1. Chapter 8
Work and Energy

2. 8.1 Work, Power, and Machines Work
What is Work
Work is done only when force cause a change in the motion of an object.
Work is calculated by multiplying an applied force by the distance over which the force is applied.
Work = force x distance
W = F x d

3. 8.1 Work, Power, and Machines Work
In the case of trying to lift a heavy object, you might apply a large force, but if the distance that the object moves is zero, there is no work done.
An object has to move at least a small amount for work to be done, because work is the product of force times distance.
Work = force x distance
W = F x d

4. 8.1 Work, Power, and Machines Work
Work is measured in Joules, J.
Because work is the product of force times distance, it is measured in units of Newtons times meters or N . m
1 N . m = 1 J = 1 kg.m2/s2

5. 8.1 Work, Power, and Machines Power
What is Power
Power is defined as the rate at which work is done.
Power = work / time
P = W / t

6. 8.1 Work, Power, and Machines Power
What is Power
Power is defined as the rate at which work is done.
Because power is the rate at which work is done, it is measured in units of Joules per second or Watts.
1 N . m /s = 1 J / s = 1 watt

7. 8.1 Work, Power, and Machines Machines & Mechanical Advantage
Machines multiply and redirect forces
Work input equals work output
Mechanical advantage tells how much a machine multiplies force or increases distance
Defined as the ratio of the output force over the input force or the input distance to the output distance.
Mechanical advantage = output force / input force
Or = input distance / output distance

8. 8.2 Simple Machines The six (6) Simple Machines The lever family
simple lever
pulley
wheel & axle
The inclined plane family
simple inclined plane
wedge
screw

9. 8.2 Simple Machines The Three Classes of Levers
First class lever
All first class levers have a fulcrum in the middle of an arm; the input force acts on one end, and the other end supplies an output force. [Claw hammer] MA > 1
Input force
Output force

10. 8.2 Simple Machines The Three Classes of Levers
Second class lever
All second class levers have a fulcrum at one end of an arm; the input force is applied to the other end. [wheel barrow] MA > 1
Output force
Input force

11. 8.2 Simple Machines The Three Classes of Levers
Third class lever
All third class levers also have a fulcrum at one end of an arm; however, these levers multiply distance rather than force. [forearm] MA < 1
Output force
Input force

12. 8.2 Simple Machines Pulleys are modified levers
The mechanical advantage of pulleys
Output force = 150 N
Input force = 150 N
MA = 1
Fixed pulley

13. 8.2 Simple Machines Pulleys are modified levers
The mechanical advantage of pulleys
Input force = 75 N
Output force = 150 N
MA = 2
Moving pulley

14. 8.2 Simple Machines Pulleys are modified levers
The mechanical advantage of pulleys
Input force = 75 N
Output force = 150 N
MA = 3
Multiple pulleys

15. 8.2 Simple Machines A wheel and axle is a lever or pulley connected to a shaft
Output force
fulcrum

16. 8.2 Simple Machines The Inclined Plane Family
Inclined Planes multiply and redirect force
Output force
Input force
An inclined plane changes both the magnitude and direction of force

17. 8.2 Simple Machines The Inclined Plane Family
A wedge is a modified inclined plane

18. 8.2 Simple Machines The Inclined Plane Family
A screw is an inclined plane wrapped around a cylinder.

19. 8.2 Compound Machines
Many devices that you use every day are made of more than one simple machine.
A machine that combines two or more simple machines is called a compound machine.

20. 8.3 Energy and Work Energy is measured in joules
While work is done only when an object experiences a change in its motion, energy can be present in an object or a system when nothing is happening at all.
There are several types and forms of energy: potential, kinetic, chemical, nuclear, electromagnetic, electrical, etc.

21. 8.3 Energy and Work Potential Energy
Potential energy is stored energy.
Gravitational potential energy depends on both mass and height:
Gravitational PE = mass x free-fall acceleration x height.
PE = m g h
Do practice problems on page 265

22. KE = ½ x mass x velocity squared
8.3 Energy and Work
Kinetic Energy
The energy that a n object has because it is in motion is called kinetic energy.
Kinetic energy depends on mass and velocity.
KE = ½ x mass x velocity squared
KE = ½ m v2

23. 8.4 Conservation of Energy
Potential energy can become kinetic energy
Kinetic energy can become potential energy
Energy can neither be created or destroyed!