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Work and Machines Chapter 3

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**Lesson 1- Work and Power What is work?**

any time a force is exerted on an object that causes the object to move the object must move in the same direction in which the force is exerted. No movement of object = no work done no matter how much force is used

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**Different directions = NO WORK...**

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Some work is done.

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Most work is done.

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**How do you calculate work?**

Work = Force X Distance Work = (N) X (m) Work = _____ N.m Therefore the SI unit for Work is called a JOULE. (1 J = force of 1N to move an object a distance of 1 meter)

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**90N X 30 m = 2700 J (force X Distance)**

180 N X 60 m = J ( 4 times more)

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**What is Work Demonstration… Worksheet 71 with discussion**

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**What is Power? the rate at which work is done**

Power = Work Force X Distance Time Time Unit for Power is watt 1J/s = 1 W

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**Pinwheel Demo… (15 seconds)**

Is work done on the pinwheel? Explain. How are the two situations different? Which situation involved more power? Explain.

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**Investigating Power: pg 72**

You and your partner will be completing this investigation. Use your time wisely!

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**Understanding Machines**

Lesson 2 Understanding Machines

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**Input force This is the force you exert.**

You exert this force over a specific distance. (input work)

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**Output Force This is the force the machine exerts**

The machine also moves a specific distance (output work)

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**Output distance and force**

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**What does a machine do? Makes work easier by**

changing the amount of force your exert OR changing the distance over which you exert your force OR changing the direction in which you exert your force

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**Machines don’t change the amount of work you do, but they do change the way you do the work.**

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Changing Force If a machine allows you to use less input force, you must apply that force over a greater distance.

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**Changing Force Work = Force X Distance Let’s Say: My force is 5 N**

and the distance is 2 m; 5 N x 2 m = 10 J of work If the amount of work stay the same, a decrease of force means an increase of distance. If I put 2N of force, then my distance would be 5m to equal 10 J of work.

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Changing Distance If a machine allows you to move your input force over a shorter distance, than you need to apply a greater input force.

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Changing Distance Large input force Large output distance

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Change Direction Some machines don’t change in either force or distance, they change the direction of the force. Small input force large input distance large output distance Small output force

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Mechanical Advantage The number of times a machine increases a force exerted on it. The ratio of output force to input force Mechanical Advantage = output force input force

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**What happens when you increase Force?**

When the output force is greater than the input force - mechanical advantage > 1 YES 15N / 10N = 1.5 Greater

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**What happens when distance increases?**

When a machine increases distance, the output force (machine’s force) is less than the input force (your force). Mechanical advantage is < 1

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**What happens when direction changes?**

Input force = output force Mechanical advantage is 1. The larger the mechanical advantage, the easier a machine makes your work.

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What is efficiency? The machine’s (output) work is always less than your (input) work because the machine has to overcome the force of friction. The less work a machine has to do to overcome friction, the more efficient it is.

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**To calculate efficiency...**

Output work X 100% Input work

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**Inclined Planes and Levers**

Lesson 3 Inclined Planes and Levers

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**Simple Machines: Inclined Planes - Flat sloped surface**

Wedge - device that is thick on one end and tapers to a thin edge at the other Screw - be thought of as an inclined plane wrapped around a cylinder.

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**Levers: Rigid bar that is free to pivot, or rotate on a fixed point.**

That fixed point is called a fulcrum Types of levers are classified according to the location of the fulcrum relative to the input and output forces.

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**Types of Levers: 1st Class**

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**Types of Levers: 2nd Class**

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**Types of Levers: 3rd Class**

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**More Examples of Levers:**

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**Putting Machines Together**

Lesson 4 Putting Machines Together

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PULLEY Systems Simple Machine made of a grooved wheel with a cable or rope wrapped around it.

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**Types of Pulleys: Fixed**

Changes the direction of the force but not the amount applied:

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**Types of Pulleys: Movable**

Decreases the amount of input force needed. It doesn’t change the direction of the force.

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**Type of Pulleys: Block and Tackle**

A pulley system make up of fixed and movable pulleys.

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Wheel and Axle Two connected objects that rotate about a common axis.

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