Power is equal to the work done divided

Power : is equal to the work done, divided by the time taken to do the work P = W/t Power is the rate of which work is done.

Power = W/t (W = Fd) So Power = Fd/t ( v = d/t) So Power = Fv P =Fv

Units to measure Power is measured in Watts (W) 1 Watt is 1 J of energy transferred in 1 s. A watt of power is relatively small. Power is usually measured in kilowatts ( 1000 W) In English system power is measured in horse power (HP) 1 HP = 746 W

Sample Problem 1 (P = W/t) A box that weighs 575 N is lifted a distance of 20. 0 m. If the job is done in 10 s, what is the power developed by the motor? (W = Fd) W = 575 N x 20 m = 11500 J P = 11500 J/10 s = 1150 watts

Sample Problem 2 You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force: a. What power do you develop? b. If you moved the turkey twice as fast, how much power is developed?

Sample Problem 2 You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force: a. What power do you develop? W = 145 N x 60 m = 8700 J P = W/t = 8700 J/25 s = 348 W

Sample Problem 2 You push a giant turkey a distance of 60 m at a constant speed for 25 s. If you exert 145 N of force: b. If you moved the turkey twice as fast, how much power is developed? Time would be 12. 5 s, twice as fast, half the time P = 8700 J / 12. 5 s = 696 W ( twice as much power)

Sample Problem 3 An electric motor develops 65 k. W of power as it lifts an elevator 17. 5 m in 35 s. How much force does the motor exert? P = W/t: 65 x 10 3 Watts = W/35 s W = 2. 28 x 106 J W = Fd 2. 28 x 10 6 J = F(17. 5 m) F = 1. 3 x 105 N

Sample Problem 4 A lawn roller is rolled across a lawn by a force of 115 N along the direction of the handle, which is 22. 5 o above the horizontal. If 64. 6 watts of power is generated in 90 s, what distance is the roller pushed? Fx = 115 cos 22. 5 Fx = 106. 3 N 115 N W = Fd 5814 J = (106. 3 N)d 22. 50 d = 54. 7 m P = W/t 64. 6 watts = W/90 s W = 5814 J W=Fd

Power Lab Objective: To use your knowledge of work and power to measure the horsepower you generate when you walk up a set of stairs. Power = W/t Power = Fv

Power Lab ( P = W/t) W = mgh P = W/t t= time to travel to top of stairs h meters mg = your weight in newtons = # lbs x 4. 45 N/lb h = vertical height of staircase

Power Lab h meters P = W/t ( in Watts or J/s) Convert power from Watts to Horsepower (HP) 746 Watts = 1 HP

Power Lab ( P = Fv) F = mg v = h/t t= time to travel to top of stairs h meters weight in Newtons = # lbs x 4. 45 N/lb

Questions 3 and 4 Food calories burned climbing the stairs 1 Food calorie = 4186 J How high would you have to climb to burn 100 food calories (100 food calories 1/3 of a candy bar)?

☺ Work in groups of three (one person time, one record data, climber). ☺ Be careful on stairs! ☺ Make sure your shoes are tied. ☺ Do not run, walk at a quick steady pace. ☺ Wait until stairs are clear before going down. ☺ Do not skip stairs. ☺ No more than two climbers on the stairs at one time ☺Wait quietly when your group is finished, our class will return to the room together. h meters Power Lab Safety!!!

WARNING!!! l Quiz next class on Work and Power!!!