Work, Power, Simple MachinesWork, Power, Simple Machines

Work - equationPower - equation--> practice problemsMachines

Work - equationPower - equation--> practice problemsMachines

When a force is exerted on an object, energy is transferred to the object.

This “energy” is called the work done on the object.

BUT…energy is only transferred if the object moves!

It DOES NOT require more work to walk up a flight of stairs than to run up them = the work is the same.

Equation---->

WORK = force X distance

Units----> N x m = 1 J

WORK is measured in joules (J).

Examples of these units

One apple = 1 N

*To raise the apple 1 meter, you do about 1 J of work.

*Doing 3 push-ups requires around 1000 J of work.

Back to the stairs…so what is the difference between walking and running up?

How long it takes you, the TIME!

The amount of TIME it takes to get work done is important.

-> How much work is done in a certain amount of time

Power = work or P = W

time t

Power is measured in SI units called wattswatts (W).

1 watt is the power required to do 1 J of work in about 1 second.

Practice Problems - WORK

1. A crane used an average force of 5200 N to lift a beam 25 m. How much work does the crane do on the beam?

2. An apple weighing 1 N falls through a distance of 1m. How much work is done on the apple by the force of gravity?

3. The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0 m. How much work have the brakes done on the bicycle?

5. While rowing in a race, Jimmy uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes?

6. While rowing in a race, Billy does 3960 J of work on the oars in 60.0 s. What is his power output in watts?

Simple Machines

• Lever family

• Inclined plane family

All machines start with a simple machine - ➠ Modify ➠ Combine

Levers

☞ move a load around a fulcrum (pivot) using a force

Three classes of levers:

*Fulcrum in the middle

*Input force acts on one end

*Other end applies an output

--> Fulcrum is b/w the effort and the load

*Fulcrum at one end

*Input force applied at the other end

-> Load is b/w the fulcrum and the effort

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*Multiply distance, not force

--> Effort is b/w the fulcrum and the load

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Modified Levers

Pulleys

Wheel & Axle

*multiply and redirect force*stationary

*Surface set at an angle, against a horizontal surface

*Allows you to overcome a large resistance by applying a small force through a longer distance

*Modified inclined plane

*does work by moving

*single downward force = two forces directed out the sides

*Functions like 2 inclined planes back to back

*A separating, holding, and stopping device with countless applications

www.mikids.com/Smachines.htm

• More pictures of simple machines

• More definitions & explanations

• Quizzes