Frictional Forces and Non-Uniform Motion

Friction

Friction is a force that opposes motion.

A force of friction acts when an object moves through air or water, or when solid surfaces slide across each other.

The faster the object moves through a fluid, the greater the force of friction.

Friction causes objects to heat up and wear away at their surfaces.

Friction is important in cars.

Tyres have to have a certain amount of tread on them, otherwise they would not grip the road. Brakes exert a large force of friction to stop the vehicle.

The faster a vehicle travels, the bigger the braking force needed to stop it in a certain distance, or a greater distance is needed if the same force is applied.

If too great a braking force is applied, the car may skid due to too little friction between the road and tyres.

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Stopping Distances

total stopping distance of a vehicle = thinking distance + braking distance

The thinking distance is how far the vehicle travels during the time it takes the driver to react and put his/her foot on the brake.

The braking distance is how far the vehicle travels after the brake has been pressed.

Factors which affect the thinking distance include:

• Tiredness
• Drugs
• Alcohol
• Poor visibility
• Speed of the vehicle

Factors that affect the braking distance include:

• Speed of the car
• Condition of the tyres
• Condition of the brakes
• Condition of the road, e.g. wet, ice or oil
• The mass of the vehicle

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Terminal Velocity

When an object falls, it will accelerate towards the Earth, due to the force of gravity acting on it. This force can also be called weight.

Frictional forces between the air and the object will act in the opposite direction.

The faster the object falls, the more air it passes through, so air resistance, or drag, gradually increases.

If the falling object has a large surface area, the force of friction will be greater.

A parachute provides a large surface area.

When the force of friction acting upwards equals the force of gravity acting downwards on the object, the resultant force will be zero.

The object then falls at a steady speed.

This speed is called the terminal velocity.

When a vehicle has a steady speed, the driving force of the engine is equal to the frictional forces due to the air and the road.

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