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Work and Machines

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What is Work? Work is force times distance. To be exact, work is force times the distance moved in the direction of the force. The unit of measurement for work is the joule. One joule is equal to one newton of force times one meter of distance.

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Work When you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books.

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Work (2) When you carry books while walking, you might think that your arms are doing work. However, in this case, the force exerted by your arms does no work on the books. The force exerted by your arms on the books is upward, but the books are moving horizontally.

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Doing Work Energy is always transferred from the object that is doing the work to the object on which the work is done. He is transferring energy from his moving muscles to the box and is increasing its potential energy by increasing its height.

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**Would each of the following be considered work? Why or why not?**

A teacher applies a force to a wall and becomes exhausted. A book falls off a table and free falls to the ground. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. A rocket accelerates through space.

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Work A rock weighing 2 newtons is lifted 3 meters. How much work was done? W = F x D W= F= D=

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Work A force of 30 newtons is used to push a box along the floor a distance of 2 meters. How much work was done? W = F x D W= F= D=

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Work A force of 200 newtons was necessary to lift a rock. A total of 1000 joules of work was done. How far was the rock lifted? W = F x D W= F= D=

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Power The rate at which work is done is called power. The power of a machine is calculated by dividing the amount of work done by the time it takes in seconds.

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Power (2) The unit of power is called the watt, named after James Watts, the inventor of the steam engine. One watt is equal to one joule of work done in one second. Another unit commonly used is horsepower. One horsepower is equal to 746 watts.

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Calculating Power Suppose you drag a box with a force of 100 newtons for 10 meters, and it takes you 10 seconds. You have done 1,000 joules of work (100 N x 10 m). Suppose your friend drags a similar box but takes 60 seconds. You and your friend did the same amount of work but you used six times more power because you did the work six times faster.

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Power = Work/ Time A set of pulleys is used to lift a large piece of furniture weighing 10,000 newtons. The furniture is lifted 4 meters in 60 seconds. How much power is used? P = W = f x d T =

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Power = work/time How much work is done using a 40-watt light bulb for 1 hour? P = W = T =

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Efficiency Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine.

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Efficiency (2) In an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent. The efficiency of a real machine is always less than 100 percent (oil or grease can be added to make it more efficient).

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Efficiency A man expends 200 joules of work to move a box up an inclined plane. The amount of work produced is 160 joules. What is the efficiency of this machine? W in= W out= Eff =

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Efficiency A girl pushes a lever down 3 meters with a force of 80 newtons. The box at the other end with a weight of 50 newtons moves up 2 meters. What is the efficiency? W in= f x d = 80n x 3m W out= f x d = 50n x 2m Eff = W in/ W out = 100/240(100%) = 42%

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Efficiency Using a lever, a person applies 30 newtons of force and moves the lever 0.5 meters. This moves a 100 newton rock at the other end by 0.1 meters. What is the Work in and work out? What is the efficiency? Win=F x D = 30 x .5 =15 Wout=F x D = 100 x .1 =10 Eff= 10/15 (100%)= 67%

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**Using 3 to 5 sentences, answer the following:**

A small girl is pushing a rather large box across the room. The box is moving rather slowly but is indeed moving in the direction the girl is pushing. Is there any work occurring? If so, how do you know? If not, why not?

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Machines A machine is a device with moving parts that work together to accomplish a task. Machines can be simple. Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier.

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Machines and Forces Machines are best understood through the concepts of input and output forces. The input force is the force applied to the machine. The output force is the force the machine applies to accomplish a task. Rule for all machines: You can never get more work out of a machine than you put into it.

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**Machines help us do work by**

Changing direction of force Multiplying the force put in Increasing the distance

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Mechanical Advantage Mechanical advantage tells how much a machine multiplies force or increases distance. It is defined as the ratio between the output force and the input force. THERE is no unit for MA.

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**Mechanical Advantage (2)**

A machine with a MA of greater than 1 multiplies the input force. Such a machine can help you move or lift heavy objects. A machine with a MA of less than 1 increases distance and speed.

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Simple Machines A simple machine is a machine that does work with only one movement of the machine. There are six types of simple machines: lever, pulley, wheel and axle, inclined plane, screw and wedge.

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1 Lever A lever is a bar that is free to pivot or turn around a fixed point. The fixed point the lever pivots on is called the fulcrum. The input arm of the lever is the distance from the fulcrum to the point where the input force is applied. The output arm is the distance from the fulcrum to the point where the output force is exerted by the lever.

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**Lever (2) First Class Lever (Screwdriver opening a paint can)**

Second Class Lever (A wheelbarrow where the fulcrum is the wheel) Third Class Lever (a bat where the fulcrum is the batter’s hand)

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MA of a Lever

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2 Pulley A pulley is a grooved wheel with a rope, chain, or cable running along the groove. A fixed pulley is a modified first-class lever. The axle of the pulley acts as the fulcrum.

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Pulley (2) A pulley in which one end of the rope is fixed and the wheel is free to move is called a movable pulley. A system of pulleys consisting of fixed and movable pulleys is called a block and tackle.

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3 Wheel and Axle A wheel and axle is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together.

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MA of the Wheel and Axle

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Inclined Plane A sloping surface, such as a ramp that reduces the amount of force required to do work, is an inclined plane.

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MA of an Inclined Plane

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Screw A screw is an inclined plane wrapped in a spiral around a cylindrical post. You apply the input force by turning the screw. The output force is exerted along the threads of the screw.

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Wedge A wedge is an inclined plane with one or two sloping sides. It changes the direction of the input force.

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Compound Machines Two or more simple machines that operate together form a compound machine.

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1. Mechanical Advantage What is the MA of the ramp below? How much force is needed to slide the 1200 N object up the ramp? 36 cm 1200 N 6 cm

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2. Mechanical Advantage What is the mechanical advantage of the lever below? What force is needed to lift the 10 N object at the end of the lever? 4 m 2 m 10 N

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**3. Which lever below has the greatest Mechanical Advantage?**

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**4. The mechanical advantage of the ramp below is 5**

4. The mechanical advantage of the ramp below is 5. What is the height of the ramp? 10 m ? m

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**5. What is the force needed to move the block up the ramp?**

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**6.What is the MA of the lever below? Why is this not a useful machine?**

Force needed to lift the object would be 10,000 N (1000 / 0.1) 1 m 1,000N 10 m

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Using 5-10 sentences, describe the relationship between the words listed below. You may use your notes Machines Mechanical Advantage Work Force Distance Direction

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**Create a concept map to organize what you know using the following terms:**

Types of Energy Law of Conservation of Energy Machines Mechanical Advantage Work Efficiency

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