# Direction of Frictional Force

1. May 14, 2014

When a body is held or rests in equilibrium in contact with a surface (e.g. a slope, the edge of a block etc.) how do we work out which direction the friction acts in? I'm used to the friction acting parallel to the surface i.e. slope but in one question I have, of a beam resting on the edge of a block, the friction seems to act along the beam (rather than along the face of the block). Why is this?

2. May 14, 2014

### UltrafastPED

Friction always opposes the motion; in the case of static friction it is in balance with the existing forces.

3. May 14, 2014

But that doesn't answer the question of which line it will act along, in a complicated system.
I can't just take components because I need to know which force to equate to the coefficient of friction * normal reaction force and I need to know the direction of that force for this to work.

4. May 14, 2014

### ZapperZ

Staff Emeritus
Why don't you actually who us this, i.e. provide a sketch? Your description is vague, and there is no way to answer this.

Try removing the friction, and see where you think the direction where the relevant part will move. The frictional force will then be in the opposite direction.

Zz.

5. May 14, 2014

### Staff: Mentor

If everything is at rest and the system is in equilibrium, you know that the net force is zero. So write down all the forces that you do know about, divide them into x, y, and z components, and the frictional forces are contributing what's needed to cancel them out.

6. May 15, 2014

### A.T.

In this case: If it would slide, would the contact point move along the beam, or along the face of the block?

In general: Idealizations like "edge" can get tricky, if you have two edges in contact. In reality of course there is always a small contact surface, and friction acts parallel to it.

7. May 16, 2014

Thanks a lot! This suggestion seems to work well for me.

And the normal reaction force itself? That is always acting perpendicular to the point or area of contact, i.e. perpendicular to the surface?

8. May 16, 2014

### jbriggs444

Yes.

When an object is in contact with a surface it is convenient (and usually meaningful) to separate the contact force into a component that is perpendicular to the surface and a component that is parallel to the surface. The component that is perpendicular is called the "normal force". In this context, "normal" simply means "perpendicular". So the normal force will always be perpendicular to the surface by definition.