Introduction to forces

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Key Points

  • A force is a push or a pull that acts on an object due to the interaction with another object.

  • Force is measured in newtons (N).

  • Forces are divided into contact forces and non-contact forces.

What is a force?

A force is a push or a pull that acts on an object due to the interaction with another object.

Force is measured in newtons (N).

There are two main categories of forces - and .

Did you know?

You cannot see a force but you can see its effect. For example, when you apply a force to push a door open.

Scalars and vectors

A physical quantity is something that can be measured.

Quantities can be a scalar quantity or a vector quantity.

Scalar quantities only have a .

Vector quantities have both a magnitude and a direction.

Vector quantities are represented using arrows. The larger the arrow the greater the magnitude of the vector.

Force is an example of a vector quantity.

The diagram below shows some examples of vector quantities. The arrow shows the direction.

The length of the arrow shows the magnitude, eg the size of the force in the first arrow.

Three different arrows pointing in different directions, the first to the left, another to the right labelled 50 km and the third pointing downwards, labelled 9.9m/s2

Contact forces

Contact forces act between two objects that are physically touching.

Here are some examples of contact forces:

Tension

Tension is a pulling force exerted on an object by a string, rope or rod.

A box hangs from a rope. Two arrows which are equal in size act upwards and downwards from the top and bottom of the rope.

Friction

When two objects slide past each other they experience friction.

Friction is a force that acts between two touching surfaces and prevents or resists them moving against each other.

A box rests on an incline. There are three arrows; one acting vertically downwards from the centre of the box’s base. One arrow acts perpendicular to the incline. One arrow acts up the incline.

Air resistance

When an object moves through the air it experiences air resistance. This force acts against the direction of movement.

The faster the object is travelling the greater the air resistance.

A box falls from the sky. An arrow labelled Air resistance acts upwards from the box.

Upthrust

Upthrust is an upwards force that acts on an object when it is in a fluid (a liquid or gas).

A boat on water with two arrows of equal length, one pointing upwards and one pointing downwards from boat. Downwards arrow is unlabelled. Upwards arrow is labelled Upthrust

Thrust

Thrust is a driving force exerted by an engine to make an object move.

A car travelling forwards with a green arrow pointing forwards from the car indicating thrust.

Normal reaction force

When an object pushes on a surface like a table, wall or the ground, the surface pushes back on the object with a balancing force.

The normal reaction force always acts at right angles to the surface.

A box rests on a table. There are two arrows, equal in size but going in opposite directions, up and down, from the point where the box meets the table. The up arrow is labelled Normal reaction force.

Friction can sometimes be useful and sometimes it can be a nuisance.

Identify whether friction is useful or not in these situations.

  • The brakes on a bike

  • Inside the hub of the wheel on a bike

  • In the moving parts of the chain and gears

Friction is useful in the brakes, but it is a nuisance inside the wheel hub, chains or gears.

Non-contact forces

Non-contact forces act between two objects that are not physically touching.

Magnetic force

Magnetic force is experienced by a magnet or a magnetic material, eg iron, when placed in a magnetic field.

This force can pull the two objects together or push them apart.

Two magnets are side by side with North poles facing each other. Two arrows between the poles of the magnets point inwards towards each other.
Two magnets are side by side with North poles facing each other. Two arrows between the poles of the magnets point outwards away from each other.

Electrostatic force

Electrostatic force is experienced by a charged particle in an electric field.

This force can be either attractive or repulsive.

Large red circle contains ‘+’ symbol and smaller blue circle contains ‘-‘ symbol. Two arrows between the particles, point inwards towards each other.
Two small blue circles containing '-' (minus) symbol. Two arrows between the particles point outwards away from each other.

Gravitational force

Gravitational force is experienced by a mass when it is sufficiently close to another mass.

Gravity always pulls two objects towards each other. It never pushes them apart.

In the diagram, Earth pulls the satellite and the satellite pulls Earth. The forces are equal in size and opposite in direction.

The Earth and a satellite point to each other in space with arrows of equal size.

A skydiver is falling from an aeroplane. Identify the two forces which are acting on her.

The gravitational force, or weight, acts downwards and air resistance acts upwards.

When forces act on objects they can have the following effects:

  • The object can speed up or slow down. Think about a race car travelling in a straight line whilst it speeds up and slows down.
  • The object can change direction. Think about a satellite orbiting Earth – the gravitational attraction between the satellite and Earth causes the circular motion.
  • The shape of the object can change. Think about squeezing a foam ball.

When a car turns around a corner, what force is causing the change of direction?

The change in direction is caused by friction between the tyres and the road.

Measuring forces

A force meter, also known as a newton meter, can be used to measure the size of a force.

Force meters have a spring inside them. The weight of the mass causes the spring to stretch.

It is important to ‘zero’ the force meter before you use it. This means adjusting it so that the reading is zero when there is no force acting on it.

If you don’t ‘zero’ the force meter, all your measurements will be inaccurate by the same amount. This is called a .

A force meter, also called a newton meter, is used to measure forces

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