• Forces can change the physical state of an object (internal energy) as well as its state of motion (kinetic energy).

• Work is the change in the energy of a system due to external forces.

Work

• Work amounts to a mechanical transfer of energy, either from a system to the environment or from the environment to a system.

Force displacement

Imagine pushing against a brick wall as shown in (a) Considering the wall as the system, is the force you exert on it internal or external?

(b) Does this force accelerate the wall? Change its shape? Raise its temperature?

(c) Does the energy of the wall change as a result of the force you exert on it?

(d) Does the force you exert on the wall do work on the wall?

• Even though the work is zero in (a), it is nonzero in (b) and (c).

• In order for a force to do work, the point of application of the force must undergo a displacement.

• The displacement of the point of application of the force is called the force displacement