Power
Name:
Period
A machine is powerful if it can do a lot of work in a short time. The same is true of a person. Power is a
measure of how much work a machine, person or animal can do in a unit of time
Power = Work / time
Power is measured in units called watts. A watt (W) is the same as one joule per second.
1 W = 1 J/s
Power can be measured in larger units called kilowatts (kW) or even larger units called megawatts (MW)
1 kW = 1 000 W
1 MW = 1 000 kW = 1 000 000 W
Examples
1. What is the power of a machine if it does 12 000 J of work in 40 s
Answer: Power = Work/time = 12 000 J/ 40 s = 300 W
2. If a runner can lift his 600 N weight to the top of a 1000 m high
hill in 50 minutes.
a) How much work does he do?
b) How may seconds in 30 minutes?
c) How much work does he do?
Answer: a) Work = Force x distance = 600 N x 1000 m = 600 000 J
b) 50 min x 60 s/min= 3000 s
c) Power = Work/time = 600 000 J / 3 000 s = 200 W
3) If your mass is 50 kg and you can run up 9 stairs, each of
which is 20 cm high, in a time of 10 seconds. What is your
power?
Ans:
50 kg = 500 N., 9 stairs x .2 m = 1.8 m
Work = F X d = 500 N x 1.8 m = 900J
Power = Work/time = 900J/10s = 90 W
Watt’s my Power
Name:
Period:
You can find out your own power in a climbing situation if you can measure (a) the force of gravity on yourself
(your weight I Newtons), (b) the height you actually climb, and (c) the time it takes you to climb. In this
experiment you will measure how powerful you are climbing a flight of stairs, or running up to the school from
the nearest creek.
Purpose: To measure your own power, in watts.
Procedure
1. Determine the force of gravity on yourself, in newtons. To do this, measure your mass in Kilograms,
(or lbs. and then divide by 2.2 lbs./kg. Once you have your mass in kilograms, multiply this by 9.8 N/kg.
Record this force (your “weight”) in a copy of the table below:
2. Measure the height of on stair and the number of stairs from the lowest step to the highest.
3. Calculate the total distance up from the bottom to the top stair.
4. Use a stopwatch to measure how long it takes you to run your fastest from the bottom step to the top.
5. Calculate your power by filling in the table below. Convert W to kW by dividing by 1000 and Convert W
to Hp by dividing by 746. One Horsepower was considered the power output of a typical horse. We
often use this to rate the power of motors.
6. Record your name and your power on the board for the whole class.
My mass ( in Kg, or divided lbs by 2.2)
= ____________kg.
My “Weight” (mass x 9.8)
=____________N.
Height of a stair
= ____________cm
Number of stairs
= ____________
Total height of stairs in meters
=____________m
Work done by me running up stairs or trail
(My weight x height if went up)
=____________J
My Power = Work done / time
=____________W
My Power in Kilowatts (Watts / 1000 Kw/W
=____________kW
My Power in Horsepower (Hp) = Power / 746 W/Hp
=____________Hp
Concluding Questions :
1) What was the highest power in the class?
2) What was the average power for the class?
3) What was the average power for girls?
4) What was the average power for boys?
5) How many 100 W light bulbs was you power equivalent?
Exercises: Show all formulas, put in the appropriate numbers with units and calculate an answer with units.
1) What is your power output when you drag a boat across the sand with a force of 500N for a distance of
100m in 25 s?
2) What is the power of an elevator that weights 10, 000 N that raises 10 floors, each of which is 3m if it
takes 50 s for the trip?
3) A cyclist rides up the Cypress Bowl highway in 30 minutes. What is their power if they rise a vertical
distance of 700 m and the combined mass of the rider and bike is 90 kg.
4) The fastest person on the Grouse Grind can run up its 903 m in 23 minutes. If the runner has a mass of
65 kg. What is their power?
5) A 1500 W motorized winch pulls a truck out of the mud in 10 s. How much work did the winch do.
6) How long will it take a person whose power is 500 W to do 10, 000 J of work.