When is work done?

1. When is work done? • You do work any time you exert a force on an object that causes the object to move in the same direction as the force. • In addition, the force you exert must be in the same direction as the object’s motion. Work is done on the bin because force and motion are in the same direction. No work is done on the bin because force and motion are in different directions. (contd.)

2. The amount of work done depends on both the amount of force you exert and the distance the object moves. The amount of work done on an object can be determined using this formula: • Work = Force × Distance • The SI unit of work is the newton-meter (N•m). 1. CalculateHow much work do rock climbers do on their backpacks if their packs weigh 90 N and they climb to a height of 30 m?

3. How do machines affect work? • A machine is a device that allows you to do work in an easier way. Machines do not reduce the amount of work you do. Instead, they just change the way you do work. • A machine makes work easier by changing at least one of three factors: • • the amount of force you exert, • • the distance over which you exert your force, or • • the direction in which you exert your force. 2. Apply It is your turn to help raise the school’s flag. As you pull down on the rope, the flag raises to the top of the pole. Which factor did the pulley at the top of the pole change in order to make work easier? (contd.)

4. When you do work, the force you exert is called the input force. You exert your input force over the input distance. • A machine exerts the output force over the output distance. Output force times output distance equals output work. Output work is never greater than input work. Input force = output force Input distance = output distance Input force = force used to pull the box Input distance = length of the ramp Output force = weight of the box Output distance = height of the ramp