# Friction query

1. Mar 24, 2015

### gracy

I have a doubt regarding the following line of post 6 of the above particular thread

According to below lines of the mentioned post
The applied force will accelerate the box towards the north, so it will move in a curve. The friction force is always along the tangent to that curve and opposite to the direction the box is moving. So the component of friction force in the south direction will increase and the component in the west direction will decrease.
the diagram would be

But the below lines of the very same post
If your box is moving east, the dynamic friction force is towards the west. If you suddenly apply a force towards the north at time t=0, the dynamic friction force at t=0 will still be towards the west.
Are contradictory of my diagram this is my opinion ,I think I am wrong I am missing something.Please guide me.

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2. Mar 24, 2015

### Staff: Mentor

Not sure what you are drawing here. To find the direction of the dynamic friction, you must draw the path of the box. It starts out moving east, then begins to curve towards the north as the northern force is applied. Your diagram seems to show the path instantly changing when the force is applied.

3. Mar 24, 2015

### gracy

If I say I have drawn the diagram of that instant when the applied force in north direction has become large enough,will my diagram be correct?

4. Mar 24, 2015

### gracy

One more query friction is
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
So ,let's take the two blocks which are in contact are moving with same velocity.That means there is not an relative motion between them so no friction?

5. Mar 24, 2015

### Staff: Mentor

It doesn't seem right to me, but I'm not sure what you are trying to illustrate.

6. Mar 24, 2015

### Staff: Mentor

If there's no relative motion between the blocks, then there is no dynamic (kinetic) friction between them. There may well be static friction between them.

7. Mar 25, 2015

### gracy

One typo is here .I meant .That means there is not any relative motion between them ,so no friction?

8. Mar 25, 2015

### gracy

As they are moving,will there be static friction?

9. Mar 25, 2015

### Staff: Mentor

If there is no relative motion, there is no dynamic friction.

10. Mar 25, 2015

### Staff: Mentor

Just because they are moving with respect to some frame does not mean there cannot be static friction between the surfaces. Depends on the situation.

For example: Say one block is sitting on top of the other. And the bottom block is being pulled along (accelerating) while the top block comes along for the ride. In this case, both blocks are moving at the same speed, yet there is static friction between them.