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Work and Power. Presented by: Ms. Quarles. Did you do work?.

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Did you do work?

- After a heavy snow storm, a neighbor’s car gets stuck in the snow. You shovel some snow away from the car and try to push it. Although you try to push as hard as you can, the car just won’t budge. This sure is hard work! You exerted a lot of force, but the car never moved. Was work done on the car?

You did some work when you shoveled the snow, but you might be surprised to discover that in scienctific terms you didn’t do any work at all on the car!

- In science you do work on an object when you exert a force on the object that causes the object to move some distance.
- No work without motion!

Calculating Work

- Which do you think involves more work: (exerting a force of 100N to lift a potted tree a meter off the ground) or (exerting a force of 200N to lift a heavier tree the same height)?
- Is it more work to lift (a bag of sand from the ground to a wheelbarrow) or (from the ground to the top of a building)?

Circle the one you think is right

Your common sense may suggest that lifting a heavier object, which demands a greater force, requires more work than lifting a lighter object. Also, moving an object a greater distance requires more work than moving an object a shorter distance. Both of these are true.

Work= Force x Distance

- The amount of work done on an object can be determined by multiplying force times distance.
- Example: To help rearrange the furniture in your classroom, you exert a force of 20N to push a desk 10m. How much work did you do?

Sample Problem

- You know the force exerted on the desk (20N) and the distance the desk moved (10m). Draw a diagram to help you understand the problem.
- Write the formula, substitute, and solve.
- W =F x D W = 20 x 10
- The work you do on the desk is 200J.

Joule

- When force is measured in newtons and distance is measured in meters, the SI unit of work is newton x meter (N x m). This unit is also called a joule (jool).
- The unit for work got its name from the physicist James Prescott Joule, who studied work in the middle 1800s.

What is one joule?

- One joule (j) is the amount of work you do when you exert a force of 1 newton to move an object a distance of 1 meter.

Power – How fast work is done

- Like work, the term power is used a lot in everyday language. Power is the rate at which work is done.

Machines and Mechanical Advantage

- When you think of machines, you may think of complex gadgets that run on electricity, but a machine can be as simple as a shovel or even a ramp.
- A machine is a device with which you can do work in a way that is easier or more effective.

Machines

- A machine makes work easier by changing the amount of force you exert, the distance over which you exert your force, or the direction in which you exert your force.

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