Studio Physics I
Work and Kinetic Energy
0.50Release
meters from rest--keep hand
out of way of motion detector
Experimental Setup
Get the file “WorkandEnergy.mbl”. This must be done by transferring it from the course web page.
This file type (.mbl) ONLY TRANSFERS CORRECTLY WITH INTERNET EXPLORER…
Don’t use Netscape for the transfer. You should right click on the file and save it to your disk.
Remember where you saved it. Open the file in LoggerPro. You should be able to do this by double
clicking on the file.
Calibrate your force probe. To do this, remove everything from the force probe and click the “zero”
button at the top of the screen.
Check to make sure that your calibration was successful. To do this, attach the 50 gram mass to the
force probe, run the line over the pulley and let the mass hang straight down. Click collect and let
the software collect data for awhile. Click the “x=?” icon on the tool bar and point the cursor at the
graph of force versus time. The force should be very close to 0.5 newtons. If it is not, ask your
TA or Professor for help.
You should not need to calibrate your force probe again less you crash the cart (hard) into something.
You can check to make sure that the force probe is reading accurately at any time by repeating the
step immediately above.
DO NOT ZERO THE FORCE PROBE AGAIN UNLESS THE HANGING MASS HAS BEEN
REMOVED FROM THE FORCE PROBE.
Set up the equipment as shown above, if it is not already done for you. Note the distance between the
cart and motion detector shown in the figure.
1. State the “Work-Energy” theorem (discussed in lecture today).
2. The frictional force acting on the cart is very small and can be ignored. The back of the cart
starts at least ½ meter away from the motion detector and is pulled with a constant force
(applied by the tension in the string, but indirectly due to the weight hanging on the end of the
string). Take 2 or 3 minutes to sketch an INDIVIDUAL prediction of what the velocity of the
cart versus time and applied force on the cart versus time graphs should look like as the weight
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falls. Compare your predictions with others in your group and make sense out of what everyone
thinks.
3.
Set up the equipment as shown above. Figure out what you will use for the starting location of
your cart- it must be at least 50 cm from the detector. WRITE THIS location DOWN. What
is the location of the back of the cart (the part closest to the motion detector) when the hang mass
hits the ground? WRITE THIS VALUE DOWN. What is the distance that the mass will fall?
4.
You are now ready to take data. You MUST hold the cord from the force probe up off of the
table in such a way that you do not interfere with the motion detector, but that you also do not
pull one way or the other on the cart. This cord cannot drag on anything as the cart moves.
Click collect and record data for the motion of the cart as the weight falls. Don’t click on the
stop button to stop taking data. It is unnecessary and sometimes messes up your results. Sketch
the actual graphs on your activity sheets. Record only relevant data. (You don’t need to sketch
the velocity squared graph). If your actual graphs do not closely correspond to your predictions,
you have a problem. Either you got “bad”, invalid data or your predictions were wrong. You
need to figure out which one it is and get valid data if you have not already done so.
5. Click on the “x=?” icon in the toolbar at the top of the page. You can use this tool to determine
exact values on your graphs by pointing your cursor to the point of interest. Determine the value
of the force applied to your cart WHILE THE CART IS MOVING and note it on your paper.
(Note, this is not 0.5 newtons because the cart is moving.---You will look at that issue in more
detail on your homework.)
6. The graph of velocity squared can be used to easily calculate the kinetic energy (KE) of the cart
at any time. The mass of the cart plus force probe is 665 grams. What is the cart’s initial KE?
(The cart starts at rest). What is the cart’s final KE? What is the change in the cart’s KE?
7. Now add a 495 gram mass to the cart. (This is one of the black bars). Doing so just about
doubles the mass of the cart. Suppose that you now repeat the experiment that you just did. You
start the cart at the same point as you noted in question 3 and keep everything else the same.
Will the applied force be the same in this case? Is the distance the object falls the same? Is the
work done on the cart the same?
8. PREDICT whether the final KE of the cart with the extra 495 grams will be the same, greater
than or less than the value you found in question 6 for the cart without the extra mass. Making
use of the work-kinetic energy theorem, carefully explain why you answered the way that you
did.
9. Now you should actually repeat the experiment with the extra 495 grams on the cart. Start the
cart at the same location as before. Click collect and don’t let the force probe’s cord drag. Don’t
pull on the cord either. Sketch your actual graphs of velocity vs. time and force versus time on
your activity sheet.
10. Use the x=? or “stats” icon at the top of the screen to determine the value of the tension in the
string WHILE THE CART IS MOVING in this case. Record it. Is the force the same as it was
for the cart without the extra mass (question 5 above) ? If it is different, how different is it? A
lot (like 50% or more) or just a little (like less than 10% different) (We will consider whether
the force should or should not be the same as part of a homework question).
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11. Is the work done on the cart by the applied force the same in this case as it was when the cart
was 495 grams lighter?
12. What is the total mass of the cart, force probe and extra mass? What is the cart’s initial KE?
What is the cart’s final KE in this case? What is the change in the cart’s KE?
13. Is the change in the cart’s KE in this case basically the same, much larger or much smaller than it
was in the case without the extra mass on the cart (question 6 above)?
14. According to the work-kinetic energy theorm, should the change in the cart’s kinetic energy
be affected by the mass of the cart? Why or why not? Use your answer to question 15 as the
basis of your answer to the following question: Are the results of these TWO experiments
consistent with the work-energy theorem? If so, how so? If not, why not?
15. The following statements are made by two students while discussing the experiments above:
(In one case, a hanging mass (m) was attached to a massive cart (M#1). In the other case, the same
hanging mass (m) was attached to a less massive cart (M#2).)
Student A: If the same force, for example the same hanging mass, acts on two different objects, the
object with the larger mass will have the smaller acceleration. That means that its velocity will change
less than the velocity of the less massive object would. Since the change in velocity is smaller, the
change in KE is smaller. Therefore, the change in KE is smaller for the more massive object.
Student B: No, way. The work energy theorem states that the net work done on an object is equal to the
change in its KE. So if the same force acts for the same distance on the two different objects, both
objects undergo the same change in KE. The same force produces the same acceleration for the two
objects and so they have the same change in velocity.
Explain/discuss what is correct and incorrect in each student’s statement.
16. Use the value of force that you measured in question #10 to calculate the work done by the
tension in the string on the cart as the weight falls. What are you using for the distance in this
calculation? (The distance the weight falls or the distance between where the cart starts out and
the end of the track?) Why did you use this distance in your calculation? Why don’t we have to
worry about the cosine term in W=Fd?
17. Do any other forces act on the cart in the horizontal direction? If so, what are they? What is the
NET work done on the cart as the weight falls? If the cord from the force probe were allowed to
drag on the table top, would your answers to these questions be the same? Why or Why not?
18. Compare the change in the cart’s KE to the work done on the cart by the tension in the string.
Are they about the same? What is the percent difference between them? If the percent
difference is greater than about 10%, speak to your TA or professor.
19. Discuss your answer above in terms of the work-energy theorem.
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If you finish early, you should do the following:
20. Add an additional 20-50 grams to the hanging mass. Keep the extra 495 gram mass on the cart.
If you now repeat this experiment, keeping everything the same except the amount of hanging
mass , PREDICT whether the final KE of the cart will be the same, greater than or less than the
value you just found for the cart with extra mass but smaller hanging mass.
21. Repeat the experiment with the extra 495 grams on the cart and the extra mass added to the
hanging mass. Measure the force applied to the cart WHILE THE CART IS MOVING and
calculate the net work done on the cart in this case. Is the work done on the cart the same as for
the cart with the extra mass but the lesser hanging mass (question 16 above)?
22. What is the cart’s initial KE? What is the cart’s final KE? What is the change in the cart’s KE?
Is the change in the carts KE the same, larger or smaller than it was in the case with the extra
mass on the cart but smaller hanging mass(question 12 above)?
23. Compare the change in the cart’s KE to the work done on the cart by the tension in the string.
Are they about the same? What is the percent difference between them?
24. Is the result of this experiment consistent with the work-energy theorem? If so, how so? If not,
why not?
COPYRIGHT2001 K. Cummings