Work and Machines

1
Work and Machines

• Chapter 8

2
The Scientific Meaning of Work
Answer No mental effort might seem like a lot of
work, but you are not using a force to move an
object.

• Definition work occurs when a force causes an
  object to move in the direction of the force.
• Use the definition above, is doing homework,
  work??

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3
Working Hard, or Hardly Working?

• Applying a force doesnt always result in work
  being done.
• For example, pushing a car that wont budge. If
  you cant get it to move you havent done any
  work!

4
Work continued

• Force and motion in the same direction
• For work to be done, the object must move in the
  same direction as the force.
• For example if you are carrying a heavy suitcase
  through the airport it is not considered work
  because the direction of force is upward, and the
  direction of motion is forward!

5
Work or Not Work?
6
Calculating Work

• Work force x distance
• W F x d
• If a man applies a force of 500 N to push a truck
  100 m down the street. How much work does he do?

7
W F x d

• Answer
• W 500 N x 100 m
• W 50,000 J
• The unit used to express work is the
  newton-meter, but is more commonly called the
  joule (J).

http//www.youtube.com/watch?vIvcOYOO0Fvwfeature
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8
Nerdy Science Joke Break

• Did you hear about the criminals who never had to
  do any work?

Answer They were joule thieves.
9
Power

• Power is the rate at which work is done.
• It is how fast work happens!
• P W / t
• Power Work
• time
• The unit used to express power is joules per
  second (J/s), which is more simply called the
  watt (W).
• For example if you do 50 J of work in 5 seconds,
  your power is 10 J/s or 10 W.

10
Try it

• Calculate Power
• What is the power of a small motor that can do
  500 J of work in 25 seconds?

http//www.youtube.com/watch?v5EsMmdaYClQ
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Nerdy Science Joke

• Q What is the unit of power?
• A Watt.
• Q I said, what is the unit of power?
• A WATT!
• Q I SAID

12
Quiz 8.1

• 1. In which situation do you do more work?
• W F x d
• a. You lift a 75 N bowling ball 2 m off the
  floor.
• b. You lift two 50 N bowling balls 1 m off the
  floor.
• 2. The rate at which work is done is
• a. Joule b. Time
• c. Power d. Work
• 3. Power is
• a. Work/time b. time/Work
• c. Force x distance c. Work/Force

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Quiz 8.1 continued

• 4. The unit used for Power is
• a. newton-meter b. Joule
• c. meter per second d. Watt
• 5. You push a 100 N box 3 m across the floor. How
  much work has been done?
• a. 33.3 J b. 300 J
• c. 33.3 W c. 300 W

14
Machines

• A machine is a device that helps make work easier
  by changing the size or direction of a force.
• The work that you do on a machine is called work
  input.
• Called input force
• The work done by the machine is called work
  output.
• Called output force

15
The force-distance trade off
W 450 N x 1 m 450 J W 150 N x 3 m 450 J
16
Mechanical Advantage

• Mechanical Advantage tells you how many times the
  machine multiplies force.
• MA output force
• input force

17
Finding Advantage

• You apply 200 N to a machine, and the machine
  applies 2,000 N to an object. What is the
  mechanical advantage?
• Remember MA output force/input force

Answer MA 2000 N 10
200 N
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18
Mechanical Efficiency

• Is a comparison of a machines work output with
  the work input.
• Mechanical Efficiency work output X 100
• work input

19
Simple Machines
20
Levers

• A lever is a simple machine consisting of a bar
  that pivots at a fixed point, called a fulcrum.
• There are 3 types of levers.

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http//www.youtube.com/watch?vwV1pYkTtsxgfeature
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21
First Class Levers

• The fulcrum is between the input force and the
  load

22
Second Class Levers

• The load is between the fulcrum and the input
  force.

23
Third Class Levers

• The input force is between the fulcrum and the
  load.

24
Inclined Planes

• A simple machine that is a straight, slanted
  surface.
• An inclined plane allows you to apply a smaller
  force over a greater distance.

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Wedges

• A wedge is a double inclined plane that moves.
• Wedges are often used to cut materials.
• They allow you to exert your force over an
  increased distance.

26
Screws

• An inclined plane that is wrapped in a spiral.
• When you turn a screw, you exert a small input
  force over a large turning distance, but the
  screw itself doesnt move very far.

27
Mechanical Advantage of Screws

• The threads on the left screw are closer together
  and wrap more times around, so that screw has a
  greater mechanical advantage that the one on the
  right.

28
Wheel and Axle

• A simple machine consisting of two circular
  objects of different sizes.
• Examples include a door knob, a crank, a fishing
  reel

http//www.youtube.com/watch?vC-UXry7OiXMfeature
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Pulleys

• A simple machine consisting of a grooved wheel
  that holds a rope or a cable.
• A load is attached to one end of the rope, and
  input force is applied to the other end.

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• Fixed Pulleys only change the direction of the
  force.

• Movable Pulleys are attached to the object that
  is being moved.

31
Compound Machines

• Machines that are made of two or more simple
  machines.