Gravity is a force that attracts objects with mass towards each other. The weight of an object is the force acting on it due to gravity. The gravitational field strength of the Earth is 10 N/kg.

The stopping distance of a car depends on two things: the thinking distance and the braking distance.

Weight

On Earth, if you drop an object it accelerates towards the centre of the planet. You can calculate the weight of an object using this equation:

weight (N) = mass (kg) × gravitational field strength (N/kg)

Weight is not the same as mass. Mass is a measure of how much stuff is in an object. Weight is a force acting on that stuff.

You have to be careful. In physics, the term weight has a specific meaning, and is measured in newtons. Mass is measured in kilograms. The mass of a given object is the same everywhere, but its weight can change.

Gravitational field strength

Weight is the result of gravity. The gravitational field strength of the Earth is 10 N/kg (ten newtons per kilogram). This means an object with a mass of 1kg would be attracted towards the centre of the Earth by a force of 10N. We feel forces like this as weight.

You would weigh less on the Moon because the gravitational field strength of the Moon is one-sixth of that of the Earth. But note that your mass would stay the same.

Falling objects

You should be able to describe the forces affecting a falling object at different stages of its fall. Usually, you need to think about two forces:

1. The weight of the object. This is a force acting downwards, caused by the object’s mass the Earth’s gravitational field.
2. Air resistance. This is a frictional force acting in the opposite direction to the movement of the object.

Three stages of falling

When an object is dropped, we can identify three stages before it hits the ground:

1. At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
2. As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.

3. Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.

Three stages of falling

When an object is dropped, we can identify three stages before it hits the ground:

1. At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
2. As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
3. Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.

Terminal velocity

What happens if you drop a feather and a coin together? The feather and the coin have roughly the same surface area, so when they begin to fall they have about the same air resistance.

As the feather falls, its air resistance increases until it soon balances the weight of the feather. The feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it reaches its terminal velocity.

An astronaut on the Moon carried out a famous experiment. He dropped a hammer and a feather at the same time and found that they landed together. The Moon's gravity is too weak for it to hold onto an atmosphere, so there is no air resistance. When the hammer and feather were dropped, they fell together with the same acceleration.

Thinking distance

It takes a certain amount of time for a driver to react to a hazard and start applying the brakes. During this time, the car is still moving. The faster the car is travelling, the greater this thinking distance will be.

The thinking distance will also increase if the driver's reactions are slower because they are:

• under the influence of alcohol
• under the influence of drugs
• tired

Braking distance

The braking distance is the distance the car travels from where the brakes are first applied to where the car stops. If the braking force is too great, the tyres may not grip the road sufficiently and the car may skid. The faster the car is travelling, the greater the braking distance will be.

The braking distance will also increase if:

• The brakes or tyres are worn.
• The weather conditions are poor, such as an icy or wet road.
• The car is more heavily laden, for example, with passengers and luggage.