0.23 Phy1210: energy -- internal and external forces  (Page 2/4)

Internal forces

Internal forces are forces that can act on an object without physically touching the object. Examples of internal forces include:

• the force of gravity
• electrical forces
• magnetic forces
• spring forces

These forces cannot change the total mechanical energy possessed by an object, but can transform that energy between potential energy and kineticenergy.

External forces

Examples of external forces include:

• applied force
• normal force
• tension force
• friction force
• air resistance force

When work is done on an object by external forces, the total mechanical energy, consisting of kinetic energy plus potential energy, will change. Thework can be positive, in which case the total mechanical energy will increase. The work can be negative, in which case the total mechanical energy willdecrease. The change in mechanical energy will be equal to the net work that is done on the object.

A Super Ball example

Assume that a Super Ball (a toy manufactured by Wham-O that bounces with great vigor) is at rest on a table. The ball has a certain amount of potential energy due to its position relative to the floor. While at rest,however, it has no kinetic energy. The total mechanical energy possessed by the ball is the potential energy due to gravity.

The ball falls

Assume that the ball is dangerously close to the edge of the table and the family cat accidently tips the ball off the table.

As the ball falls towards the floor, it will be losing potential energy because its height above the floor will be decreasing. At the same time, it willbe gaining kinetic energy because its speed will be increasing.

A transfer of energy

Assuming that we can neglect air resistance and that no forces other than the force of gravity are acting on the ball, during its fall, it will be subjectedonly to the internal force of gravity. During that time interval, its total mechanical energy cannot change. The loss in potential energy will bereplaced by an increase in kinetic energy and the total mechanical energy possessed by the ball will remain constant.

An external force is applied

Suddenly the ball reaches the floor. At that instant, an external upward force will be applied to the ball by the normal force exerted by thefloor. There will be a change in the total mechanical energy possessed by the ball, if for no reason other than the fact that some of the energy is convertedinto sound wave energy. Both the kinetic energy and the gravitational potential energy will go to zero for a very short period of time.

Negative work

The work done on the ball by the floor will be negative because the force will be in the opposite direction of the direction of the displacement. Thus,the total mechanical energy will be decreased.

Conversion into elastic potential energy

However, some of the mechanical energy will probably be converted into elastic potential energy as the ball is compressed.

Shortly thereafter, that elastic potential energy will be converted into kinetic energy as the ball expands causing it to bounce up.