The distance it takes for a vehicle to stop depends on two factors:

• – thinking distance
• – braking distance

These factors also depend on a third factor: the speed of the car.

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Forces on a vehicle

When a vehicle is travelling at a steady speed the resultant force is zero. The driving force from the engine is balanced by a number of resistive forces including air resistance.

• Gravity pulls the vehicle downwards.
• The reaction force coming from the road pushes upwards against the wheels.
• The driving force coming from the engines propels the vehicle along the road.
• Air resistance pushes from the front, opposite the direction which it’s travelling.
• Friction, between the tyres and the road.

The sum of all those forces is the resultant force. Don’t forget to notice in what direction the forces are acting.

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Force and terminal velocity

When an object falls the speed at which it travels downwards is affected by two main forces:

• – Weight: this is the force that acts downwards. It’s due to the Earth’s gravitational field.
• – Air resistance: this is a frictional force and acts in the opposite direction to the falling object.

Falling can be split up into three stages:

• – First of all the object is accelerated downwards due to its weight. As there’s no air resistance the resultant force is down.
• – As the object starts to gain speed the air resistance increases. However, it’s weight doesn’t change. The resultant force remains down.
• – A point is then reached in which the weight of the object becomes balanced out by the air resistance. There’s no longer a resultant force and the object now travels at a steady speed. This is known as terminal velocity.