Momentum and EnergyEnergy |
What energy transfers are involved when a ball is tossed? |
If you lift an object like a ball you increase the energy in Earth’s gravitational field. Energy is transferred from you to the field, resulting in a decrease in your stored chemical energy (see p. 62). Suppose you toss the ball up. When you toss it you do work on the ball, transferring energy from your body to the ball, increasing its kinetic energy as well as the gravitational field energy. Once you let go of the ball it continues to rise, but its velocity decreases as the gravitational field energy increases and the ball’s kinetic energy decreases. It reaches its maximum height; at that instant the kinetic energy is zero—all energy is in the field. On its way back down it speeds up, so its kinetic energy increases but the energy stored in the gravitational field decreases. While you are not touching it the sum of the kinetic energy of the ball and the gravitational field acting on it is a constant. Energy changes from one form to the other and back again.
As you catch it, stopping its motion and thus reducing its kinetic energy to zero. The ball does work on you. But, your stored chemical energy does not increase.