Work has a specific definition in physics. Work is done anytime a force is applied through a distance.

W = F·d The unit is the newtonmeter, or joule.

A component of the force must be in the direction of the displacement. Only this component is used when calculating the work.

Example: How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0° above the horizontal?

Work can be positive or negative depending on whether the force is in the same direction as displacement or in the opposite direction.

For example: If you push a box, the work is positive; but the work the friction does is negative.

Kinetic energy is the energy of motion. KE = 1/2 mv 2

KE is a scalar and the unit is the joule.

Example: A 7.00 kg bowling ball moves at 3.00 m/s. How much kinetic energy does the bowling ball have? How fast must a 2.45 g table tennis ball move in order to have the same kinetic energy as the bowling ball? Is this reasonable?

The net work done by a net force acting on an object is equal to the change in the kinetic energy of the object.

This is called the work-kinetic energy theorem: W = ΔKE

Example: 25 joules of work is done to a 2 kg ball. If the ball is not moving initially, how fast is it moving after the work is done?

Example: A person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10?

Potential Energy is stored energy, energy of possible motion, energy of position.

One type of potential energy is gravitational potential energy. PE = mgh

Example: A 5 kg bunch of bananas is at the top of a 6 meter palm tree. What is the gravitational potential energy of the bunch of bananas?

PE = mgh is just W = Fd.

The unit of potential energy is the joule. (The unit of all types of energy and work is the joule.) It is always measured relative to a zero level.

Another type of potential energy is Elastic Potential Energy. This energy is stored in a compressed or stretched object.

The formula for elastic potential energy is: PE elastic = 1/2 kx 2 The symbol k is the spring constant, x is the distance compressed or stretched.

The spring constant is a measure of the force needed to stretch or compress a spring. It is measured in newtons per meter, N/m.

Example: How much energy is stored in a spring with a spring constant of 10 N/m if it is stretched 2 meters?

Example: A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps from a height of 50.0 m. When he stops, the cord has stretched to m. If k for the cord is 71.8 N/m, what is the total PE relative to the ground when the man stops falling?