# Homework Help: Question about power and output power.

1. Jun 11, 2014

### vegetables102

1. How could you measure your output power developed while pushing a car?

2. If you developed a certain amount of power running up lets say 2 flights of stairs, could you develop the same power if you ran up twice as many stairs?

3. What factors affect a persons output power running up stairs?

2. Jun 11, 2014

### mafagafo

You need to try to solve the problem before we can help you. Show us your progress.

3. Jun 12, 2014

### dean barry

1.
Rough guess :
Level road assumed, tyres at recommended pressure.

m = mass of the car in kg
g = local gravity rate ( use 9.81 (m/s)/s )
Crr = tyre on ashphalt rolling resistance co-efficient ( use 0.03 )

Rolling resistance force (f) ( constant, regardless of speed ) = m * g * Crr
( answer is in Newtons )

Find the maximum speed (v) you can push the car in metres per second
(mph * 0.44704 = metres per second)

Then :
The power provided (W) to push the car :
W = f * v
(answer is in Watts, divide by 745.7 to get horsepower)

Example :
v = maximum push speed = 4 m/s (example)
m = 1,500 kg
g = 9.81 (m/s)/s
Crr = 0.03

f = m * g * Crr
f = 1,500 * 9.81 * 0.03
f = 441.45 Newtons

W = f * v
W = 441.45 * 4
W = 1,765.8 Watts ( about 2.37 hp )

2.
Energy change (Joules) = m * g * h
m = your mass in kg
g = local gravity rate (use 9.81 (m/s)/s)
h = vertical height moved in metres (floor to floor difference)
Then :
Average Power = energy change / time in seconds
3.
Power output is roughly proportional to your size, but includes factors like sex, fitness level, nutrition etc.

4. Jun 12, 2014

### BvU

Hello vegie, and welcome to PF.

You show us the questions that were put to you. Which question are you putting to PF ?

Anyway, for # 1 I would use a scale between my hands and the car to have guess for the force. Push a certain distance in a certain time and there you are !

(How many tyres does Dean's car have? And does he really think he can push hard enough to get a meaningful furlongs/fortnight -- oh, sorry, mph -- reading from the speedometer ?)