Friction is a contact force that slows down motion. Whenever the surface of an object slides over the surface of another object, each object exerts a frictional force on the other. This is because of the irregularities of the two surfaces. Even if there is no relative motion between the two objects, frictional forces exist between the two objects. Friction has negative and positive effects.
Effects of Friction
Friction has both negative and positive effects.
The common methods used to minimise the negative effects of friction are:
using highly polished surfaces for moving parts;
introducing a layer of lubricant between the two surfaces to prevent the surface irregularities from holding on to one another;
using steel balls bearings to enable surfaces to roll over rather than slide over each other.
Friction on the road
For any vehicle travelling at a constant velocity, the resultant force is zero. This is because the force of its engine is balanced by the resistive forces . The braking distance needed to stop a vehicle in a certain distance depends on :
the speed of the vehicle when the brakes are first applied;
the mass of the vehicle.
resultant force = mass × acceleration, in which the braking force is the resultant force.
. The greater the speed, the greater the deceleration needed to stop the vehicle in a certain distance. Therefore, the braking force must be greater than at low speed. 2. The greater the mass, the greater the braking force needed for a given deceleration.
Stopping distances
The stopping distance is the safest distance a vehicle can stop in, and is in two parts:
The thinking distance: the distance travelled by the vehicle during the drivers reaction time.
The braking distance: the distance travelled by the vehicle during the time the braking force acts.
stopping distance = thinking distance + braking distance
Factors affecting stopping distances
1. Tiredness, alcohol and drugs all increase reaction times. These factors increase the thinking distance (thinking distance = speed × reaction time).
2. The faster a vehicle is travelling, the further it travels before stopping, because both the thinking and braking distance increase with in creased speed.
3. In adverse road conditions, on wet or icy roads as drivers have to brake with less force to avoid skidding.
4. Poorly maintained vehicles, worn brakes or tyres, take longer to stop because they are less effective.