1. Work, Power, and Simple Machines
Physical Science
Chapter 12

2. Work In science work is done ONLY when a force moves an object.
A force acting through a distance is considered work.
If the object does not move, work is NOT being done, no matter how tired you get. Work might be on a cellular level, but not a macroscopic level.

3. Rules for Work Work = Force X Distance
Force is measured in newtons, distance is measured in meters.
The unit of work in newton/meters or joules
1 joule =1 newton of force applied to an object that moves 1 meter.
Work not only has to move the object, but the object must move in the direction of the force applied.

4. Power
Power is the rate at which work is done, or amt of work per unit of time.
Power tells us how fast something is happening.
Power = Work / Time or,
Force x Distance / Time
The unit of power is the watt.
1 watt = 1 joule / second

5. Machines A machine is an instrument that makes work easier.
Machines change either the size or the direction of the force put into the machine, and…..
Any change in the size of a force is accompanied by a change in the distance through which the force is exerted.

6. Distance and Work Two types of work are involved in a machine:
-Work Input –work that goes into a machine or force applied to a machine
-Work Output – the work done by the machine or force applied by the machine
How a machine can change force:
If a machine multiplies input force, the distance over which the work output is exerted decreases.
If distance increases, then force must decrease.
Therefore, machines make work easier by multiplying either force or distance, but never both.

7. Work Output
Work output is used to overcome the force you and the machine are working against; also called the resistance force.
Ex. Weight of object

9. How Helpful is a Machine?
Work output can never equal work input due to friction.
Work output is compared to work input to determine the efficiency of a machine.
The closer the two amounts are the more efficient a machine is.

10. Mechanical Advantage
Mechanical advantage (MA) of a machine=the number of times a machine multiplies effort force.
The more times a machine multiplies the effort force, the easier it is to do the job.
MA = output force = Fout input force Fin
Two kinds of MA, ideal & actual - Why?

12. Simple Machines There are six types of simple machines:
The inclined plane
The wedge
The screw
The lever
The pulley
The wheel and axle

13. MA = length of incline
height of incline
The longer the incline, the
less force you need

14. Wood or metal that is thinner at one end.
Is a form of inclined plane that moves to raise an object or split one.
Wood or metal that is thinner at one end.
The longer and thinner, the more force it exerts.
Ex. Knife or Zipper or Axe
MA = length of wedge
width
Longer & thinner, more MA

15. Screw
The screw is an inclined plane wrapped around a central bar or cylinder to form a spiral.
A screw multiplies the force by acting through a long distance.
Ex. Bolt and nut, and jar lids
MA =The closer the threads are, the more you have to turn the screw, so you increase the distance, which increases output force.

16. The Lever
A lever is a rigid bar that is free to pivot, or move about, a fixed point.
The fixed point is called the fulcrum.
When force is applied to the bar by pushing or pulling, the lever swings on the fulcrum & overcomes resistance force.
MA = d from fulcrum to input force
d from fulcrum to output force

18. Wheel and Axle
Simple machine made up of two circular objects of different sizes.
The wheel is the larger object and it turns about the smaller object called the axle.
Because the wheel is larger, force is multiplied when it is applied to the axle.
MA = wheel radius axle radius
Ex. Screwdriver, Ferris Wheel

19. Pulley
A pulley is a rope, belt or chain wrapped around a grooved wheel.
A pulley can function two ways:
Change direction of the force
Change amount of the force
A fixed pulley (attached to a stationary object) changes the direction of the effort force.
A movable pulley is attached to the object you are trying to move and thus the effort force is multiplied.
Fixed and movable pulleys can be combined to accomplish even more work.
MA = # ropes that support the object.

20. MA of Pulleys

22. Compound machines are
combinations of simple machines.
Mechanical advantage = the sum of all
the MA of each simple machine.