1. Chapter 5 Power 1

2. Power is the rate at which work is done. More generally, power is the rate of energy transferred by any method. 2

3. Power If the work done on an object is W in a time interval Δt, then the power is the energy delivered to the object over the time interval. 3

4. Power P = W Δt The SI unit of power is watt, W or J/s 4

5. Power In terms of the definition of W(work) the equation is: P =F d Δt or P = Fv Force x velocity 5

6. Power The horsepower, hp, is another unit of power that is sometimes used. 1 hp = 746 watts 6

7. Power The watt is perhaps the most familiar to you from your everyday experience with light bulbs. Electricity used in your homes is measured in kilowatts (kW). 7

8. Power Machines with different power ratings do the same work in different time intervals. So the more powerful motor could do the work in a shorter time interval. 8

9. Power A 193 kg curtain needs to be raised 7.5 m, at a constant speed, in as close to 5.0 s, as possible. The power ratings for 3 motors are 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job? 9

10. Power In order to solve, we need to list our knowns: m = 193 kg ∆t = 5.0 s d = 7.5 m 10

11. Power So we have the following formula: P =F d Δt we know that F = mg so we can insert this in the above formula P = (mg)d Δt 11

12. Power Now we can plug-in our knowns in the formula: P = (193kg)(9.8m/s 2 )(7.5m) 5.0 s 12

13. Power Answer: 2,837 W *We need to convert since the power ratings are in kilowatts. 13

14. Power We use dimensional analysis to convert: 2837 W x 1 kW = 2.8 kW 1000W 14

15. Power So based on this, the best motor for the job would be the one with the power rating of 3.5 kW 15