1. Aim: How do we define power?

2. Definitions POWER Units Amount of WORK DONE PER UNIT TIME
Joule/second (J/s)
-OR-
Watts (W)

3. Power
Mr. Rao’s mass is 65 kg and he jogs up some stairs that have a height of 6 meters in 5 seconds. What is his power?
What if a rabbit made the climb in the same time? How does the rabbits power compare?
It’s less since he’s
less massive

4. How does the jeep make it up the hill?
Jeep Climbing
How does the jeep make it up the hill?
What was another way for the jeep to reach the top of the hill?
How is this way different?

5. A team of dogs pulls a sled with an average force of 4000 Newtons across a distance of 300 meters in 150 seconds. How much power do the dogs produce?

6. Elevator Motor A motor pulls an 8000 Newton elevator upward at an
average speed of 3.0 meters per second.
How much power is used by the motor?
How can the demand for power from the motor be
reduced?
Pull the cars up at a slower speed or
Use lighter elevator cars.

7. Stations Activity: 2 Stations
Remember all your energy equations and how they can best be applied!
Remember energy must be conserved!

8. A box of 5 kg is accelerated across the floor at a rate of 2
A box of 5 kg is accelerated across the floor at a rate of 2.0 m/s2 for 7.0 seconds. What is the net work done on the box? How does this comply with the energy theorem?
WNC = ΔKE + ΔPE = FFd
ΔPE = 0 J
Vf = Vi + at Vf = 14 m/s
KEf = (1/2)mv2 = 490 J
WNC = (10N)(49m) = 490J

9. How high will a 1.85 kg rock go, if thrown straight up by someone who does 80.0 J of work on it? Neglect air resistance. Explain how energy is conserved. The rock must have 80 J of work done on it to reach a maximum height and the force of gravity must do J to stop it. Since force is down and displacement is up, the angle between them is 180o J = FGd = mgdCos(180o) d = 4.41 m

10. What is the minimum work needed to push a 950 kg car 810 m up along a 9o incline?
Assume the coefficient of friction retarding the car to be 0.25.
Assume the car is not accelerated.
Draw a FBD
Set up Newton’s 2nd Law in x and y direction.
Sum Fx = FP – Ff – mgSinθ = 0
Multiply each term by d for Work… W = 3x106 J