# Calculating friction forces

• H4NS
I calculated it to be 0.0908.In summary, the problem involves a 1326kg car on a flat surface that requires a force of 160N to overcome static friction and start moving, and a force of 120N to stop it. The question asks about calculating the friction forces or coefficients of friction, with the coefficient being a ratio. The direction of friction is always opposite of the motion of the object and can be calculated by multiplying the Normal force by the kinetic coefficient. Without knowing the coefficient, it is calculated to be 0.0908.

## 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.

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.

CWatters said:
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

azizlwl said:
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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