LAB 8

PhysicsLab8 June 16, 2010 Name __________________
Dr Dave Menke, Instructor

I Title: Potential vs. Kinetic Energy

II Purpose: To investigate the difference between potential energy and kinetic energy by using “elastic” bouncing spheres.

III Equipment:
· Meter stick
· Stopwatch
· Writing tool
· Superball of 8.5 grams
· Masking tape

IV Procedure
1. Measure 2.0 meters above the floor, and mark it with masking tape
2. Raise the superball to h1 = 2.0 meters above the lab floor.
3. Calculate the potential energy of the superball, PE = mgh1, by placing the first superball on the ground, and then lifting it up two meters. Record. This will be in JOULES.
4. Calculate the amount of time that it takes for the superball to fall 2.0 meters. Remember the “free fall” relationship: h1 = ½ g t2. You already know h = 2.0 meters. And g = 9.8 m/s2. Record. t = √[(2h)/(g)]
5. Calculate the final velocity for the superball, vf, using the relationship that vf2 – vi2 = 2 g h1. Since vi = 0, vf = √(2 g h1) = √[(2)(9.8)(2.0)]. Record.
6. Calculate the Kinetic Energy of the superball just before impact. KE = ½ m vf2. Record.
7. Compare the KE in #6 with the PE in #3. Which is larger, or are they equal? Explain why.
8. Place the superball at h1 = 2.0 meters above the floor. Release the superball. Measure how high the superball rebounds after it hits the floor, h2. Record.
9. Repeat 4 more times, for a total of 5 times. Take the average of the rebound, h2. Record.
10. Using DPE = m g (Dh), find out how much energy was “lost” by the superball when it bounced up. Remember, Dh = h1 – h2.
11. List places that the energy “went.”

V Data & Calculations
1. Mass of superball: _________grams; convert to kg____________

2. Initial PE = mgh1 = ____________ joules

3. Time for sphere to fall 2.0 meters: _________ seconds.

4. The final velocity, vf : ____________ m/s.
PhysicsLab8, page 2

5. Kinetic energy: ___________ Joules

6. The difference between PE and KE, in joules: ________J.

7. The average change in height (the average of the 5 trials): _________ meters.

8.The final PE after the rebound, m g h2 : ________

9. Subtract the final PE in #9 from the initial PE in #3. DPE = __________ joules.


VI Results
Explain if there were a difference between PE and KE, and why. Also, explain where the “lost energy” went.


VII Error Analysis
A. Quantitative Error –
The true laboratory value of final velocity, vf(true) = 6.261 m/s; the lab value for the mean velocity is vave = 3.131 m/s.

% Error = [ |v(true) – v (yours)| / v(true) ] x 100%

B. Qualitative Error: Sources of Error
1. Personal
2. Systematic
3. Random

VIII Questions
1. Spheres made of rubber cause collisions that are somewhat elastic. a. name objects where the collisions would be almost perfectly elastic; b. name objects where the collisions would be almost entirely inelastic.

2. In light of these concepts, why do you think that some football players are “light” and others are very heavy and muscular?