I recently developed this lab for my online class. Students would bounce a bouncy ball such as a ping pong ball or a tennis ball, measure the KE and PE at different points and explain what is happening.

Here is what I hoped would happen:

Students would make a video of the ball bouncing using the Hudl app and get a slow motion video of the ball near a ruler. The students could measure the PE/m by measuring the height of the ball. PE = mgh. Most folks did that just fine. For KE, I was hopping that the students would measure the change in distance over some very small time interval, determine the velocity, then find KE/m using the equation KE = 1/2 m v².

When you plot the KE, PE , and E it should look like this:

Each time this ball hits the ground there is an inelastic collision with the ground and some energy will change form, converted into heat, drive sound waves, etc etc. There may be some loss of energy while the ball is in the air due to drag but that effect will likely be smaller than the collisional change, so it will look like a step.

There were a number of common errors in the lab:

• Some folks read the height at top of the trajectory and time it took to get to the bottom. This does give the average velocity of the fall, but it does not give the instantaneous velocity. You need the instantaneous velocity for this lab, to determine the total energy at one moment.
• Some folks found the PE at the top and the KE at the bottom and added them together. The PE and KE only add together at the same moment of time.
• Some folks found the KE by solving E=KE at the bottom. That’s ok. BUT many said that E = KE + PE and that energy was conserved and solved for the velocity at different heights, not realizing that energy had been removed from the system. Then they recalculated the energy with the new KE. The PE plus KE now had an increasing trend. Which can never happen unless you are driving the system somehow.

AND there were two wonderful students who won the day. Instead of finding the instantaneous velocity directly, they used the time it took from the top height to the bottom and the constant acceleration equations to determine the final velocity. Not perfect, but brilliant.