External Forces and Internal Energy Changes
As the ice skater in pushes herself away from a railing, there is a force ii on her from the railing at angle 4> to the horizontal. This force accelerates her, increasing her speed until she leaves the railing . Thus, her kinetic energy is increased via the force. This example differs in two ways from earlier examples in which an object’s kinetic energy is changed via a force.
1. Previously, each part of an object moved rigidly in the same direction. Here the skater’s arm does not move like the rest of her body.
2. Previously, energy was transferred between the object (or system) and its environment via an external force; that is, the force did work. Here the energy is transferred internally (from one part of the system to another) via the external force ii. In particular, the energy is transferred from internal biochemical energy of the skater’s muscles to kinetic energy of her body as a whole.
Next let us consider the energy transfers in the ice skater, which we now take as the system. Although an external force acts on the system, the force does not transfer energy to or from the system. Thus. the total energy £ of the system cannot change £ = O.
This equation means that as increases for the ice skater, decreases by just as much. Substituting.
If the driver applies the brakes, still holds. Now F due to the frictional forces is toward the rear . Energy is now transferred from kinetic energy of the car’s center of mass to thermal energy of the brakes.