⇒ Work is done when a force moves an object in the direction of the force

⇒ Work (J) = force (N) x distance moved in the direction of the force (m)

• This equation defines the joule: 1J = 1N x 1m

⇒ Energy is the capacity to do work or to heat something up - these quantities are measured in joules

⇒ Power is the rate of doing work or transferring energy

• Power = energy / time

⇒ This equation defines the watt: 1 W = 1 J/s^-1

⇒ Energy is conserved. This means that energy cannot be created or destroyed, but it can be transferred from one form to another

⇒ It is possible to use energy but do no work (just like at school!)

⇒ In the first picture, the lady is holding some weights but not moving them. She gets tired holding them, converting chemical energy into heat energy, but no work is done because the weights do not move. However, she does work when she lifts the weight

⇒ In the second picture, three people are halping to push a car. Salim and Anne are pushing at the back of the car, and therefore doing work as they are pushing along the direction in which the car is moving. However, Jim does no work as he is pushing at right angles to the direction of motion.

⇒ Sometimes an object will move in a different direction to the applied force - to calculate the work done, the force must be resolved into parallel and perpendicular components to the direction of motion

• Only the component parallel does the work

⇒ This lady is wheeling her luggage with a force (F), at a particular angle (θ) to the ground

⇒ The component Fsinθ does no work (it simply supports the bag), but the force Fcosθ does do work against frictional forces

⇒ The general formula for work is: work done = F s cosθ

• F is the applied force, s is the displacement of the object, and θ is the angle between the force and the displacement

⇒ Sometimes the force we are working against does not remain constant, which means the calculation of work done is a little more complicated

⇒ Below is an example of a force increasing as an object is displaced...

⇒ We calculate work done by a vairable force that changes at a constant rate by finding the average force x the displacement

⇒ For everything else, you can calculate work done on an object by calculating the area under teh curve of a force displacement graph

⇒ Also see our notes on:

• Calculating the Energy
• The Principle of Conservation of Energy
• Efficiency
• Power