# How to specify the direction of Friction force

How to specify the direction of Friction force ,

Hello,

Could you tell me how to specify the direction of the friction force ?

For example see the question below, I couldn't solve the question
http://img291.imageshack.us/img291/7808/25857191iu3.png [Broken]

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Gib Z
Homework Helper
Well, what does friction do? It impedes motion!

So the direction is opposite to the direction of motion!

i solved it and the answer is 1.71 m/s2 is it correct ??
sorry i can post the solution steps because the short of time.

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Here is the free body diagram

http://img384.imageshack.us/img384/3640/46900974mn2.png [Broken]

Is there any mistake ??

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PhanthomJay
Homework Helper
Gold Member
Here is the free body diagram

http://img384.imageshack.us/img384/3640/46900974mn2.png [Broken]

Is there any mistake ??
I see a couple of errors. Since you've broken up the weight into its x (Wsin10) and y (Wcos10) components, that "W" acting down no longer belongs in your FBD.

It appears that you are using a value of g=10m/s^2, which is fine, but you had better check the direction of your friction force. The problem is asking for the maximum acceleration of the truck. Static friction forces oppose the direction of imminent relative motion between the 2 surfaces.

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when they ask you a freebody diagram they only asking you only the force ,the friction force and force of gravity which is mg. you can split mg in two 3 parts but some teachers and Ap exm. don't give you creadit. any way.......
friction is always is opposit of the force action up on it. so you are right on the FBD .

PhanthomJay
Homework Helper
Gold Member
friction is always is opposit of the force action up on it. so you are right on the FBD .
No, friction is always opposite the direction of relative motion or pending relative motion between the two objects. Consider the case where the box is sitting on the rough truckbed, and the truck is on a horizontal level surface. If the truck accelerates to the left, which way does the friction force act on the box?

look at the first picture I posted.
the truck is moving and the box it on it so the box is moving too.
so the direction of the friction is opposite that movement !

PhanthomJay
Homework Helper
Gold Member
look at the first picture I posted.
the truck is moving and the box it on it so the box is moving too.
so the direction of the friction is opposite that movement !
No, friction is always opposite the direction of relative motion or pending relative motion between the two objects. Consider the simpler case where the box is sitting on the rough truckbed, and the truck is on a horizontal level surface. If the truck accelerates to the left, which way does the friction force act on the box? I hope you will answer "to the left".....which is in the direction of the acceleration relative to the ground, and the direction of its movement relative to the ground (it may not be moving at all with respect to the truck).

Okay, Thank you PhanthomJay .

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