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Work and Machines

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Work and Machines Chapter 8 – PowerPoint PPT presentation

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Title: 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??

http//www.youtube.com/watch?vpbsSiFgW-Xwfeature
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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
11
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

13
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
http//www.youtube.com/watch?voWiZ_5qvs7Ifeature
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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.

http//www.youtube.com/watch?vPW7ztbwJKBkfeature
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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.

http//www.youtube.com/watch?vpG_kT565-XQfeature
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25
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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29
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.

30
  • 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.
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