Sec 14.2 Work and Machines. Objectives

1. 14.2.1 Describe what a machine is and hoe it makes work easier to do. 14.2.2 Relate the work input of a machine to the work output of the machine. Sec 14.2 Work and Machines.Objectives

2. All cars come equipped with simple machines designed to make changing a tire a fairly easy task.

3. How do machines make work easier? A machineis a device that changes a force. Machines make work easier to do. They change 1)the size of a force needed, 2)the direction of a force, or 3)the distance over which a force acts. Machines Do Work

4. Machines Do Work Increasing Force Turning the jack handle allows the man to raise the car.

5. Machines Do Work Each complete rotation of a jack handle applies a small force over a large distance. ( work = force x distance) • A small force exerted over a large distance becomes a large force exerted over a short distance. • Each rotation lifts the car only a very short distance.

6. Machines Do Work Increasing Distance A rower pulls an oar through a small distance. The end of the oar in the water moves through a large distance. The increased travel of the oar through the water requires the rower to exert a greater force. A machine that decreases the distance through which you exert a force increases the amount of force required.

7. Machines Do Work Changing Direction Some machines change the direction of the applied force. Pulling back on the handle of the oar causes its other end to move in the opposite direction. Machines can change the amount of force and the distance the force acts through as well as the direction of the force.

8. How are work input and work output related for a machine? Because of friction, the work done by a machine is always less than the work done on the machine. Work Input and Work Output

9. Work Input and Work Output The oars of the boat act as machines that increase the distance over which the force acts. Boat moves in this direction. Input force Input distance Output force Output distance

10. Work Input and Work Output Work Input to a Machine • The force exerted on a machine is the input force. • The distance the input force acts through is the input distance. • The work input equals the input force multiplied by the input distance.

11. Work Input and Work Output For an oar, the input force is the force exerted on the handle, and the input distance is the distance the handle moves. The work input is the work done to move the handle.

12. Work Input and Work Output Work Output of a Machine • The force exerted by a machine is called the output force. • The distance the output force is exerted through is the output distance. • The work output of a machine is the output force multiplied by the output distance.

13. Work Input and Work Output The output work of the oars results from the oars pushing against the water so that the water pushes back against the oars. Although nearly equal, the output work is less than the input work because of friction. All machines use some amount of input work to overcome friction.

14. Work Input and Work Output The only way to increase the work output is to increase the amount of work you put into the machine. You cannot get more work out of a machine than you put into it!

15. Assessment Questions • What is the output distance of a machine that requires 2 newtons of force exerted over 6 meters and whose output force is 4 newtons? • 2 meters • 3 meters • 6 meters • 12 meters

16. Assessment Questions • What is the output distance of a machine that requires 2 newtons of force exerted over 6 meters and whose output force is 4 newtons? • 2 meters • 3 meters • 6 meters • 12 metersANS: B

17. Assessment Questions • The work output of a machine is always greater than the work input to the machine. TrueFalse

18. Assessment Questions • The work output of a machine is always greater than the work input to the machine. TrueFalse ANS: F, never