# Homework Help: Friction force

1. May 29, 2006

### TSN79

Below is a problem I had on a test recently. The task is to find out what the friction coefficient must be in order to prevent the big box from moving. Friction between the little box and the slope is neglected.The red markings are my ideas. I figured if I sum the forces in the x-direction I'll find the friction force, which I can replace by uN, since F=uN. Will N be the same as G? If so, finding u is easy...but I think it's more complex than that, can someone explain?

#### Attached Files:

• ###### friksjon.jpg
File size:
15.2 KB
Views:
73
2. May 29, 2006

### Hootenanny

Staff Emeritus
I cannot see the image however, there is one thing the I always to when solving these type of problems: Resolve all forces so they are either parallel or perpendicular to the inclined plane. Then just sum the forces.

The normal reaction force will only be equal to mg if the plane is horizontal. Otherwise you must resolve the forces to find N (which you should have done if you follow my advice above), the normal reaction force is always perpendicular to the surface.

Hope this helps.

~H

Last edited: May 29, 2006
3. May 30, 2006

### TSN79

N is perpendicular to the box, or in other words, parallel to G, since the box simply rests on flat ground, but there is a force pushing on the box sideways, so I'm just wondering if that will have an effect on N? I'm thinking and hoping not...

4. May 30, 2006

### Hootenanny

Staff Emeritus
Then no, if the applied force is perpendicular to g, then it will have no effect on the normal reaction force of the surface on the block.

I'm afraid theres no mentors online at the moment that I can ask to approve your attachment.

~H