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Going Up!
October
Activity Overview
Investigate energy conservation of a rubber ball
Kit contents: paper strip, tape, rubber ball, tape measure, colored pencils, safety glasses
Key concepts: energy, kinetic energy, potential energy, energy transfer
Science Standards: The student knows that energy can be described as stored energy
(potential) or energy of motion (kinetic). (SC.B.1.2.2.5.1)
The student understands the importance of accuracy in conducting measurements, and
uses estimation when exact measurements are not possible. (SC.H.1.2.2.5.2)
Key Knowledge
Vocabulary
energy: the ability to cause a change in matter
kinetic energy: energy of motion. The faster the motion, the more kinetic energy. The
more massive the object, the more kinetic energy.
potential energy: the energy an object has because of its condition or position. The higher
the object, the more potential energy. The more massive the object, the more potential
energy.
energy transfer: the movement of energy from one place or object to another
Concepts:
Law of the conservation of energy: energy can never be made or destroyed, but it can
change forms
A pogo stick spring stores energy as potential energy. A car uses the energy in gasoline to
go.
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Going Up!
October
Script Idea
Today we are going to talk about energy.
Hold up a ball. Tell me about the energy this ball has. Kids should get to something about it
having potential energy. You could lead them to it by asking what would happen if you let the
ball go. It has the potential to attack the earth.
Hold the ball on the ground. What about its energy now? They should say it has no potential
energy, of course assuming you don't want to destroy the ball by burning it.
So, if the ball up high has potential energy, and the ball on the floor has no potential energy,
where does the energy go when you drop it? It goes into motion, kinetic energy.
Okay, so I've dropped the ball, lost my potential energy, gained kinetic energy. Then the ball
returns to me. What is happening with the energy now? Kids should get somewhere close to the
fact that as it went up it lost kinetic energy but regained potential energy.
This idea is the law of conservation of energy. And it is not a law that says you have to turn the
lights off when you leave the room.
Let's do some observations about the conservation of energy. (This is where we begin the official
activity.) Have the kids follow the lab as written, noting the ball's height on the return bounce at
50 cm and 100 cm. Alternatively, give them the materials and ask them to make observations
about the ball, then continue through the method, including recording results. Try to steer them
pretty much toward the experiment as written. It is possible that some will conclude that energy is
not conserved. Perfectly valid conclusion, based on their limited tests. (And it is fair for them to
question authority, even if we call it a law.) We hope to dispel that notion in the next section.
Regain the kids' attention. What did they observe? Did they regain all of their potential energy?
But didn't we just say that energy has to be conserved? What ideas/hypotheses do kids have about
where the energy went? Why did the bounce generate heat, sound, etc? Suggestions for
hypotheses include:
Heat energy is created as the ball flexes on impact.
Energy is used in making the bouncing sound.
Heat energy is created as the ground absorbs the impact. (Remember Newton's 3rd law
from the balloon rocket?)
The friction of air going past the ball heats the air.
The friction of air going in and out of the ball uses energy to generate heat.
Now have the kids think of an experimental plan to test one of the ideas. Emphasize that they
should try to only change one variable at a time, if possible. If they have to change more than one
variable, they ought to try to account for the two differences. Ideas for testing the above
hypotheses could include:
Testing balls of two different flexibilities, a flat ball and a blown up ball or a baseball
with a tennis ball, a bb or a pea, a cold ball and a hot ball.
Still thinking on the sound experiment.... Suggestions welcome.
Bounce the ball on different surfaces--tile floor, wood, sand.
Compare a tennis ball and one with a hole drilled into it.
Maybe compare a golf ball to a ping pong ball, with different surfaces friction should be
different.
Let the kids test their plans. Come prepared with some different balls, a pie pan with sand, etc. so
they could at least test the above ideas. If you don't have the materials to perform the experiment
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Going Up!
October
they plan, see if they can come up with an alternate test with the materials you provide. You
might want to steer them mostly to testing different types of balls, different types of surfaces.
Have the kids report their results and conclusions.
Now see if the kids can extend what they have learned. Why don't we play tennis with a baseball
or volleyball with a golf ball? Why do baseballs hurt while soccer balls don't (as much)? How do
grass courts and clay courts different when playing tennis? Do you want your running shoe soles
to have a lot of elasticity or a little? What might be the difference between an aluminum or wood
baseball bat? What happens to the ball's energy when we drop it in a bucket of water?
I LOVE Science
Going Up!
October
Energy Science
If a ball is perfectly elastic then no other energy changes will happen. In practice, some of the
elastic energy will be converted into heat, by internal friction, as the molecules move against each
other in the ball. A small amount of kinetic energy will also be converted into sound. These other
energy changes are the reasons why the ball does not return to the same height.
The more pressure a basketball has inside it, the less its surface dents during a bounce and the
more of its original energy it stores in the compressed air. Air stores and returns energy relatively
efficiently during a rapid bounce, so the pressurized ball bounces high. But an underinflated ball
dents deeply and its skin flexes inefficiently. Much of the ball's original energy is wasted in
heating the bending skin and it doesn't bounce very high. In general, the higher the internal
pressure in the ball, the better it will bounce.
The ball isn't the only thing that is affected by the bounce--the ground is also impacted. Soft
ground, such as sand, will absorb most of the energy of the bounce by moving the sand grains out
of the way. Harder surfaces will deform less, thereby conserving energy for the bouncing ball.
Where does the energy come from to put the initial potential energy into the ball? Your muscles
lifted the ball. Your muscles got their energy from the food you eat. The food you eat got its
energy from the sun. All energy (except nuclear) can in some way be traced to the sun.
Butyl rubber and similar ones can absorb kinetic energy instead of changing it into potential
energy. In running shoes, the superior shock-absorbing ability of the "no bounce" rubber helps
alleviate problems caused by the tremendous pounding suffered by feet, legs and ankles.
Aluminum is more elastic than wood. Wood will absorb a larger amount of the kinetic energy of a
baseball, reducing the distance it will go when hit. Wood bats should show dents where they hit
the ball, probably more noticable on bats used in the Major League....
Kinetic energy is 1/2 mass x velocity2 . Potential energy is mass x gravity x height.
Additional Demonstrations
A popular demo is to hold a small ball on top of a large ball and bounce them together. The small
ball will bounce quite high.
Cut a racquet ball in half. Invert one half. Drop the ball bulge-side up. The ball should return to a
height greater than the original height. What happened? You used some energy to turn the ball
inside out. That energy was returned when the ball struck the floor, providing an increase in
kinetic energy, which led to an increase in the potential energy, or the height.
For instructions on a bouncing water balloon, which shows the deflection of the ball, see
http://exploratorium.com/sports/ball_bounces/ballbounces1.html.
A fun activity exploring potential and kinetic energy is a roller coaster. Kids can explore who can
make the most hills, the highest hills, etc. Check out this site for one way to do it:
http://www.thinkingfountain.org/t/tubesandspheres/tubesandspheres.html