# Homework Help: Work And Power Problem

1. Dec 21, 2004

### DethRose

Hey this is my last work and power problem haha...this is the hardest of them all.

the question is

A tractor pulls a loaded sled, massing 4110 kg, along a level snow packed road at a constant speed of 45 km/h. If the amount of friction between the road and the sled is 585 N find the power required.

I found the weight of the load in newtons to be 40319.1 Nand subtracted the friction to get a net force of 39734.1 N but i dont know how to find the power from here.

2. Dec 21, 2004

### Andrew Mason

I am not sure why you are doing that. The mass of the sled is irrelevant since you know the actual force of friction.

Power is: Work (against the friction force) / unit time. Work is: Force (friction) x distance. So power is Force (friction) x distance/unit time = Force (friction) x speed.

AM

3. Dec 21, 2004

### mjfairch

Make sure you use consistent units as well. If you're going to use Newtons, then you need to use m/s as well instead of km/hr. More generally, for reasons pointed out by Andrew Mason, power is given by:

$$P=\vec F \cdot \vec v = |\vec F| |\vec v| \cos{(\theta)}$$