Chapter 8: Work, Power, Simple

machines

Section 1: Work and Power

1) In science, work occurs when a force causes an object to move in the direction of the force.

20N Work has happened because the box moved

2) Work is done when a)the object moves as the force is

applied and b) the direction of the object’s motion

is the same as the direction of the force applied.

3) Work can be calculated:W=F x dWork equals force times distance

4) Force is measured in Newtons, and distance is measured in meters: so the unit for work is the Newton-meter or the joule.

5) Calculating work:a) A man applies a

force of 500N to push a truck 100m down the street.

How much work does he do?

b) In which situation do you do more work?1- You lift a 75N bowling ball 2 m

off the floor

2- You lift two 50 N bowling balls 1m off the floor.

Complete the following work problems. The first one has been done as an example for you.

Work (J) = Force (N) X Distance (m) 1. 20J = 10N X 2m

2. 20J = force? X 2m

3. work? = 10N X 50m

4. work? = 0.5N X 600m

5. 500J = 500N X distance?

6. work? = 80N X 7m

6) Power is how fast work is done. It can be calculated:

P=W/t the units for power are the watt (W)

7) Power is increased ifa) work is done in less

time, or b) the amount of work is increased.

Review game

Section 2: Machines

1)A machine is a device that helps make work easier

Machines change either the size or direction of a force.

2) Machines do not make work less; they make work easier,

and this is called the mechanical advantage.

3) Work done on a machine is called work input, work done by the machine is the work output.

Work output is never greater than work input, because some of the work done by the machine is used to overcome friction.

4) The less work a machine has to do to overcome friction the more efficient it is.

Section 3: Types of Machines

1) All machines are made from six simple machines: the lever, inclined plane, wedge, screw, pulley, and wheel and axle.

2) Levers: a bar that pivots at a fixed point called a fulcrum

A) First class levers: fulcrum between the input force and load (see-saw)

B) Second class levers: load between the fulcrum and the input force (wheel barrow)

C) Third class levers: input force between the fulcrum and the load (hammer)

3) Inclined Plane: straight slanted surface - the longer the plane the greater the mechanical advantage (ramp)

4) Wedges: double inclined plane that moves (knife or scissors)

5) Screw: inclined plane wrapped in a spiral

6) Wheel and axle: two circular objects of different sizes (door knobs)

7) Pulleys: grooved wheel that holds a rope or cable, with a load attached to one end, input force applied to another

8) Compound machines: machines made of two or more simple machines