Friction

Friction is a contact force that is present whenever the surfaces of two materials are in contact with each other. It is the force that resists the movement of the material over another. For instance, the wheels of vehicles rolling on the road and the soles of your shoes touching the ground as you walk involve friction. The tires of the car and the road are the two surfaces that rub together. The bumps and hollows cause resistance in the movement of the surfaces over each other. The irregularities on the surfaces can be more obviously seen on through rough surfaces than on smooth surfaces. The attraction of the particles of two materials when their bumps and hollows rubs against each other is friction.

The amount of friction affects the motion of an object. Try rolling a ball on a smooth surface. Do the same on a rough surface. You will most probably observe a difference on the motions of the ball. A smooth surface like a newly polished floor produces less friction, which enables objects to move fast. A rough surface such as that of a bumpy road produces greater friction, causing objects to move slowly.

Observe the running vehicles on an expressway. They can travel on the expressway faster than on a rocky rough road. They can speed up from a minimum of 60 km/h up to 100 km/h because of the smooth road; there are no holes and humps that slow down their speed. If these vehicles would travel on a rocky rough road, they would not be able to run with those speeds. Rugged rocky roads would prevent them from running too fast. The vehicles would slow down because they would have to hurdle over the rocks on the road.