1. Unit 3, Lesson 4: Power!

2. Power: the rate of doing work, measured in Watts
Power = work/time = W/t
1. After moving to Vancouver for university, Hannah starts a habit of climbing the Grouse Grind every week. Find Hannah’s power output if she can do the 800 m elevation hike in 50 minutes and she weighs 60 kg.
P = W / t
= ΔPE / t
= mgh / t
= 60 • 9.8 • 800 / (50 • 60)
= 157 Watts

3. 2. If Hannah is 30% efficient, how much energy does she burn on her hike? How many 80-calorie Oreo cookies does she burn off (1 calorie = 4200 J)?
EOUT = mgh
= 60 • 9.8 • 800
= 470,000 J
Eff = EOUT / EIN
0.3 = 470,000 / EIN
EIN = 1,570,000 J (1.57 MJ)
1 cookie = 80 x 4200 = J
4.7 cookies!

4. (Answer in W and in horse power. 1 hp = 746 W.)
3. Brad becomes a successful engineer and spends his first year’s salary on a sports car. Find the power output when his 1200 kg car accelerates from 20 m/s to 30 m/s in 5 seconds.
(Answer in W and in horse power. 1 hp = 746 W.)
W = ΔKE
= ½ (1200)(30)2 - ½(1200)(20)2
= J
P = W / t
= / 5
= W
= 80 hp

5. Recommended Questions:
Energy Practice Problems #4-6
You can now do all of the Energy Practice Problems sheet!
This is the end of the first half of the unit, on energy. Next class we will start the second half, on momentum.
45 min