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Chapter 5 Work and Machines

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

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**Work – Two Conditions 1. The object must move.**

2. The motion must be in the same direction as the applied force.

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**Is Work Done? Picking up a stack of books. Holding a stack of books.**

Walking while holding a stack of books.

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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

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

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**Power Is the rate at which work is done.**

Amount of work done in a certain amount of time.

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**Calculating Power Power = Work/time P = W/t W = J t = sec**

P = J/sec = Watts (W)

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**Power and Energy Energy can be transferred without involving work.**

Ex. Light bulb transfers energy into heat and light. P = E/t

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**Machines A device that makes doing work easier. Work by:**

Increasing F (car jack) Increasing d (ramp) Changing direction (ax)

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**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.

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**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.

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**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.

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

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**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?

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**Mechanical Advantage (MA)**

It is the number of times a machine multiplies the effort force. IMA vs AMA

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**Calculating MA MA = resistance force/effort force MA = Fr/Fe**

MA has no units. Why?

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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?

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**Calculating Efficiency**

Efficiency = (Wout/Win) X 100%

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**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.

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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

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

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**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)

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**IMA of Levers IMA = length of effort arm/length of resistance arm**

IMA = Le/Lr

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

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**Fixed Pulleys Attached to something that does not move**

Only change the direction of the force IMA = 1

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

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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.

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

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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

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

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

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**Wedge Moving inclined plane with one or two sloping sides.**

Changes the direction of effort force

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

Examples?

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