1. Chapter 5 – Work, Power, & Machines
“Give me a lever and a place to stand and I will move the earth.” Archimedes

2. WORK
Work: Transfer of Energy that occurs when a force makes an object move.
NOT every force results in work
The direction of the force and the motion of the object must be taken into account
2 conditions for work to happen
Object must move
Force and movement are in
the same direction
Equation for work:
W = F × d
Work (J) = Force (N) × Displacement (m) × cosө
F = ma

3. Examples of work a soccer player kicking a goal
a pitcher throwing in the first pitch
a student lifting a backpack full of books upon her shoulder
a father picking up a child off the floor
a track star launching the shot-put
The force exerted on the object and its motion are in the same direction.

4. Is this really work? Draw force vector & motion vector to verify answer.
A body builder lifts 350 pounds
above his head.
A mother carries her baby from
room to room.
Busy mom pushes a grocery cart.
A woman carries a 20 kg grocery
bag to her car
A man pushes on a truck and it doesn’t move
Yes No

5. Work Example Problems
A crate is moved across the floor by a rope. The tension in the rope is 50 N. How much work is done in moving the crate 10 m?
A 3,000-kg car accelerates at 2.4 m/s2. How much work is done over 500m? (calculate force first)

7. Power Power: the rate at which work is done.
P = W SI unit for Power: Watt (W) T
Power (W) = Work (J)
Time (sec)
work and power
Symbol W = Work
Ex. W = 5J
Unit W = Watt
Ex. P = 5W

8. Example Problem: Work & Power
2 soccer players work out in the off season by sprinting up a 100-m hill before stopping, resting, and walking back down. Mia and Julia weigh the same, 588N. It takes Mia 42 sec and Julie 38 sec to reach the top of the hill.
Which athlete did more work?
Which athlete produced more power?

10. Bill Nye – Simple Machines
link works