0.21 Phy1190: energy -- kinetic and mechanical energy  (Page 4/8)

An exchange of energy

In all such cases, the object doing the work possesses either potential energy or kinetic energy or both that is exchanged with the object upon whichthe work is performed. (When you lift the child and place the child into a feeding chair, you are expending potential energy that you possess in order toincrease the potential energy possessed by the child.)

The type of energy expended

For example, the energy being expended may be in the form of

• chemical energy such as the energy stored in food, fuel, or TNT
• atomic energy such as is used in a nuclear power plant
• radiant energy such as energy received from the sun
• gravitational potential energy and/or kinetic energy as in the case of water rushing downhill through a turbine,
• elastic potential energy such as the energy stored in the spring that closes the gate to my back yard after someone opens that gateand twists the spring

Work causes objects to gain energy

Whenever work is done on an object, that object gains energy (as in the case of lifting a child into a feeding chair, thereby increasing the child'sgravitational potential energy).

Mechanical energy

According to The Physics Classroom (see (External Link) ), the energy acquired by the objects upon which work is done is known as mechanical energy .

Mechanical energy is the energy that is possessed by an object due to its motion or due to its position (where position includes the deformation,stretching, compressing, etc., involved in elastic potential energy). Mechanical energy can be either kinetic energy resulting from motion or potential energy resulting from the position of the object.

A speeding car has kinetic energy. An airplane in flight has both kinetic energy and gravitational potential energy. A stretched or compressed or twistedspring has elastic potential energy. (The spring on my gate doesn't obviously stretch or compress when the gate is opened. Instead, it twists.)

The ability to do work

Mechanical energy is often defined as the ability to do work because an object that has mechanical energy can do work. For example, the book on the topbookshelf has the ability to do work on your head if it falls on your head.

When a falling book strikes your head

When at rest on the bookshelf, the book has mechanical energy in the form of gravitational potential energy. On the way down, some of that potential energy is converted to kinetic energy.

When the book strikes your head and causes your head, the bone in your head, or possibly both, to be displaced, it gives up some of its kinetic energy. Thekinetic energy given up by the book is absorbed by your head.

At that point, the book still has mechanical energy in the form of potential energy, and possibly some kinetic energy as well. The remaining mechanicalenergy could do work on your toe if the book happens to land on your toe.

In summary...

The mechanical energy possessed by an object makes it possible for the object to apply a force to some other object to cause the other object to bedisplaced, thus doing work on the other object.