LAB 7

PhysicsLab7 Monday August 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:
* One “elastic” bouncing sphere of 8.5 gram mass or 32.4 grams
* Metric ruler
* Pen, calculator, lab book, etc.

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

V Data & Calculations
1. Mass of elastic sphere: _________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.

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 #8 from the initial PE in #2. 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
Personal
Systematic
Random

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