Work, Power, and Machines

Last night, you read a chapter for your language arts homework. Did you do work??

Not really! According to science, you only do work when: A force is applied to an object That object moves in the same direction as the force applied Note: You did do work when you turned the pages in the book, but your overall process of reading was not work.

What is Work? When you apply a force and the object moves in the same direction as the force, you do work. Ex: You push a cart and it moves forward. Reading a book is not doing work because you are not applying a force on the book or making it move. When you transfer energy to something, you do work. Ex: Someone bowling is doing work because they have applied a push force on the bowling ball which makes it move forward. The bowler transferred energy to the ball, causing it to start moving.

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Don’t get tricked! In order to do work on an object, it has to move in the same direction as the force you apply… When you pick up books off the top of a desk, you do work: Force applied? YES Object moves? YES Direction of force: UP Direction of motion: UP

Don’t get tricked! In order to do work on an object, it has to move in the same direction as the force you apply… When you walk through the hallway and carry your books, you do not do any work on the books: Force applied? YES Object moves? YES (with you) Direction of force: UP Direction of motion: FORWARD The direction of the force and distance traveled are not the same, so no work is done.

Is It Work? Situation Work? (Yes or No) Carrying a bag of groceries Picking up a bag of groceries Pushing in your chair Sitting in your chair Pulling a wagon Pushing against a brick wall

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Same Work, Different Forces Rock climb straight up the mountain Shorter distance Greater force required to pull yourself straight up Hike along winding slope to the top Greater distance Less force required to make it to the top Same amount of work done!

Calculating Work Imagine you are lifting weights. You are able to lift a186 Newton bar a distance of 2 meters. How much work do you do? Remember that work occurs when an object moves in the direction of the force applied, so… Work = Force x Distance or W = F x d Units: Newtons x meters so N x m = Joules (J) Unit for work: Joules (pronounced like jewels)

Calculating Work Imagine you are lifting weights. You are able to lift a186 Newton bar a distance of 2 meters. How much work do you do? CLUES FORMULA SUBSTITUTE ANSWER You do 372 J (joules) of work.

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Power! What is more powerful…you knocking down a house with a hammer, or a bulldozer knocking down a house? The bulldozer, because it applies more force and does more work in less time! Anything that does the same amount of work in less time has more power! Anything that does more work in the same amount of time has more power!

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What is Power? Power is defined as the rate at which energy is transferred or work is done. In simple terms, this means the amount of time it takes to do work. If you do the work in less time, you are more powerful and have a high power output. If you do the work in more time, you are less powerful and have a low power output.

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Calculating Power Because power is the amount of work you do in a certain amount of time, it is calculated with the formula: Power = work divided by time P = W/t The unit of power is: Joules/second, represented as watts (w)

Calculating Power A light bulb is on for 12 seconds and uses 1,200 J of electrical energy during that time. What is the wattage (power) of the light bulb? CLUES FORMULA SUBSTITUTE ANSWER

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How can work get easier? We use machines to make work easier. A machine is a device that makes work easier. This is because they change the size or direction of the force you need to apply in order to do the work. cutting a steak with a sharp knife or tearing it by hand bulldozer vs. breaking apart by hand snow blower vs. shoveling electric mixer vs. mixing cookie dough by hand

Types of Simple Machines Lever Inclined Plane Wedge Screw Pulley Wheel and axle Description A bar that pivots at a fixed point A straight, slanted surface Consists of two inclined planes An inclined plane wrapped in a spiral around a cylinder A grooved wheel that holds a rope or chain Consists of two circular objects of different sizes Purpose Raise an object or load Raise a heavy object Used for cutting or splitting Used to fasten things together Used to lift something Used to turn or open/close an object Examples Bottle opener, hammer Ramp Knife, ax Flagpole hoist, open/close blinds Door knob, steering wheel, wrench

Lever Wheel and Axle Pulley Wedge Inclined Plane Screw

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Compound Machines Also known as complex machines Made of two or more simple machines Examples: bicycle, can opener, pencil sharpener, lawn mower, wheel barrow

Rube Goldberg was an inventor who created unique devices made out of many different types of simple machines.

How many machines can you identify in the following video?

Now, take a Work and Power calculation sheet and practice your math skills! ~Be sure to show your work using the 4 steps. ~Be careful with your new units and formulas! ~Complete both sides of the sheet.