Competency #2
Answer and Explanation
Some common answers are filled in below:
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Pretend you are holding a basketball over your head. What kind(s) of energy does the ball have?
As you stand with the basketball over your head, the basketball has gravitational potential energy. The ball also has thermal energy (related to kinetic energies associated with the random motion of the atoms in the basketball). This energy is related to the mass and temperature of the ball.
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If you release the ball, as it is falling towards the ground, how does the energy change?
As you drop the ball, the gravitational potential energy is converted into kinetic energy (the energy of motion). When the ball is halfway down, it has half as much gravitational potential energy as it did before. The other half has been converted to kinetic energy.
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When the ball bounces, it reaches a maximum height that is only halfway to its original release point. The law of conservation of energy says that energy is not created nor destroyed, but is merely transformed from one form to another. Where did the ‘missing' energy go?
When the ball reaches its peak after its first bounce, the gravitational potential energy is one half of what it was when you were holding the ball over your head. The kinetic energy is zero since the velocity of the ball at the peak is zero. At first glance, the total energy of the system appears to be less than it was originally.
However, when the ball collides with the ground, some of the kinetic energy was converted into sound energy (that is why we hear it bounce) and thermal energy (though not enough to noticeably change the temperature of the ball in this case). Thus the total amount of energy has stayed the same, in agreement with the law of conservation of energy.