1. Work , Power and Efficiency
April 13th 2011

2. Power
Power: is the rate of doing work. It is the rate at which energy is transferred. Power is the work done divided by the time it takes.
Formula: P = W/∆t
∆t = the time interval in which the work has occurred. (s)
Power is measured in watts (W) with
1W = 1 J/s
Power is very often measured in kilowatts (kW),
1kW = 1000W

3. Power cont. Average racing lawn mower ~ 14 hp 14 hp = 10,500 Watts
For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750 Watts.
Something to think about…. Average lawn mower has 6 hp.
So, if 1 hp = 750 Watts,
6 hp = 4,500 Watts
Average racing lawn mower ~ 14 hp
14 hp = 10,500 Watts

4. Example.
An electric motor lifts an elevator that weighs 1.20x104N a distance of 9.00m in 15.0s.
a) What is the power of the motor in watts?
b) What is the power in kilowatts?
Given: F = 1.20x104N Unknown: power, P
d = 9.00m Equation: P = W/t
t = 15.0s

5. Solution

6. Example 2.
Two Montreal Canadien fans, Mike and Jon are working out during the intermissions. Mike lifts the 100-kg barbell over his head in 5.0s; Jon lifts the same barbell over his head in 2.9 s. (They both lift it a height of 2.3m)Which student does the most work? Which student delivers the most power? Explain your answer.

7. Solution.

8. Efficiency
Energy can be, and is, converted into forms that do no work or do not serve the intended purpose. Transforming energy involves some “loss” of useful energy. (think of driving in your car)
Efficiency is the ratio of useful energy or work output to the total energy or work input.

9. Efficiency Efficiency = Woutput X 100% Winput OR
Efficiency = Eoutput X 100%
Einput

10. Example.
A 100 W light bulb produces 90000J of light energy in 30 minutes. What is the efficiency of the light bulb?

11. Homework.
Pg. 266 #’s 41, 42, 43
Pg. 270 #’s 44, 46, 47, 48