallel to the track.
+
4. Carefully sketch your graphs on the axes above, or print them out and label them.
0
velocity
Q2-1: Does either graph –velocity or acceleration— resemble the force graph? Which one?
Explain how you reached this conclusion.
0
acceleration
+
0
-
Q2-2: Based on your observations, does it appear that there is a mathematical relationship
between probe output (i.e. amount of “pull”) and velocity, probe output and
acceleration, both, neither? Explain.
Activity 2-2: Speeding up again
You have seen in the previous activity that probe output and acceleration seem to be related. But
just what is the relationship between probe output and acceleration? In this activity, you will
time (s)
examine what
happens as a cart pulled by a string accelerates.
Prediction 2-2:
Suppose that you have a cart with very little friction and you pull this cart
with a constant amount of “pull” as shown below on the force-time graph. On the axes below,
sketch your predictions of the velocity-time and acceleration-time graphs of the cart’s motion.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
FINAL RESULTS
+
force
Name: ______________________________
0
-
velocity
+
Forces and Motion, p. 8/11
1. Test your predictions. Set up the ramp, pulley, cart string motion detector and probe as shown
below. The cart should be the same mass as before. Be sure the cart’s friction is at a minimum.
(If the cart has a friction pad, it should be raised so it doesn’t contact the ramp.) Prepare to
graph velocity, acceleration and force. (If you’ve followed the directions to this point, this should
already set up).
0
acceleration
+
2. It is important to choose the amount of falling mass so that the cart doesn’t move too fast to
observe the motion. Experiment with different hanging masses until you get the cart to move
across the track in about 1-2 sec after the mass is released.
Record the hanging mass that you use: ____________ (Don’t forget the units)
0
-
Also test to make sure that the motion detector “sees” the cart throughout the motion.
3. Zero the force probe with the string hanging loosely so that no force is applied to the probe.
Zero the probe before each run.
time
(s)cart after you hear the clicks of the motion detector. Be sure that the cable from the force
4. Release
the
probe is not “seen” by the motion detector, and that it doesn’t drag or pull the cart. Repeat until you get good
graphs over its whole motion. Keep this data on the screen. You will need it for Activity 2-3.
5. Carefully sketch your graphs on the axes above, or print them out and label them.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 9/11
Q2-3: After the cart is moving, is the amount of “pull” applied to the cart by the string
constant, increasing, or decreasing? Explain based on your graph.
Q2-3: How does the acceleration graph vary with time? Does this agree with your
prediction? Does a constant “pull” produce a constant acceleration?
Q2-4: How does the velocity vary in time? Does this agree with your prediction? What kind
of change in velocity corresponds to a constant applied “pull”?
Q2-4a: Based on your observations so far, is it possible to identify the quantity that the
probe measures as the “force” referred to in Newton’s laws? Explain.
è Check your answers with an instructor.
Activity 2-3: Acceleration from different forces
In the previous activity you examined the motion of a cart with a constant amount of “pull” applied
to it. But what is the mathematical relationship between “pull” and acceleration? If you apply a larger
pull to the same cart (while the mass of the cart is not changed) how will the acceleration change? In
this activity you will try to answer these questions by applying different pulls to the cart and
measuring the corresponding accelerations.
Prediction 2-3:
Suppose that you pulled the cart with a pull about twice as large as before.
What would happen to the acceleration of the cart? Explain.
1. Test your prediction. Keep the graphs from Activity 2-2 displayed on the screen.
2. Accelerate the cart with a larger force than before. To produce a larger force, hang a mass about
twice as large as in the previous activity.
3. Record the hanging mass: __________ (Don’t forget units)
4. Graph force, velocity, and acceleration as before. Don’t forget to zero the force probe with nothing
attached to the hook before graphing.
5. Sketch your graphs as well as the graphs from Activity 2-2 on the axes on the next page, or print
them out and label them.
6. Use the analysis tools of the software to determine the average pull and average acceleration for
the cart in this activity and Activity 2-2, and record the values in the table below. Note: To find
the average pull, you will probably want to use the statistics feature on the graph of the probe’s
output; to find the average acceleration, you may want to use the fit tools on the velocity
graph… Make sure that you only consider the time intervals when the force and acceleration were nearly
constant.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
FINAL RESULTS:
Activity 2-2 and Activity 2-3
force
+
Name: ______________________________
Average Force (N)
0
Forces and Motion, p. 10/11
Average Acceleration (m/s2)
Activity 2-2
Activity 2-3
-
velocity
+
0
acceleration
+
0
-
time (s)
Q2-5: How did the force applied to the cart compare to that with the smaller force in
Activity 2-2?
Q2-7: How did the acceleration of the cart compare to that caused by the smaller force in
Activity 2-2? Does this agree with your prediction? Explain.
Activity 2-3: Relationship between acceleration and force
1. Gather data for average acceleration and average force for several other forces applied to the
same cart. Complete the table on the previous page.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004
Name: ______________________________
Forces and Motion, p. 11/11
2. Use the software to create a graph of the data in the table. Use the fit routine to determine the
mathematical relationship between the acceleration of the cart and the force applied to the cart.
3. Print the graph with the fit equation and label it.
Q2-8: Does there appear to be a simple mathematical relationship between the acceleration
of a cart (with fixed mass and negligible friction) and the amount of pull measured
by the probe? Write down the equation you found and describe the mathematical
relationship in words.
Q2-9: If you increased the force applied to the cart by a factor of 10, how would you
expect the acceleration-time graph of the cart’s motion to change? Explain based on
your graphs.
Q2-10: If you increased the force applied to the cart by a factor of 10, how would you
expect the velocity-time graph of the cart’s motion to change? Explain based on your
graphs.
Q2-12: Based on your observations so far, is it possible to identify the quantity that the
probe measures as the “force” referred to in Newton’s law? Explain.
Q2-13: What is the value of the slope of the force-acceleration graph? How does this value
compare to the mass of the cart (as measured by a balance)?
Q2-14: Based on your observations so far, it is possible to conclude that the slope of the
force-acceleration graph and the mass represent the same quantity? If so, explain
how the observations you have made so far support the conclusion. If not, describe
an experiment that could be used to help substantiate this claim.
è Check your answers with an instructor.
Adapted from RealTime Physics (Sokoloff et al.) for P13
Revised: 1/2004