2003 Virginia SOLs
Motivation for Learning
As a second demonstration of energy transfer, you can line up four tennis balls on the table and ask students, "what will happen to the row of tennis balls if a fifth tennis ball is made to collide into the first one?" Again the energy on impact will be transferred through the ball to the last one and it will move. (This experiment is difficult to do, because it is difficult to hit the balls straight on.)
It doesn't take much effort to lift a ball off the ground. However, work is being done to the ball as it is being lifted, giving it energy. We call this energy potential energy. When the ball is dropped, the ball begins to move. The potential energy begins to be converted into kinetic energy - the energy of motion. There is obviously a very close association between work and energy. Energy is defined as the ability to do work and both work and energy are measured in Joules. To help understand this concept, scientists have classified energy into two types or states. Potential energy is the energy acquired as work is being done to an object and kinetic energy is the energy released by the object as it is doing work. The amount of work put into an object, its potential energy, must always be equal to the amount of work the object can do, its kinetic energy. For example; the higher the ball is lifted off the ground, the higher it will bounce after hitting the ground. Experience tells us that the ball can never bounce back to its original height. The falling ball looses some of its energy to air friction, to internal forces within the ball, and to friction between the ball and the ground on impact. After impact, the ball and the spot directly under the ball are slightly warmer, as some of the energy is lost as heat.
The gravitational potential energy of an object, like a rubber ball, is related to its mass and the height to which the ball is lifted and can be expressed by the formula:
G.P.E.= Weight X Height = mgh
You can see from the formula that the greater the weight and the higher the position of the ball, the greater the potential energy. The kinetic energy of the falling ball is related to the mass of ball (m) and its velocity (v). This mathematical relationship is expressed as:
K.E. = 1/2 mv2
According to the equation, the heavier the ball and the faster it is moving, the greater the impact on the ground. Neglecting friction for the ball we're using, the potential energy before you drop the ball will be equal to the kinetic energy just before it hits the ground.
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|Drop Height (cm)||Bounce Height (cm)||Bounce/Drop ratio|
Conduct and experiment with two balls of similar material, each suspended from a piece of string. Pull the balls apart at equal angles and let them collide with each other. How many bounces do they undergo before stopping? Repeat the experiment with two balls of different material, let them collide and record the bounces for each one.
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1. Describe the shape of the lines plotted for both graphs.
2. As long as you were using the same type of ball, were the bounce ratios the same for all the heights?
3. Are the bounce/drop ratios the same or different for both balls?
4. Which ball retained the greatest percentage of its kinetic energy on each bounce?
5. Explain why the balls never bounce as high as the original bounce height.
6. Explain why the shape of the graphs was similar for each ball used.
7. Suppose you had carried out your investigations on a carpeted floor. How would your results have been affected?