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Work, Power, Simple Machines

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Title: Work, Power, Simple Machines


1
Work, Power, Simple Machines
2
Work
Define work done on an object by a force
as (Work) (Force) X (Distance traveled) Unit
is Newton Meter (Nm) or Joule (J) Force
acting in direction of motion Positive
work. Force acting in opposite direction
Negative work. Force perpendicular to motion
Zero work
3
Check Your Self
  • A weight lifter lifts a 500 N barbell to a height
    of 2 m. Calculate the amount of work done by the
    Weightlifter?
  • 500 N x 2 m 1,000 J (Nm)
  • If he carries the same barbell 10 m across the
    room, how much more work would he do?
  • 0 silly. The net force on the barbell would be 0,
    thus resulting in no additional work being done

4
  • Work
  • The amount of work is set and can be calculated.
  • No matter how you go about the task of work the
    amount will be the same for that task (does not
    depend on time).

5
Try these
  • 1. Which of the following is an example of work
    bowling or reading?
  • Bowling
  • 2. A man pushes against a brick wall, which does
    not move. Is this an example of work?
  • No, because there was no movement.
  • 3. A student carries her books to class. Is this
    an example of work?
  • No, force not applied in the direction of
    movement.
  • 4. A woman raises and lowers dumbbells at the
    gym. Is this an example of work?
  • Yes, force is in the direction of motion.
  • 5. A book falls off a table and lands on the
    floor. Is this an example of work?
  • Yes, the work is done by gravity.

6
This is Work
  • 1. d W / F 2.7J / 4.5N 0.6m
  • 2. d W / F 4.35 E -2J / 7.25 E -2N 0.6m
  • 3. d W / F 8.8 E -4J / 3.4 E -4N 2.6 m
  • 4. F W / d 9.8 E 7J / 35 m 2.8 E 6 N
  • 5. F W / d 405J / 15m 27 N
  • 6. F W / d 1.8 E 6J / 2.0m 9 E 5 N
  • 7. W F d (3150N)(75.5m) 2.38 E 5J
  • 8. W F d (1.6 E 6N)(2000m) 3.2 E 9J
  • 9. W F d (0.25)(1.5 E 6N)(15m) 5.6 E 6J
  • 10. d W / F 3650J / 2470N 1.48m

7
  • 11. d W / F 4365J / 1302N 3.35m
  • 12. F W / d 1.67 E 4J / 3.05 5.48 E 3 N
  • 13. W F d (2.23 E 4N)(436m) 9.72 E 6 J
  • 14. d W / F 1.10 E 9J / 2.53 E 6N 435 m
  • 15. W F d m a d
  • (70kg)(4.90 E 2m/s2)(8.05m)
  • 2.76 E 5 J

8
Power
  • How fast work is completed
  • or
  • The Rate at which work is completed
  • Work is in Joules (J)
  • Time is in seconds (s)
  • Power is in J/s or Watt
  • More Common is the Horsepower
  • 1 horsepower 746 watts

Work
Power
Time
9
1 Horsepower Car
10
Check Yourself
  • How much work is done on a 100 N boulder that you
    lift 1 m?
  • What if you carry it across a 10 m room?
  • What Power is expended if you lift the boulder a
    distance of 1m in 1 s?
  • What total power would you have if you did this
    in a ½ s?

W F x d 100 N x 1m 100 J
0J
P 100 J / 1 s 100 W
P 100 J / 0.5 s 200 W
11
More Power To You
  • 1. Power is the rate at which work is done. This
    means Power Work / Time. The units for power
    are J / S which is also called watt.
  • 2. P W / t 2405J / 35.0 s 70 W
  • 3. P W / t 250J / 25s 10W
  • 4. P W / t 9000J / 25s 360W
  • 5. P W / t 3750J / 15s 250W
  • 6. W Pt (500W)(300s) 1.5 E 5 J
  • 7. W Pt (60W)(3600s) 2.16 E 5 J
  • 8. W Pt (750W)(12s) 9000J
  • 9. t W / P 2.16 E 5J / 60W 3600s
  • 10. t W / P 810J / 45W 18s

12
Force Dist. Time Work Power
10 6 4 60 15
62.5 4 5 250 50
30 20 2 600 300
500 10 50 5000 100
4 16 8 64 8
100 0.5 2 50 25
200 0.5 2 100 50
30 50 30 1500 50
800 100 20 80,000 4,000
13
What is a Simple Machine?
  • A simple machine has few or no moving parts.
  • Simple machines make work easier

14
The principles of Work and Powerare the
fundamentals behind simple machines
  • Six Simple Machines
  • Lever
  • Wheel and Axle
  • Pulley
  • Incline Plane
  • Wedge
  • Screw

15
Why do we need simple machines?
  • Work means that we are exerting force and moving
    something. The 6 simple machines make our work
    easier. We wont have to use as much force with
    our simple machines.

16
Levers
  • All use the same principle of work
  • Type 1
  • Type 2
  • Type 3

17
Levers-First Class
  • In a first class lever the fulcrum is in the
    middle and the load and effort is on either side
  • Think of a see-saw

18
Levers-Second Class
  • In a second class lever the fulcrum is at the
    end, with the load in the middle
  • Think of a wheelbarrow

19
Levers-Third Class
  • In a third class lever the fulcrum is again at
    the end, but the effort is in the middle
  • Think of a pair of tweezers

20
Wheels and Axles
  • The wheel and axle are a simple machine
  • The axle is a rod that goes through the wheel
    which allows the wheel to turn
  • Gears are a form of wheels and axles

21
Pulleys
  • Pulley are wheels and axles with a groove around
    the outside
  • A pulley needs a rope, chain or belt around the
    groove to make it do work
  • Pulleys can also change the direction of the
    force being applied

22
Types of Pulleys
  • Fixed Pulley
  • Change the Direction of Force
  • Moveable Pulley
  • Reduce the amount force, by increasing the
    distance

23
Inclined Planes
  • An inclined plane is a flat surface that is
    higher on one end
  • Inclined planes make the work of moving things
    easier

24
Wedges
  • Two inclined planes joined back to back.
  • Wedges are used to split things.

25
Where can we find wedges?
  • Wedges that split
  • Your teeth!
  • Every time you bite into something, you are using
    a wedge.
  • Wedges that cut
  • A saw!
  • A knife!
  • Wedges that tighten
  • A doorstop to keep the door open.
  • Wedges that hold things together
  • A nail holds wood to a wall or other wood.

26
Screws
  • A screw is an inclined plane wrapped around a
    shaft or cylinder.
  • The inclined plane allows the screw to move
    itself when rotated.

27
The 6 Simple Machines
Lever
Inclined Plane
Pulley
Wheel and Axle
Wedge
Screw
28
Simple Machines
  • Once work is known we can manipulate Force and
    Distance to accommodate our needs
  • An incline plane is a perfect example of this
  • If the distance traveled is increased the Force
    applied must go down

W F x D
W F x D
29
Self Check
  • Who does more Work?

(small force) X (LONG DISTANCE)
(BIG FORCE) X (short distance)
They do the same work in different ways.
30
Figure out how each of the following simple
machines utilize WORK
  • Screw
  • Wheel and Axle
  • Wedge

All Increase the distance traveled thus reducing
the amount of force required
31
  • Mechanical Advantage
  • Ratio of output force to input force for a machine
  • Efficiency
  • Ratio of useful work output to total work input
  • Will never be 1 because energy is always
    converted to other forms

Useful Work Output
Efficiency
Total work input
Output force
M.A.
Input force
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