Activity 5a
Potential and Kinetic Energy
Name:_____________________________
PHYS 010
Date:______________________________
Partners:__________________________
__________________________
__________________________
Purpose:
To investigate the relationship between potential energy and kinetic energy.
Materials:
1.
2.
3.
4.
5.
6.
Super-balls, or hard bouncy rubber balls
Metre stick and tape
calculator
table or desk
scale
stopwatch
Diagram:
Ruler
Prediction/Discussion:
1. In layperson terms, something has potential if it has the ability to do something in
the future. In physics terms, we often use the word potential to describe different
forms of stored energy. In your own words, describe what you know about the
potential energy of a falling object. How can you increase the potential energy of
an object?
Rev 09-01
1/6
Activity 5a
Potential and Kinetic Energy
PHYS 010
2. Kinematics is the study of objects in motion. In your own words, describe what
you know about the kinetic energy of a moving object. How can you increase the
kinetic energy of an object? What is the kinetic energy of an object at rest?
Part 1:
Procedure:
1. Tape the metre stick to the side of the table or desk, so the 0-cm mark is on the
floor.
2. Hold the ball so its bottom edge aligns with the 40 cm mark, then drop the ball.
Have someone in your group measure the height to which the ball bounces.
Record this bounce height in the table below.
3. Measure the time the ball takes to drop from the starting point to the floor and
record in table.
4. Repeat steps 2 and 3 (ie. do two trials)
5. Drop the ball twice each for other initial heights: say 70cm and 100cm.
6. Calculate the average bounce height (m) and average drop time, as indicated in
the table (in m).
7. Calculate the speed of the ball just before it hits the ground: Vground = (g)*(t) =
(9.81 m/s2) * (average drop time), and record in last column of the table (m/s).
Data:
Initial
height of
ball
Trial
Bounce
Height
(m)
Average
Bounce Height
(m)
Drop
Time
(s)
Average
Drop Time
(s)
Speed
at Ground
(m/s)
1
0.4 m
2
1
0.7 m
2
1
1.0 m
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2
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Activity 5a
Potential and Kinetic Energy
PHYS 010
Mass of ball: ____________ kg
Energy Calculations:
Average
Speed at
Potential
Potential
Bounce
Ground
Initial
Energy
Energy
Kinetic Energy
Height
(m/s)
height of
(initial height) (bounce height)
(J)
(m)
*from 1st
ball
(J)
(J)
*from 1st
table*
table*
0.4 m
0.7 m
1.0 m
Questions:
1. The equation for potential energy (PE) is: Potential Energy = (mass) x (height) x
(acceleration of gravity), where mass is in kilograms, height is in meters,
acceleration of gravity is 9.8 m/s2, and potential energy is in joules.
a) Weigh the ball on a scale, and record the mass in kilograms where
indicated above the 2nd table.
b) Calculate the potential energy of the ball at its initial height, and at its
average bounce height, and fill in your answers in the second table.
Aside: The formula for potential energy should “make sense” to you. To lift an object
to a certain height, you need to do work on it.
The amount of work is W = Force  distance. W  F  d
But from Newton’s law: Force = Mass  Acceleration. F  m  a and in this case a is
acceleration due to gravity, usually called g. Then:
PE  W  m  d  g
2. What happens to the average bounce height as you increase the initial height?
Does this make sense? Does the bounce height equal the initial height? Why or
why not?
Rev 09-01
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Activity 5a
Potential and Kinetic Energy
PHYS 010
3. What happens to the potential energy of the ball as you increase the initial height?
Does this make sense? What is the difference in potential energy for the initial
and “bounce” height? Where does this energy go?
4. At what point in a drop does the ball have maximum potential energy? At what
point does the ball have minimum potential energy?
5. Calculate the kinetic energy (KE) of the ball just before it hits the ground:
KE = ½ mv2 where v is the speed of the ball just before it hits the ground, which
you calculated in the first data table. Fill in your results in the last column of the
2nd data table.
Aside: The formula for kinetic energy should also “make sense” to you (even if it
looks completely different from the formula for potential energy). The work gained to
go from zero velocity to the final velocity v is still W  m  d  a as above, but made
more general for any acceleration a. But acceleration is just average velocity divided
by time, and in this example, average velocity = (initial velocity + final velocity)/2,
v
v
and so we can write a  . Hence, W  m  d  . But remember that distance/time
2t
2t
is just velocity so one can replace the d / t term with v to end up with the kinetic
1
energy, KE  m  v 2
2
Rev 09-01
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Activity 5a
Potential and Kinetic Energy
PHYS 010
6. Compare the KE of the ball with its initial PE. Do they increase together as the
drop height increases? Does this make sense?
7. At what point in a drop does the ball have maximum kinetic energy? At what point
does the ball have minimum kinetic energy?
8. What can you conclude then about the relationship between gravitational potential
energy, and kinetic energy?
Part 2
Predict/Observe:
1. Suppose you are able to drop two superballs, one above the other as shown. What
do you think will happen?
Rev 09-01
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Activity 5a
Potential and Kinetic Energy
PHYS 010
2. Try it… (good luck). You can try a variety of ball size combinations to see which
works best. Summarize what you see. If you get a good bounce (well lined up
when they hit), how does the maximum height of the upper ball’s bounce compare
to the case of just dropping a single ball from the same height?
3. Can you explain qualitatively what you have observed?
Summary and Suggestions for the Future:
a.
What were the important concepts of physics/science that you learned from this
activity? What else did you learn?
b.
Can you think of alternative hands-on ways in which these concepts could be
demonstrated?
c.
What changes (if any) would you make to teach these activities in a Grade 7-8
classroom
Rev 09-01
6/6