# Homework Help: Power output

1. Nov 14, 2011

### AllenHe

1. The problem statement, all variables and given/known data
In order for a car of 4000kg to travel at 10m/s with a frictional force of 400N, find the power output of the engine.

2. Relevant equations
P=Fv

3. The attempt at a solution
I found
40000*10+400*10=400400W,
is this right?

2. Nov 14, 2011

### cepheid

Staff Emeritus
No, not really.

For the car to be travelling at a constant speed, Newton's second law says that the net force on it must be 0. We know that there are two forces acting on the car: the friction force and the force from the engine. For the net force to be 0, these two must balance each other out. That tells you what the applied force from the engine must be. You can then use that applied force in the equation you posted above in order to determine the engine's power.

3. Nov 14, 2011

### AllenHe

oh. SO it's 4000N?
But what if the question did not state whether the car started from rest or with an initial velocity of 10m/s?

4. Nov 14, 2011

### cepheid

Staff Emeritus
No, it's not 4000 N. Write the force balance equation for the horizontal direction:

sum of forces in horizontal direction = 0

and use this equation to solve for the applied force from the engine.

The initial condition is irrelevant here. You're told that, right now the car is moving at a constant rate of 10 m/s. This, along with the applied force, is enough info to determine the power output right now.

5. Nov 15, 2011

### AllenHe

oh.. mistake I wanted to type 4000w.
do you have any pdf book which can help me at as level?
Sometimes I just use the formula to answer the question, without looking at the question carefully?Do you have any tips?