1. Task (1) What is gravitational potential energy, Eg?
(2) How do we calculate Eg?
(3) Estimate your own gravitational potential energy relative to the lowest floor in the school when you are standing at the top of the stairs of the highest floor.

2. SPH 4C – Energy Transformations Miss Miner
POWER
SPH 4C – Energy Transformations
Miss Miner

3. Problem
Two monkeys of equal mass climb identical rope ladders 12 m in length, one in 4.0 s and the other in 6.0s. How does the work done by each monkey against gravity compare?
(Recall: the work done equals the gravitational potential energy at the top relative to the bottom, Eg=mgh.)

4. The monkey who does the work in a shorter time generates more power!
Problem Continued…
The work done by both monkeys is the same!
But if the times were different, there must be some other factor to explain the difference?
POWER!
The monkey who does the work in a shorter time generates more power!

5. Power
Definition: Power (P) is the rate of doing work or transforming energy.
power = work or power = energy transferred
time interval time interval
Symbolically,
P = W or P = Δ E
Δ t Δ t

6. What unit do power we measure in?
P = W work and energy are measured in joules (J)
Δ t
time is measured in seconds (s)
P = Δ E
Therefore power is measured in joules per second (J / s).

7. SI Unit watt (W): 1 W = 1 (J / s)
Named after Scottish physicist and inventor, James Watt ( )
He designed and built the first practical steam engine
He compared the power of his steam engine to the rate at which a horse could pull coal out of a mine
The imperial unit “Horsepower” (hp) is the average power output of a working horse
1 hp = 746 W

8. Example 1
A speedboat transforms 2.25 x 105 J of energy in 5.00 s. What is the speedboat’s power? Express the answer in watts and kilowatts.

9. Example 2
A 52- kg student climbs 3.0 m up a ladder in 4.7 s. Calculate
(a) the student’s gravitational potential energy at the top of the ladder relative to the bottom
(b) the student’s power for the climb

10. Example 3
A 1.0 x 103 kg elevator lifts a 85 kg man and a 68 kg woman from the ground floor of a high-rise building upwards at a constant speed of 3.0 m/s for 6.4 s. Calculate
(a) the gravitational potential energy of the elevator
at 6.4 s.
(b) the power needed to lift the elevator

11. Lab Activity
You will calculate the work and power output of a machine… you!

12. Homework Complete Pre-Lab for tomorrow’s Work and Power Lab:
Calculate your mass in kg (2.2 lb = 1 kg)
Find all necessary constants and formulas you will need for calculations
Bring running shoes to tomorrow’s class
Section 4.1, page 177
# 1, 2, 3, 4
(Nelson, Physics 12 College Preparation)