1. TOPIC 4 The Laws of Friction

2. 1. The FRICTIONAL FORCE between two surfaces opposes their relative motion. 2. The FRICTIONAL FORCE is just sufficient to prevent motion up to a certain limit, hence the name LIMITING FRICTION. 3. The LIMITING FRICTIONAL FORCE is proportional to the NORMAL REACTION (pressure with which the surfaces are forced together) between the bodies in contact. So: F limiting = µR whereF limiting is the limiting frictional force (N) µ is the co-efficient of friction R is the normal reaction (N) µ varies depending on what type of surfaces are in contact with each other.

3. The Laws of Friction EXAMPLE A body of mass 4.8kg accelerates along a rough surface at 1.8m/s 2 under the action of a horizontal force of magnitude 20N. Find the co-efficient of friction.