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Name: _________________________ Date: ____________ Period: ______
Investigating Kinetic Energy and the
Work-Energy Principle Lab Part I
Background: What happens when you apply an external force to an object that is free to
move and has no frictional forces on it? According to Newton’s second law, it should
experience acceleration and end up moving with a different velocity. Can we relate the change
in velocity of the object to the amount of work that is done on it?
Consider a fairly simple situation. Suppose an object is lifted through a distance and then
allowed to fall near the surface of the Earth. During the time it is falling it will experience a
constant force as a result of the attraction between the object and the Earth—glibly called
gravity or the force of gravity. You discovered how to find the work done by this force, called
kinetic energy. You will see that as the object falls, its kinetic energy increases as a result of the
work done by the gravitational force, and that it increases by an amount exactly equal to the
work done.
Materials:
Rubber ball
Rubber ball with about twice the mass
Procedure:
1. Toss the less massive rubber ball back and forth, slowly at first and then faster. Then
alternate between throwing slowly and faster. Notice how much effort it takes to throw it
and to catch (stop) it when it is moving quickly or slowly.
a. Does the effort needed to stop the ball seem to change as its speed increases?
How does it change? Explain.
b. Does the effort needed to throw the ball seem to change as its speed increases?
How does it change? Explain.
2. Now throw the more massive ball. Toss the ball back and forth at the same speed. Then
alternate between the heavier and lighter ball. Again notice how much effort it takes to
throw and stop the ball.
a. Does the effort needed to stop the ball seem to change as the mass increases?
How does it change? Explain.
b. Does the effort needed to throw the ball seem to change as its mass increases?
How does it change? Explain.
*When an object moves, it possesses a form of energy because of the work that was done to
start it moving. This energy is called kinetic energy. You should have discovered that the
amount of kinetic energy increase with both mass and speed. In fact, the kinetic energy is
defined as being proportional to the mass and the square of the speed. The mathematical
formula is
KE = ½ mv²
The unit of kinetic energy is the joule (J), the same as the unit of work.
Your Kinetic Energy
In this activity you will examine how you can graph the kinetic energy of an object such as your
body in real time.
Materials:
 Computer-based laboratory system
 Motion software
 RealTime Physics Mechanics experiment configuration files
 Motion detector
Procedure:
1. Open LoggerPro. Connect to the interface. This will display velocity vs. time axes.
2. To display kinetic energy you will need to know your mass in kilograms. Use the fact
that 1.0 kg weighs 2.2 lb. on Earth to find your mass in kilograms.
a. Mass: _________ kg
3. Modify the software with a KE column calculated from one –half of your mass times the
square of the velocity measured by the motion detector. Then both velocity and kinetic
energy will be graphed in real time.
4. Record your velocity and kinetic energy as you walk. Begin graphing while walking away
from the motion detector slowly, then more quickly, and then back toward the motion
detector slowly and then more quickly. Sketch graph on axes that look like the ones
below. BE SURE TO LABEL AXES!
Conclusion:
1. In what ways does the kinetic energy graph differ from the velocity graph? Is it possible
to have negative kinetic energy? Explain.
2. Which would have a greater effect on the kinetic energy—doubling your velocity or
doubling your mass? Explain.
When you apply a force to an object in the absence of friction, the object always accelerates.
The force does work and the kinetic energy of the object increases. Clearly, there is some
relationship between the work done on the object and the change in its kinetic energy.
Part II
Background: Suppose that an object of mass m is lifted slowly through a distance y. To cause
the object to move upward at a constant velocity, you will need to apply a constant force
upward just equal to the gravitational force, which is downward.
 Based on your knowledge from part I, how much work will you do to lift the
object through a distance of y? Explain.
We choose to define the gravitational potential energy, GPE, of an object of mass m to be equal
to the work done against the gravitational force to lift it:
GPE = mgy
Now you can use this equation to calculate the potential energy of a ball.
Materials:
 Computer based laboratory system
 Motion software
 Motion detector
 Large rubber ball (e.g., a basketball)
Procedure:
1. Set up the motion detector on the floor with the screen on top of it.
2. Open LoggerPro and connect to the interface.
3. Measure the mass of the ball: _______ kg
4. Modify the software column called Gravitational Potential calculated from mass
times gravitational acceleration, g times the position measured by the motion detector.
Plug in the calculation for gravitational potential (mass of ball*9.8*position graph).
Then the gravitational potential energy of the ball will be graphed in real time. Display
Position on the top and gravitational energy on the bottom vs time (you will
need two graphs).
5. Hold the ball from the sides or above with your hands. Starting with the ball about 0.5 m
below the motion detector (keeping your hands and body out of the way of the motion
detector), begin graphing position and gravitational potential energy as you raise the
ball to about 2.0 m above the motion detector. Be sure that the ball remains directly
above the motion detector. Have me check your work at this time.
6. Sketch your results in your lab books. Label the axes!
Questions:
1. Do the two graphs look similar? Explain why this is reasonable.
2. Gravitational potential energy is always measured with respect to a particular height
where its value is defined to be zero. In this case what has been chosen as this reference
level? In other works, for what location of the ball would its gravitational potential
energy be zero?
3. Suppose that the ball is dropped and you know its velocity at a certain time. What
equation would you use to calculate the kinetic energy of the ball?
4. Suppose that the ball is dropped from some height. What equation would you use to
calculate the mechanical energy (the sum of the gravitational potential energy and the
kinetic energy)?
5. As the ball falls, how will the kinetic energy change? How will the gravitational potential
energy change? How will the mechanical energy change?
Mechanical Energy
We can check the last predictions by measuring the two types of mechanical energy and their
sum as the ball falls.
1. Clear the data present in the graph. You will need to calculate the graphs for the next
two. You will be graphing Mechanical Energy vs. Position and GPE and KE (both on
same graph) vs. Position.
2. You will need to enter the mass of the ball (which you measured in the previous part)
into the formulas for kinetic energy and gravitational potential energy.
a. Mechanical energy is calculated as GPE + KE.
3. You are now ready to examine how kinetic energy, gravitational potential energy and
mechanical energy vary as the ball drops. Hold the ball about 2 m directly above the
motion detector. Be sure that it will fall on a straight path directly toward the motion
detector.
4. Begin graphing, and as soon as the motion detector starts clicking, release the ball. Be
sure that your body and hands are out of the path of the motion detector after the ball
is released.
5. Print your graphs and affix them over the axes.
6. Label the interval during which the ball was falling with arrows at the beginning and
end.
Conclusion:
1. How did the variation of kinetic energy and gravitational potential energy compare to
your predictions? Have me check your result.
2. We expect that mechanical energy which is defined as the sum of the kinetic energy and
the gravitational potential energy to be conserved. Did you find mechanical energy to be
conserved? If not, what was the percentage change during the period that the ball was
falling? What could be the cause of this decrease in mechanical energy?