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Chapter 5 Work and Machines.

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Presentation on theme: "Chapter 5 Work and Machines."— Presentation transcript:

1 Chapter 5 Work and Machines

2 Work Transfer of energy that occurs when a force makes an object move.

3 Work – Two Conditions 1. The object must move.
2. The motion must be in the same direction as the applied force.

4 Is Work Done? Picking up a stack of books. Holding a stack of books.
Walking while holding a stack of books.

5 Work and Energy When work is done, a transfer of energy occurs (energy = ability to do work) Ex. When you pick up a box, you transfer energy from your muscles to the box, increasing its PE

6 Calculating Work Work = Force X distance W=Fd F = N d = m W = N-m = J

7 Power Is the rate at which work is done.
Amount of work done in a certain amount of time.

8 Calculating Power Power = Work/time P = W/t W = J t = sec
P = J/sec = Watts (W)

9 Power and Energy Energy can be transferred without involving work.
Ex. Light bulb transfers energy into heat and light. P = E/t

10 Machines A device that makes doing work easier. Work by:
Increasing F (car jack) Increasing d (ramp) Changing direction (ax)

11 Work done by Machines Two forces involved
Effort Force (Fe) – the force applied to a machine. Resistance Force (Fr) – the force applied by the machine to overcome resistance.

12 Work done by Machines Two types of work involved:
Input work (Win) – work done by you on a machine. Output work (Wout) – work done by the machine.

13 Conserving Energy What does the Law of Conservation of Energy state?
You transfer energy to the machine, and the machine transfers energy to an object.

14 Conserving Energy A machine cannot create energy, so Wout is never greater that Win. Can Wout = Win ? Why?

15 Ideal Machines What is an ideal Machine?
If a machine allows you to apply less force, then how do you get the same amount of work out of the machine?

16 Mechanical Advantage (MA)
It is the number of times a machine multiplies the effort force. IMA vs AMA

17 Calculating MA MA = resistance force/effort force MA = Fr/Fe
MA has no units. Why?

18 Efficiency A measure of how much of the work put into a machine is converted into useful output work. Why is work output always less than work input? How can you  efficiency?

19 Calculating Efficiency
Efficiency = (Wout/Win) X 100%

20 Simple Machines A machine that does work with only one movement.
Six Types: 1)lever, 2)pulley, 3)wheel and axle, 4) inclined plane, 5) screw, and 6) wedge.

21 Lever A bar that is free to pivot about a fixed point or fulcrum. Effort arm = distance from fulcrum where effort force is applied. Resistance arm = distance from fulcrum where resistance force is applied

22 Types of Levers Based on positions of effort force, resistance force, and fulcrum.

23 Types of Levers 1st class – fulcrum is in the middle (crowbar)
2nd class – resistance is in the middle (wheelbarrow) 3rd class – effort is in the middle (baseball bat)

24 IMA of Levers IMA = length of effort arm/length of resistance arm
IMA = Le/Lr

25 Pulleys A grooved wheel with a rope, chain or cable running along the groove. How is a pulley a modified 1st class lever?

26 Fixed Pulleys Attached to something that does not move
Only change the direction of the force IMA = 1

27 Moveable Pulleys One end of the rope is fixed and the wheel is free to move. Multiplies effort force IMA = 2

28 Block and Tackle A system of pulleys made of fixed and moveable pulleys IMA = # of supporting rope segments. Only count effort segment if effort force is in the direction of the object’s movement.

29 Wheel and Axle Machine consisting of two wheels of different sizes that rotate together. Modified form of a lever. IMA = rw/ra

30 Gears Modified Wheel and Axle with two wheels of different sizes with interlocking teeth. Large wheel = effort gear, small wheel = resistance gear. Larger effort gear = more turns of resistance gear =  effort force

31 Inclined plane Sloping surface that reduces the amount of force required to do work. Increases distance IMA = length/height

32 Screw Inclined plane wrapped, in a spiral, around a cylinder.

33 Wedge Moving inclined plane with one or two sloping sides.
Changes the direction of effort force

34 Compound Machines When two or more simple machines are used together.

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