Energy and Work: Working Together
"In science, energy is the ability to do work. Work is done when a force causes an object to move in the direction of the force. How do energy and work help you play tennis? The tennis player in Figure 1 does work on her racket by exerting a force on it. The racket does work on the ball, and the ball does work on the net. When one object does work on another, energy is transferred from the first object to the second object. This energy allows the second object to do work. So, work is a transfer of energy. Like work, energy is expressed in units of joules (J).
Kinetic Energy
In tennis, energy is transferred from the racket to the ball. As it flies over the net, the ball has kinetic (ki NET ik) energy. Kinetic energy is the energy of motion. All moving objects have kinetic energy. Like all forms of energy, kinetic energy can be used to do work. For example, kinetic energy allows a hammer to do work on a nail, as shown in Figure 2.
Potential Energy
Not all energy has to do with motion. Potential energy is the energy an object has because of its position. For example, the stretched bow shown in Figure 3 has potential energy. The bow has energy because work has been done to change its shape. The energy of that work is turned into potential energy.
Gravitational Potential Energy
When you lift an object, you do work on it. You use a force that is against the force of gravity. When you do this, you transfer energy to the object and give the object gravitational potential energy. Books on a shelf have gravitational potential energy. So does your backpack after you lift it on to your back. The amount of gravitational potential energy that an object has depends on its weight and its height.
Calculating Gravitational Potential Energy
You can find gravitational potential energy by using the following equation:
gravitational potential energy = weight × height
Because weight is expressed in newtons and height in meters, gravitational potential energy is expressed in newton-meters (N•m), or joules (J).
Recall that work = force × distance. Weight is the amount of force that you must use on an object to lift it, and height is a distance. So, gravitational potential energy is equal to the amount of work done on the object to lift it to a certain height. Or, you can think of gravitational potential energy as being equal to the work that would be done by the object if it were dropped from that height.
Height Above What?
When you want to find out an object’s gravitational potential energy, the “ground” that you measure the object’s height from depends on where it is. For example, what if you want to measure the gravitational potential energy of an egg sitting on the kitchen counter? In this case, you would measure the egg’s height from the floor. But if you were holding the egg over a balcony several stories from the ground, you would measure the egg’s height from the ground! You can see that gravitational potential energy depends on your point of view. So, the height you use in calculating gravitational potential energy is a measure of how far an object has to fall.
Mechanical Energy
How would you describe the energy of the juggler’s pins in Figure 4? To describe their total energy, you would state their mechanical energy. Mechanical energy is the total energy of motion and position of an object. Both potential energy and kinetic energy are kinds of mechanical energy. Mechanical energy can be all potential energy, all kinetic energy, or some of each. You can use the following equation to find mechanical energy:
mechanical energy = potential energy + kinetic energy
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