Friction

If we try to pull a box across the floor there is a friction force between the box and the

floor.

If the box does not move the friction force will be equal to the force P

and as P increases from 0 N the friction force will also increase from 0 N until it

reaches its maximum value F_max, after which the box will no longer be in

equilibrium.

When friction force is at its maximum and the box is on the point of moving the box is

said to be in limiting equilibrium.

N.B. The direction of the friction force is always opposite to the direction of motion (or the

direction in which the particle would move if there was no friction).

Example: A particle of mass 2 kg rests in equilibrium on a plane which makes an angle of

25^o with the horizontal.

Find the magnitude of the friction force and the magnitude of the normal reaction.

Solution: DRAW A DIAGRAM SHOWING ALL FORCES - the weight 2g N, the

friction F N and the normal reaction R N. Remember that the particle would move down

the slope without friction so friction must act up the slope.

Then draw a second diagram showing forces resolved along and perpendicular to the

slope.