1. Work done = Energy Changed
Whenever work is done, energy is transformed / changed.
Energy in one form
WORK is done
Energy in another form

2. Work
Work
Work is the force necessary to move an object a distance.
Not Work

3. Work = Force x distance moved
Work is measured in newton-meters. A special name is given to this unit:
Joule (J) = 1 Newton-meter

4. “Working” out
A man benches presses 245 kg. The distance from his chest to the top of the lift is 0.75 m.
Find the work done by the man for one lift.
Distance (d) = .75 m
weight lifted = mg = 245kg x 10m/s2
W = Fd
= (2450N x .75m) x 2
= 3675 J mg d
.5FL
.5FL

5. How much work is necessary to lift 10 kg 5m in the air?
50 J
500 J
5000 J

6. Power Power is the rate at which work is done P = average power
= work/time
= W/t
Unit: Watt(w) = Joule/sec (J/s)

7. Power Example: Running Stairs
A 70 kg student runs up a flight of stairs in 4.0s.
The vertical height of the stairs is 4.5 m.
Estimate the student’s power output in watts
Know: mass = 70 kg; time = 4.0 s; d = 4.5 m
The work is done against gravity:
W= Force x distance; where force = mg
And distance equals vertical distance d
Work = (mass) x (gravity) x (distance)
W = (70 kg)(10m/s2)(4.5m)
W = 3150 J
P = Work/time
P = 3150 J/ 4.0 s
P = W