- #1

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## Homework Statement

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.

## Homework Equations

P=Fv

## The Attempt at a Solution

I found

40000*10+400*10=400400W,

is this right?

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- Thread starter AllenHe
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- #1

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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.

P=Fv

I found

40000*10+400*10=400400W,

is this right?

- #2

cepheid

Staff Emeritus

Science Advisor

Gold Member

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## Homework Statement

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.

## Homework Equations

P=Fv

## The Attempt at a Solution

I found

40000*10+400*10=400400W,

is this right?

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

- #3

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But what if the question did not state whether the car started from rest or with an initial velocity of 10m/s?

- #4

cepheid

Staff Emeritus

Science Advisor

Gold Member

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oh. SO it's 4000N?

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.

But what if the question did not state whether the car started from rest or with an initial velocity of 10m/s?

The initial condition is irrelevant here. You're told that,

- #5

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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?

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