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”. You can get this from the Studio Physics CD (Physics1
folder) or from our web site on the Activities page under Activity 10 as LoggerPro File A. Copy
the file to your hard drive. Then double-click on it and launch LoggerPro with 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. Only zero the force probe, not all sensors.
Check to make sure that your calibration was successful. To do this, attach the 52 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 N. 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.
1. State the “Work-Kinetic Energy Theorem” (discussed in lecture today).
2. 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. (If you are using a “Motion Sensor
II” the cart can be as close as 15 cm.) What is the location of the back of the cart (the part
closest to the motion detector) when the hang mass hits the ground? What is the distance that
the mass will fall? Record your values.
3. 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 graphs on your activity sheets. Record only relevant data. (You don’t need to
sketch the velocity squared graph.) We have done this type of measurement before, so you
should know if you have good data or not.
4. 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 N because the cart is accelerating.)
COPYRIGHT2001 K. Cummings; Rev. 2004 Bedrosian
5. The graph of velocity squared can be used to calculate the kinetic energy (KE) of the cart
easily at any time. The mass of the cart plus force probe is 665 grams. What is the cart’s
initial KE? What is the cart’s final KE? What is the change in the cart’s KE?
6. 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 approximately the same in this case? Is the distance the
object falls the same? Is the work done on the cart approximately the same?
7. 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 your answer.
8. Now you should repeat the experiment with the extra 495 grams on the cart. Start the cart at
the same location as before. Click collect. While the cart is moving don’t let the force
probe’s cord drag or pull on it. Sketch your actual graphs of velocity vs. time and force
versus time on your activity sheet.
9. Determine the final KE of the cart and compare it with your answer to 5. Is the KE much
larger, much smaller, or about the same? Is your result consistent with the Work-Kinetic
Energy Theorem (within measurement error)?
10. Use the value of force that you measured in step 4 to calculate the work done by the tension
in the string on the cart (without the black bar) as the weight falls. What are you using for the
distance in this calculation and why did you use it? Why don’t we have to worry about the
cosine term in W=Fd?
11. 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?
12. Compare the change in the cart’s KE (step 5) 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.
13. Discuss your answer above in terms of the Work-Kinetic Energy Theorem.
Exercise
14. Use Newton’s 2nd Law to calculate the tension in the string when the hanging mass is
dropping. Use 52g for the hanging mass and calculate the tension with and without extra
mass in the cart. Are the two answers approximately or exactly equal?
(Note: If you think the answer is T = (9.8)*(0.052), you are on the wrong track.)
15. Use the Work-Kinetic Energy Theorem to predict the final KE of the cart (with and without
extra mass) when the hanging mass hits the floor. Use the same distance as in step 10.
16. If you did the calculations correctly, your predicted KE will be greater than the measured KE.
What would account for that? Calculate the KE difference: predicted minus measured.
17. Optional Challenge: Assume there is another constant force acting on the cart that we have
not measured, decreasing the net force on the cart. Estimate the magnitude of this force using
the result of 16 and the Work-Kinetic Energy Theorem. What might this force be?
COPYRIGHT2001 K. Cummings; Rev. 2004 Bedrosian