Work and Power

1
Chapter 14

• Work and Power

2
Work

• Requires movement
• A quantity of energy transferred by a force
  acting on an object in the direction the object
  moves

Is this weight lifter doing work? What about when
the barbell is over his head? Is he working?
3
Work

• Can be calculated by the formula
• Work Force X Distance
• Units of work
• newton-meters
• or Joules (J)

4
Calculating work

• How much work is done when a 25 Newton force
  lifts a potted plant to a shelf that is 1.5
  meters high?
• Work Force X Distance
• Work 25 N X 1.5 m
• Work 38 Newton-meters or J

5
Calculating work

• Jake, a 235 N track athlete completes his race,
  which totals 1575 J, what is the total distance
  Jake ran?
• Work distance
• force
• 1575 J 6.70 m
• 235 N

6
Calculating Work

• Joey, performed 900 J of work, while lifting a
  box 12 meters. What force did Joey exert on the
  box?
• Force work
• distance
• Force 900 75 N
• 12

7
Power

• The rate of doing work
• Power increases when
• Work is increased
• Time is decreased

8
Power

• Can be calculated by the formula
• Power Work
• time
• Units of Power Joules/sec or Watts (W)
• 1 W 1J/sec so a 40 watt bulb requires 40 Joules
  each second that it is lit.
• 1 horsepower (hp) 746 Watts

9
Calculating Power

• You lift a large bag of flour from the floor to a
  1 meter high counter, doing 100 Joules of work in
  2 seconds. How much power does it require?
• Power work/time
• Power 100 J
• 2 s
• Power 50 W

10
Calculating Power

• Using a jack, a mechanic does 5350 J of work to
  lift a car 0.500 m in 50.0 s. What is the
  mechanics power output?
• Power 5350J
• 50 s
• Power 107 W

11
Machines

• Definition
• - a device that changes a force to make work
  easier
• ex. jack, nutcracker, an oar
• How does a machine change force?
• - change the size of force needed
• - the direction of a force
• - the distance over which a force acts

12
Changing the Size of the Force

• Increasing the force you applied
• ex. jack applied to a car
• - small force exerted over a large distance
  becomes a large force exerted over a short
  distance

13
Changing the Distance

• Increasing distance
• ex. rowing a boat with oars
• small movement of oar at the hands makes a large
  distance the oar in the water will move.
• remember the trade off small distance large
  force

14
Changing the Direction

• Changing the direction of the applied force
• ex. rowing a boat
• - pulling back on the handle of the oars
  causes its other end to move in the opposite
  direction.

15
Work Input

• Work Input
• - the work done on a machine as the input force
  acts through the input distance
• - input force force exerted on a machine
• - input distance distance the input force acts
  through
• - work input input force x the input distance
• ex. rowing a boat
• - input distance lt output distance
• - input force gt output force

16
Work Output

• Work Output
• - the work done by a machine as the output force
  acts through the output distance
• - output force force exerted by a machine
• - output distance distance the output force is
  exerted through
• due to friction the work done by a machine is
  always less than the work done on the machine

17
Mechanical Advantage

• Definition
• - a quantity that measures how much a machine
  multiplies force or distance
• Two types
• Actual measures the actual forces acting on
  a machine
• - AMA input force
• output force
• Ideal measures the mechanical advantage in the
  absence of friction
• - IMA input distance
• output distance

18
Mechanical Advantage Problems

• Q Alex pulls on the handle of a claw hammer with
  a force of 15 N. If the hammer has a actual
  mechanical advantage of 5.2, how much force is
  exerted on a nail in the claw?
• A output force (5.2)(15N) 78 N
• Q If you exert 100 N on a jack to lift a 10,000
  N car, what would be the jacks actual mechanical
  advantage (AMA)
• A AMA 10,000 N / 100 N 100

19
Mechanical Advantage Problems

• Q Calculate the ideal mechanical advantage (IMA)
  of a ramp that is 6.0 m long and 1.5 m high?
• A IMA 6.0m / 1.5m 4.0
• Q The IMA of a simple machine is 2.5. If the
  output distance of the machine is 1.0 m, what is
  the input distance?
• A Input distance (2.5)(1.0m) 2.5 m

20
Efficiency of Machines

• Definition
• - a quantity, usually expressed as a
  percentage, that measures the ratio of useful
  work input
• Formula
• - Efficiency work output
• work input
• - of work input that becomes work output
• - due to friction, efficiency of any machine is
  always less than 100

21
Efficiency Problems

• Q Alice and Jim calculate that they must do 1800
  J of work to push a piano up a ramp. However,
  because they must also overcome friction, they
  must actually do 2400 J of work. What is the
  efficiency of the ramp?
• A 1800 J/ 2400 J x 100 75
• Q If the machine has an efficiency of 40, and
  you do 1000 J of work on the machine, what will
  be the work output of the machine?
• A Work Output (Efficiency x work input) / 100
• Work Output (40 x 1000 J) / 100 4.0 x
  102 J