Transfer Of Energy

Age

Elementary School, Middle School, High School

Format

Hands-on, Stage Show

Materials

   2 balls (preferably of different sizes - a soccer ball and tennis ball work well) 

Safety Precautions

Be in a fairly open area; balls can move in unexpected directions. Have observers sit/stand at least 2-3 feet away.

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Preparation

Collect needed supplies. Make sure observers are a proper distance away.

Demonstration

If you would like, begin by holding the tennis ball. Drop it from rest and explain that it won’t bounce any higher than the height that you dropped it from. You could do the same for the soccer ball.

Now, show how you can make it go higher. Begin by holding the soccer ball above the ground. The higher you hold it, the more energy it will have. A good height is about 3 feet. Now, either you or another person needs to hold the tennis ball above, close to, and centered on the soccer ball. Holding it about 1-2 inches above works well. Now, drop both balls at the same time. If you are working with another person, you might want to drop them on the count of three (or such). After falling and bouncing off the soccer ball, the tennis ball should go higher than where it was initially held. The soccer ball shouldn’t go any higher after its collision with the tennis ball.

Note: Try to keep the two as centered as possible throughout the entire demonstration. If the tennis ball hits the soccer ball at an angle, it will bounce at an angle rather than go straight up and could injure someone. Also, make sure that you DROP the balls from rest. Do NOT throw them or add velocity to their motion. If you do this, the ball(s) will have more initial energy than just the potential energy. This means that the ball would bounce higher than its initial height regardless of energy being transferred to it or not.

What to Say: Begin by explaining that when the tennis ball is being held above the ground, it has potential energy (PE). Explain that PE is stored energy, related to height above the ground, and that it is changed into kinetic energy (KE) as the ball falls toward the ground. Say that KE is energy of motion (i.e. a moving object has KE). This is why the ball moves and gets faster as it falls. The farther it falls, the more PE is converted to KE and, therefore, the faster it moves. You can drop the tennis ball to show them this. Explain that you are providing PE to the ball when you lift it.

Next, explain conservation of energy. Say that energy can’t be lost, only transferred between two objects (i.e. two balls colliding which I’ll show you shortly) or changed into different forms of energy (i.e. PE to KE which we just saw). Explain that since energy is conserved, the tennis ball can’t bounce higher than the initial height that it was dropped from. If it went higher, its final total energy would be greater than its initial PE which was its initial total energy. The conservation of energy says that the initial and final total energy need to be equal.

Now, you can show them the demo using both balls. It should be easy to see that the tennis ball goes higher than its initial height. Ask if anyone knows why that happened. Wouldn’t it be breaking the law of conservation of energy? If no one answers correctly, remind them about what you said earlier about energy being transferred between objects (otherwise, something such as “That’s right; remember earlier I said…”). Now, explain that when the soccer ball hits the tennis ball, it transfers its energy to the tennis ball. This gives the tennis ball more energy than it initially had. However, the soccer ball now has less energy than it originally had. Explain that conservation of energy still holds. The total energy of both balls combined has stayed the same from beginning to end. You just changed the amount of energy in each ball.

Why It Is

After dropping the balls, the soccer ball hits the ground first. Because we held the tennis ball above, but not touching, the soccer ball, it doesn’t collide with the soccer ball until the soccer ball has left the ground on its way up. The soccer ball hasn’t gone very high after colliding with the ground and didn’t lose much energy in the ground collision, so most of its initial energy is in the form of KE that is transferred to the tennis ball when the two collide. This gives the tennis ball more energy than it initially had, so it can bounce higher than the initial height that it was dropped from. The closer you hold the two balls, the higher the tennis ball should go because you are reducing the amount of time that the soccer ball will spend moving up after bouncing off the ground (decreasing the PE or, in other words, increasing the KE it has at the point of collision). Also, starting the two balls higher off the ground will provide more energy and, therefore, the tennis ball will bounce higher. Potential energy is given by U=mgh, where U is PE, m is the object’s mass, g is gravitational acceleration, and h is height.

Real Life Examples

Double-bouncing someone on a trampoline or diving board – In these cases one person begins by jumping up. When they land near the second person, their energy is transfer into the trampoline (or diving board). The second person begins jumping as the trampoline moves “up” with the energy transferred into it. If the second person leaves the trampoline at the same time as its highest point, all of the energy that the first person put into it will be transferred to the second person, giving them more energy. This allows them to bounce higher.