# Physics Lab - Work & Kinetic Energy

(uses Science WorkshopTM Interface)

## Purpose:

The purpose of this lab is to compare the work done on an object with the object's change in kinetic energy.

## Discussion:

The Work/Energy Equation says "The work done on an object by the net force on it equals the object's change in kinetic energy." or, in symbols:

In this lab, you let gravity do work on a dynamics cart and compare the work done on the cart to the cart's kinetic energy. Here's how it's done.

As just mentioned, a hanging mass will be used to supply the force, so if the mass is m, then Fnet = mg. First, you can start the cart from rest, so that its initial kinetic energy is zero.

You might want to try the Work & Kinetic Energy Simulation before you do this lab. It will give you a good idea of what quantities you need to measure and calculate.

## Setup:

1. Your instructor may have already assembled the apparatus, but if not:
1. Attach the Science WorkshopTM interface to the computer and switch the interface on.
2. Attach the photogate plug to Digital Channel 1 on the interface.
3. Attach the bumper and pulley assembly to the dynamics track as shown.
2. Open the "Simple Timer" experiment file. (Ask your instructor where to find this file).
3. Level the dynamics track using the screw(s) on the bottom of one end.
4. Attach one end of the string to the dynamics cart and the other end to a weight hanger (which you can make out of a paper clip). The weight hanger should be just below the pulley when the cart is at the opposite end of the track. Measure and record the mass of the dynamics cart, string, and weight hanger.
5. Construct a "timer flag" for the dynamics cart from the index card or stiff paper. Measure and record the width of the flag, d, in the data table, then attach the flag to the dynamics cart with a small piece of tape.
6. Pick and record a starting position for the cart near the non-pulley end of the track. You might want to use a small piece of tape to mark this position.
7. Now, pick a stopping position for the cart near the pulley end of the track. Be sure that the cart reaches the stopping position on the track before the weights touch the floor!

8. Position the photogate on the track so that the LED on the photogate comes on just as the dynamics cart reaches the stopping position.
9. Place a small mass on the weight hanger Record the mass, m, in the data table.
Note: In the data table, the mass hanging from the weight hanger is called "Hanging mass, mh". It is the weight of this mass that pulls the dynamics cart. The column labeled "Added mass, ma" is where you record any mass added to (riding on) the cart in a trial.
10. Try a practice run. You need to release the cart from rest at the starting position. The photogate LED should light just as the cart reaches the stopping position, and you should catch the cart before it hits the bumper.

## Procedure:

1. Position the cart at the starting position on the track.
2. Click "Start" in the experiment window.
3. Release the cart.
4. Catch the cart BEFORE it hits the bumper.
5. Press "Stop" in the experiment window.
6. Record the time in your data table.
7. Repeat to check the consistency of your results. When you are satisfied, change the hanging mass, or add mass to the dynamics cart and repeat. For convenience, don't change the starting or stopping positions.

## Results:

Note about units: In this lab, it is most convenient to measure distances in centimeters and masses in grams. Instead of converting these to kilograms and meters, the data tables are set up to accept the original units. This means that forces will have units of gm cm/s2. The name for this force unit is "dyne". The dyne is not used as often these days as the Newton, but it is a convenient unit for small forces. In this system, work and energy will have units dyne cm or gm cm2/s2, which is called an "erg". Although the erg is not as commonly used as the Joule, it is still a convenient unit for small energies.

For each trial. (Show a sample calculation.)

1. Calculate the total mass set in motion, M = mc + ma + mh.
2. The force, F, that pulls the cart is the weight of the hanging mass = mh. (Note: g = 980 cm/s2)
3. The velocity, v, of the cart equals the width of the flag divided by the time for the flag to pass through the photogate = d/t.
4. The work done by gravity = .
5. The kinetic energy of the system = .
6. The percent of difference between the work done on the system and its final kinetic energy.

## Conclusions:

How does the work done on a system by the net force compare to the change in kinetic energy of the system? Why do you think so? What do you think accounts for the discrepancies in your results?

last update November 21, 2007 by JL Stanbrough