1. Pearson Prentice Hall Physical Science: Concepts in Action
Chapter 14
Work, Power and Machines

2. 14.1 Work and Power Objectives:
1. Describe the conditions that must exist for a force to do work on an object
2. Calculate the work done on an object
3. Describe and calculate power
4. Compare units of watts and horsepower as they relate to power

3. Conditions for Work Def: work is the product of force times distance
For a force to do work on an object, some of the force must act in the same direction as the object moves
If the object does not move, no work is done
Work depends on direction
Any part of a force that does not act in the direction of motion does no work on the object

4. Calculating Work Work = Force x Distance
The units for force are Newtons, N
Recall from chapter 12 that 1 N = 1 kg*m/s2
The unit for distance is the meter, m
The unit for force is 1 N*m or 1 kg*m2/s2 which equals one joule, abbreviated J

5. Calculate Power Def: power is the rate of doing work
Doing work at a faster rate requires more power
To increase power, increase the amount of work done in a given time OR do a given amount of work in less time
Power = Work/Time
The unit of work is joules (J)
The unit of time is seconds (s)
J/s = watts (W) & the unit of power is watts

6. Watts and Horsepower
One horsepower equals 746 watts
James Watt defined horsepower as the power output of a very strong horse
Watt did not want to exaggerate the power of steam engines

7. 1. Describe what a machine is and how it makes work easier to do
14.2 Work and Machines
Objectives:
1. Describe what a machine is and how it makes work easier to do
2. Relate work input of a machine to work output of the machine

8. What a Machine is & How it Makes Work Easier
Def: a machine is a device that changes a force
Machines make work easier to do
Machines change the size of a force needed, the direction of the force, or the distance over which a force acts
Some machines increase distance over which to exert a force, decreasing the amount of force needed
Some machines exert a large force over a short distance
Some machines change the direction of the applied force

9. Work Input and Work Output
Because of friction, the work done BY a machine is always less than the work done ON a machine
Def: work input is work done by the input force acting through input distance
Def: work output is force exerted by a machine
Def: output distance is the distance of the output force

10. 14.3 Mechanical Advantage and Energy
Objectives:
1. Compare a machine’s actual mechanical advantage to it ideal mechanical advantage
2. Calculate the ideal and actual mechanical advantages of various machines
3. Explain why efficiency of a machine is always less than 100%
4. Calculate a machine’s efficiency

11. Actual and Ideal Mechanical Advantage + Calculations
Def: mechanical advantage is the number of times that a machine increases an input force
Actual MA = output force/input force
Def: ideal mechanical advantage is the MA in the absence of friction
Friction is always present, so the actual MA of a machine is always less than the ideal MA
Ideal MA= input distance/output distance
There are no units with MA

12. Efficiency Calculation & Why it is Less Than 100%
Def: efficiency of a machine is the percentage of work input that becomes work output
Efficiency is always less than 100% since friction is always present
Efficiency = work output/work input x 100%

13. 1. Describe the six types of simple machines
Objectives:
1. Describe the six types of simple machines
2. Explain what determines the mechanical advantage of the six types of simple machines

14. Six Types of Simple Machines & MA
The six types of simple machines are the lever, wheel and axle, inclined plane, wedge, screw and pulley
Def: a lever is a rigid bar free to move about a fixed point
Def: a fulcrum is the fixed point a lever moves around
Def: the input arm is the distance between the input force and the fulcrum

15. Def: the output arm is the distance between the output force and the fulcrum
For a lever: MA = input arm/output arm
There are 3 classes of levers: first, second and third class
For first class levers the fulcrum is located between the input force and the output force
MA for first class levers is =, < or > 1
Examples: seesaws, scissors, tongs, screwdriver

16. For second class levers, the output force is located between the input force and fulcrum
MA is always >1 for second class levers
Example: wheelbarrow
For third class levers, the input force is located between the fulcrum and output force
MA is always <1 for third class levers
Examples: baseball bats, hockey sticks, golf clubs & brooms

17. Def: a wheel and axle consists of 2 disks or cylinders, each one with a different radius
Example: steering wheel
To calculate MA for wheel and axle, divide the radius (or diameter) where the input force is exerted by the radius (or diameter) where the output force is exerted
Def: an inclined plane is a slanted surface along which a surface moves an object to a different elevation
Example: ramp in front of buildings
The ideal MA for an inclined plane is the distance along the plane divided by its height

18. Def: a wedge is V-shaped object whose sides are two inclined planes sloped toward each other
Example: flat head screwdriver
A thin wedge of given length has a greater ideal MA than a thick wedge of the same length
Def: a screw is an inclined plane wrapped around a cylinder
Screws with threads closer together have a greater ideal MA

19. Def: a pulley consists of a rope that fits into a groove in a wheel
The MA of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted
Def: a fixed pulley is a wheel attached in a fixed location
The ideal MA of a fixed pulley is always 1
Def: a movable pulley us attached to the object being moved
The ideal MA of a movable pulley is 2

20. Def: a pulley system is a combination of fixed and movable pulleys that operate together
MA depends on pulley arrangement
Def: a compound machine is a combination of two or more simple machines that operate together
Examples: cars, washing machines, clocks