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Work and Machines

Section 1: Work. Work and Machines. Learning Goals. Explain the meaning of work. Describe how work and energy are related. Calculate work. Calculate power. Warm-up:.

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Work and Machines

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  1. Section 1: Work Work and Machines

  2. Learning Goals • Explain the meaning of work. • Describe how work and energy are related. • Calculate work. • Calculate power.

  3. Warm-up: Backpacking is a lot of fun, but it also can be a lot of work. Whether hoisting the pack onto your back to start the hike or trudging up a long hill, you’ll need to exert a good deal of effort to get to the next camp. Compare the effort exerted by a backpacker moving over level ground to that exerted by a backpacker moving uphill. How do you think the weight of the backpack affects the amount of force needed to move it?

  4. Work • Work: transfer of energy that occurs when a force makes an object move

  5. Work • For work to occur, two conditions must be met: • an object must move • the motion of the object must be in the same direction as the applied force on the object.

  6. Work and Energy • Work and energy are related. • Energy is always transferred from the object doing the work to the object on which the work is being done.

  7. Work and Energy

  8. Work and Energy • Work is done on an object only when a force is being applied to the object and the object moves. • If the object does not move, then no work is being done!

  9. Calculating Work • Work equals force (in newtons) times distance • W = Fd • W  work, measured in joules (J) • F  force, measured in newtons (N) • d  distance, measured in meters (m)

  10. Calculating Work • You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work are you doing?

  11. Calculating Work • F = W/d • d = W/f

  12. Calculating Work • A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed?

  13. Calculating Work • The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the force the brakes exert on the car?

  14. Power • Power: amount of work done in a certain amount of time • rate at which work is done

  15. Calculating Power • Power equals work divided by time. • P = W/t • P  power, measured in watts (W) • W  work, measured in joules (J) • t  time, measured in seconds (s)

  16. Calculating Power • You do 900 J of work in pushing a sofa. If it took 5 s to move the sofa, how much power did you use?

  17. Calculating Power • W = Pt • t = PW

  18. Calculating Power • If a runner’s power is 130 W as she runs, how much work is done by the runner in 10 minutes?

  19. Calculating Power • The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

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