Work and Machines

1. Work and Machines How machines make our lives easier

2. Machines • A machine is a device that changes a force • Machines make work easier to do • Machines change the size of the force needed, the direction of the force, or the distance over which a force acts • Example a lug wrench and jack allow a person to change a car tire

3. Increasing Force • A small force exerted over a large distance becomes a large force over a short distance • Example a jack lifts a car a small bit at a time but the distance the jack moves is large

4. Increasing Force • If a machine increases the distance traveled then it lessens the force required • Example moving books one at a time takes less force but requires more distance than moving the whole stack at once

5. Increasing Distance • A machine may decrease the distance the force moves but increases the force required • Example is a crew team rowing a boat • Each oar pull is short but the distance the boat moves is long, the distance is shorter and the force needs to be greater

6. Changing Direction • Machines can also change the direction of the force • For example, when pulling on an oar, one end of the oar goes in the opposite direction • This has changed the direction of the force

7. Work Input • The force you input on a machine is called input force • The distance the input force acts through is known as the input distance • The work done by the input force acting through the input distance is called the work input • The work input equals the input force times the input distance (think: work = force x distance)

8. Work Output • The force exerted by a machine is the output force • The distance the output force is exerted is the output distance • The work output is the output force times the output distance

9. Work Input and Work Output • Output and input work are nearly equal • The output work will be less • All machines need to use input work to overcome friction