Section 15'1 What it means to do work

1
Chapter 15
2
Section 15.1 What it means to do work

• A. Work
• 1. Occurs when a force moves an object
• 2. A force acting through a distance
• 3. Force applied to object
• a. object moves means work is done
• b. object does not move means no work
• 4. Object must move in direction of force

3

• B. Calculating Work
• 1. Work Force
  Distance
• Newtons (N) meters (m)
• Joule Newton meter
• 2. Homework problem 4 pg. 375
• Work Force distance
• Work 900 N 100 m
• Work 90000 N m
• Work 90000 Joules

4
Section 15.2 Power

• A. Powers Definition
• 1. Society definition is strength
• 2. Sciences definition
• a. rate at which work is done
• b. amount of work per unit of time
• 3. Power work
• time
• 4. Power force distance
• time
• 5. Power Newton meter
• second

5

• 6. Power Joule
• second
• 7. Watt Joule
• second
• 8. Power is measured in watts

6

• B. Measurement of power
• 1. Electric companies use watts
• a. larger amounts of power measured in
  kilowatts
• 2. Engine power uses horsepower in English
  system
• a. 1 horsepower (hp) 746 watts

7
Section 15.3 Machines

• A. Human nature
• 1. Use devices to make work easier
• a. get it done as fast as possible
• b. cars, mowers, snow blowers, power
  tools
• 2. machines make work easier
• B. Work input
• 1. work going into machine
• 2. work force distance
• 3. work input effort force effort
  distance

8

• C. Work output
• 1. work machine does
• 2. work output resistance force
  resistance distance
• a. resistance force is usually weight
  of object
• D. Machines do not increase work
• 1. output work is always less than the input
  work
• 2. Friction - output work is less than
  input work
• 3. machines change size or direction of force

9

• 4. Change in size is accompanied by change
  in distance
• of force exerted
• a. put in 100 N to move 400N object
• b. 100 N moves 4 meters to 400 N object
  1 meter
• 5. No free lunches when work is involved
• a. increase in force costs distance
• b. increase in distance costs force
• 6. No machine can increase
• both force and distance

10

• E. Determining how helpful a machine is
• 1. Why is output work always less than input
  work?
• a. friction
• 2. Efficiency
• a. comparison of output work to input work
• b. expressed as a percentage (grade in science
  class)
• c. can never be greater than 100
• d. Eff output work x 100
• input work
• e. Eff AMA x 100
• IMA

11

• 3. Mechanical Advantage
• a. number of times a machine multiplies effort
  force
• b. Ideal MA perfect MA (hint meters)
• c. Actual MA MA with friction (hint
  Newtons)
• d. Ideal MA effort distance
• resistance distance
• e. AMA Resistance
• Effort

12
Section 15-4 Simple and Compound Machines

• A. Six types
• 1. Inclined plane 4. lever
• 2. wedge 5. pulley
• 3. screw 6. wheel axle
• B. Inclined Plane
• 1. Ramp
• a. decreases the effort force
• b. increases the distance of effort force
• 2. IMA effort distance
• resistance distance

13

• 3. Wedge
• a. inclined plane that moves
• b. 2 inclined planes put together
• c. knife, ax, doorstop, splitting wedge,
  zippers, skates
• d. longer thinner (sharper) the greater the
  MA
• 4. Screw
• a. inclined plane wrapped around a nail
• b. multiplies effort force by acting through
  long
• distance
• c. closer the threads the higher the MA

14

• C. Lever
• 1. rigid bar that pivots around a fixed point
• 2. fulcrum - fixed point of support
• 3. crowbar, seesaw, 2-wheel dolly, wheelbarrow
• 4. three classes of levers
• a. scissors, pliers, seesaw
• b. wheelbarrow, nutcracker
• c. brooms, rakes, shovels

15

• D. Pulley
• 1. can change direction or amount of force
• 2. fixed pulley
• a. attached to a structure
• b. changes direction of force
• c. distance pulled equals distance moved
• 3. Movable pulley
• a. increases effort force (mechanical
  advantage)
• b. effort put out for longer distance
• 4. Block Tackle system
• a. fixed and movable pulley system
• b. MA number of pulleys
• c. MA number of supporting ropes

16

• E. Wheel Axle
• 1. simple machine made up of 2 circular
• objects of different sizes
• 2. wheel is larger moves a greater distance
  than axle
• 3. MA radius of wheel
• radius of axle
• 4. bicycles, ferris wheels, gears,
  doorknobs,
• wrenches, steering wheels, screwdrivers
• F. Compound Machines
• 1. combination of 2 or more simple machines
• 2. car, lawn mower, recliner, VCR