1. CHS Physics Work Done on a System by an External Force

2. Work Work is energy transferred to, or from, a system by an external force acting on the system. If the system contains only one particle, then the only energy transformation is a loss or gain of kinetic energy of the particle. This is the realm of the Work-Kinetic Energy Theorem.

3. Complicated Systems For systems which contain two or more particles additional energy transformations can occur. Changes in Kinetic Energy as well as Potential Energy are possible. Consider the following situations: No friction is present in the system. Friction is present within the system.

4. System with No Friction Bowling-ball hurling contest. Fig. 8-12, page 152. A person throws a bowling-ball straight up into the air. The work done by the person (an external force) results in a change in mechanical energy. Of what system?

5. The Work Done The work done on the system by the person is equal the change in mechanical energy.

6. System with Friction Consider a block pulled across a table by a constant force. Newton’s Second Law and the Kinematic Equations can be used to analyze the system.

7. The Analysis

8. General Statement A more general expression can been written if we consider a possible change in potential energy. For example, pushing up a hill.

9. Friction Does Work A common misconception is that only forces that cause motion can do work. But recall that work is a transfer of energy. Does friction transfer energy? Yes it does. What type of Energy? Thermal Energy

10. Wrap Up The work done by friction is equal to the change in thermal energy. The work done by the force is the change in Mechanical Energy plus the change in thermal energy.