# Frictional force problem(box sliding down between two walls)

1. Sep 20, 2014

### Kararr

The full problem is: "A 2.0kg box slides down at constant velocity between two identical walls. What is the frictional force exerted on the box by one of the walls?"

The answer I got was zero because I equated Normal force (of the wall on the box) at zero.
Am I wrong? ): Does the weight of the box press against the wall as well?

2. Sep 20, 2014

### Staff: Mentor

Since the velocity is constant, what can you say about the net force on the box?

3. Sep 20, 2014

### Kararr

oh so is frictional force= 20 N? What about the normal force though? Sorry a bit confused

4. Sep 20, 2014

### Staff: Mentor

Yes, the total friction force is 20 N.

Who cares? ;)

5. Sep 20, 2014

### CWatters

I can see the confusion about the normal force...

In most friction problems gravity provides a normal force. However in this problem something (unspecified) is causing the walls to press on the sides of the box creating a normal force and hence friction with them. Perhaps the box is made of rubber and is just a tight fit between the walls? Whatever the cause you don't need to care about it.

The problem isn't a problem about friction at all. It's about Newtons laws.