PHYS 2425 – Engineering
Physics I
Conservation
of Mechanical Energy
Leader: _________________________ Recorder: __________________________
Skeptic: _________________________ Encourager: ________________________
Materials
Lab Jack Laptop
(For graphs only)
Introduction
In this activity we will investigate
energy transformation in a freely falling object. We will determine the gravitational potential
energy at the beginning of an object’s fall to the top of a table, and at an
intermediate point along the fall. We
will also determine the kinetic energy at the intermediate point and will
quantitatively investigate the transformation of potential into 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 about
1 m above the table 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 height of the ball release mechanism above
the table top, h1, as shown in Figure 1, and the height of the touch
pad, h2 above the table top as well.
|
h1 Meter Stick
Figure 1. h2 |
6.
Turn the screw to
release the ball. Record the time it
took the ball to fall in the table below.
7.
Make sure that
the lab jack is completely closed and place the touch pad on top of the lab
jack so that the touch pad lies directly below the release mechanism. Measure the height of the touch pad above the
tabletop. Release the ball and record
the heights 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
|
|
Height of
Release Mechanism (m) |
Height of
Touch Pad (m) |
Elapsed Time (s) |
|
1 |
|
|
|
|
2 |
|
|
|
|
3 |
|
|
|
|
4 |
|
|
|
|
5 |
|
|
|
Analysis
Q1. From the
mass of the ball and the height in meters, determine the gravitational
potential energy for each trial at the release point, GPEi and at
the point at which it hits the touch pad, GPEf. Your reference point for the determination of
the GPE should be the top of the table.
Determine DGPE for each trial.
Q2. Was ΔGPE
positive or negative? What does this
mean about the gravitational potential energy of the ball as it fell?
Q3. From the
time it takes the ball to fall, determine the speed at the bottom for each
trial. Calculate the 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. Determine DKE for each trial.
Q4. Was
ΔKE positive or negative? What does
this mean about the kinetic energy of the ball as it fell?
Q5. Determine
the change in mechanical energy for each trial.DE = DKE + DGPE
|
GPEi = mgh1 (J) |
GPEf = mgh2 (J) |
DGPE (J) |
v = gt (m/s) |
KEi (J) |
KEf = 1/2mv2 (J) |
DKE (J) |
DE = DKE + DGPE (J) |
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
0 |
|
|
|
Q6. Does the
last column seem to be consistent with a value of 0 J? Explain.
Q7. If the last
column is 0 J, did the mechanical energy change from the beginning to end in
each trial?
If the mechanical energy doesn’t change, we say it is conserved.
Q8. Complete the following. If the change in mechanical energy was 0 as
the ball fell, then the mechanical energy was ___________.
Q9. If there
was significant air resistance, how do you think it would affect the last
column? I.e., if there were significant
air resistance, would the last column be greater or less than zero. Explain.
Q10. Use Excel or
LoggerPro 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.
Q11. Describe in
words the relationship between kinetic, potential, and mechanical energy shown
by your graph as the ball fell.
Q12. 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?
Q13. If air
resistance was appreciable, do you think the change in total mechanical energy
in this experiment would be 0?
Q14. Air
resistance is an example of a non-conservative
or a dissipative force. Give another example
of a non-conservative force.
Q15. 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?
Q16. If the
change in mechanical energy was very close to 0, then did non-conservative
forces produce a significant effect in this experiment?