• The units of work and of energy are joules.
• Work can be negative ... this just means that the force is applied in the direction opposite
displacement.
This means that the kinetic energy of an object equals one-half the object’s mass times its speed squared.
The net work done on an object is equal to that object’s change in kinetic energy. Here’s an application:
A train car with a mass of 200 kg is traveling at 20 m/s. How much force must the brakes exert in oder
to stop the train car in a distance of 10 m?Here, because the only horizontal force is the force of the brakes, the work done by this force is W (^) net .
Let’s pause for a minute to think about what this value means. We’ve just calculated the change in kinetic
energy of the train car, which is equal to the net work done on the train car. The negative sign simply
means that the net force was opposite the train’s displacement.
To find the force: