PHYS 1405 – Conceptual Physics I

Conservation of Mechanical Energy

 

Materials

Pasco Free Fall Apparatus                                Ring stand

Meter stick                                                       Lab Jack

 

Introduction

      In this activity we will investigate energy transformation in a freely falling object.  We will determine the change in the gravitational potential energy of an object as it falls and compare that to the corresponding the kinetic energy.

 

Procedure

1.      Determine the mass in kg of the steel ball:  mass = _________________ kg

2.      Set up the ball release mechanism on a vertical stand and place the touch pad on the table top directly beneath the release mechanism.

3.      Place the steel ball in the mechanism and set the tightening screw so that the ball is secure.

4.      Make sure that the touch pad lies directly underneath the ball.  

5.      Measure the distance, h, between ball release mechanism and the table top, as shown in Figure 1.

 

     Ball Release Mechanism

 

 

 

 

 

 

 

 


                                                  h             Meter Stick

 

 

 


                         Touch Pad

Figure 1.                             

6.      Make sure that the timer is on and reads 0.  If it doesn’t press the red reset button.  Turn the screw to release the ball.  Record the time it takes the ball to fall in the table below.

7.      Place the touch pad on top of the lab jack and make sure that the touch pad lies directly below the release mechanism.  Measure the new distance between the touch pad and the bottom of the release mechanism.  Release the ball and record the height and the time in your data table below.

8.      Repeat the experiment 3 more times each time raising the height of the touch pad by .08 m.  You should have a total of 5 data points.

 

 


Data

 

 

Distance between Release Mechanism and Touch Pad

(m)

Elapsed Time

 

 

(s)

1

 

 

2

 

 

3

 

 

4

 

 

5

 

 

 

Analysis

1.  From the mass of the ball and the height in meters, determine the change in gravitational potential energy for each trial.  Record in the table below.

 

Q1)  Did the gravitational potential energy increase or decrease?  How will you show this in the table?

 

 

2.  From the time it takes the ball to fall, determine the speed at the bottom for each trial.  Calculate the change in kinetic energy at the bottom for each trial.  Since the ball is essentially released from rest, we assume the kinetic energy at the top is 0 J.  Use 9.8 m/s/s for g.

 

Q2)  Did the kinetic energy increase or decrease?  How will you show this in the table?

 

 

 

3.  The change in mechanical energy is the sum of the changes of the kinetic energy and the gravitational potential energy.  Determine the change in mechanical energy for each trial.  DE = DKE + DGPE

 

DGPE = mgh

(J)

v = gt

(m/s)

DKE = 1/2mv2

(J)

DE = DKE + DGPE

(J)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q3.  Does the last column seem to be consistent with a value of 0 J? Explain.

 

 

 

Q4.  If the last column is 0 J, did the mechanical energy change from the beginning to end in each trial?

 

 

Q5.  If there was significant air resistance, how do you think it would affect the last column? 

 

 

Q6.  If there were significant air resistance, would the last column be greater or less than zero.  Explain.

 

 

8.  Use LoggerPro 3.1 to make a graph of the change in the potential energy of the ball, the change in kinetic energy of the ball and the change in the total mechanical energy of the ball vs. the height of the touch pad.  All three graphs should appear on the same axes.  Check with your instructor if you need help doing this.

 

 

Q7.  Describe how the potential energy changes compared to the kinetic energy.

 

 

Q8.  Is the change in mechanical energy a horizontal line?  What is the value of the change in mechanical energy?  What does this say about the total mechanical energy?

 

 

 

Q9.  If air resistance was appreciable, do you think the change in total energy in this experiment would be 0?

 

Q10.  Air resistance is an example of a non-conservative or a dissipative force.  Give another example of a non-conservative force.

 

 

Q11.  We call non-conservative forces dissipative because they turn mechanical energy into another form of energy.  Into what form of energy do dissipative forces convert mechanical energy?  (Hint:  Try rubbing your hands together briskly.)

 

 

Q15.  If the change in mechanical energy was very close to 0, then did non-conservative forces produce a significant effect in this experiment?

 

 

 

Q16.  Complete the following.  If we ignore ______________ then the total mechanical energy is ________________.