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Work and Power

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What is work?

- Definition: energy transferred by a _____________ to move an object a _____________ (when both the force and the motion of the object are in the same direction.)

Work or Not?

- According to the scientific definition, what is work and what is not?
- a teacher lecturing to her class
- a mouse pushing a piece of cheese with its nose across the floor

Work or Not?

- According to the scientific definition, what is work and what is not?
- a teacher lecturing to her class
- a mouse pushing a piece of cheese with its nose across the floor

Work or Not

- Why?

What’s work?

- A scientist delivers a speech to an audience of his peers.
- A body builder lifts 350 pounds above his head.
- A mother carries her baby from room to room.
- A father pushes a baby in a carriage.
- A woman carries a 20 kg grocery bag to her car?

What’s work?

- A scientist delivers a speech to an audience of his peers. No
- A body builder lifts 350 pounds above his head. Yes
- A mother carries her baby from room to room. No
- A father pushes a baby in a carriage. Yes
- A woman carries a 20 km grocery bag to her car? No

Formula for work

Work = _________ x Distance

- The unit of work is newton x meters
- One ____________ is equal to one ______
- So, the unit of work is a __________
- J= 1 N x m = 1 kg x m2/s2

W=FD

Work = Force x Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

W=FD

Work = Force x Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules

(W = 20N x 10m)

Power

- Power is the __________ at which work is done.
- Power = Work*/Time
or P= W/t

- The unit of power is the watt.
- 1 watt= ______________

Work vs. Power

- Work vs. Power

Concept Check

- 1. Describe the difference between work and power?
- 2. How is work being done in the following situation
- Powerlifting
- Dropping your textbook
- Doing a handstand
- What is the unit for lightbulbs? Why is that unit used?

Math Practice

- 1. A crane use an average force of 5300 N to lift a girder 25 m. How much work does the crane do on the girder?
- 2. Aylssa powerlifts Kyle over her head 1 m with a force of 250 N. How much work was done?
- 3. While rowing in a race, John 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?

Math Practice

- 4. While rowing in a race, John does 3960 J of work on the oars in 60.0 s. What is his output in watts.
- 5. Steve using a jack does 4320 J of work to lift a car 1.5 m in 100.0 s. What is the power output?
- 6. Mr. Farrell walks up the stairs on his way to make copies. He weight 622 Newtonsand the stairs go 5.75 vertically.
a. Calculate my output if I climb the stairs in 8.9s.

b. What is I climb the stairs in 7.2 s?