# Calculating friction forces

## Homework Statement

A 1326kg car lies on a flat surface. To make it move very slowly (a~0), one needs a force of 160N parallel to the surface. To stop it, the force must be reduced to 120N.

## Homework Equations

Calculate the friction forces of the car

On Tuesday I have an exam regarding this and related topics. I missed some classes due to sickness and now I am a little lost.

According Newton's 2nd law,

Fnet=ma

Fnet==F1+F2+F3+ ...Fn

Forces are vectors, so they have magnitude and direction.
If you take vector force pointing to the right is positive(vector as an arrow) , then vectors pointing to the left as negative.
As for the friction, the direction is always opposite of the motion of the object.
The value can be calculated by multiplying the Normal force to the kinetic coefficient μ.

CWatters
Homework Helper
Gold Member
If I have understood correctly, the problem statement is telling you that to overcome the static friction you need to apply 160N. However once it's moving you only need to apply >120N to keep it moving. (eg this is a problem about static and dynamic friction).

However is the question really asking you to calculate the "friction forces" or the "coefficients of friction" ?

If the latter, look at the wikipedia entry for coefficient of friction. Hint: It's a ratio.

If I have understood correctly, the problem statement is telling you that to overcome the static friction you need to apply 160N. However once it's moving you only need to apply >120N to keep it moving. (eg this is a problem about static and dynamic friction).

However is the question really asking you to calculate the "friction forces" or the "coefficients of friction" ?

If the latter, look at the wikipedia entry for coefficient of friction. Hint: It's a ratio.

Yes, I meant the coefficient.

And also, the stop the car, you reduce the applied force to 120N

According Newton's 2nd law,

Fnet=ma

Fnet==F1+F2+F3+ ...Fn

Forces are vectors, so they have magnitude and direction.
If you take vector force pointing to the right is positive(vector as an arrow) , then vectors pointing to the left as negative.
As for the friction, the direction is always opposite of the motion of the object.
The value can be calculated by multiplying the Normal force to the kinetic coefficient μ.

I don't have the kinetic coefficient

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