Here we focus on those scientific concepts that are the hardest to explain to children. We break it down into what pupils need to know and outline the background science. Even though much of the background science does not need to be taught to primary aged children, it is useful for you as a teacher when addressing misconceptions and children’s challenging questions. 

Please refer to your national curriculum documents when planning your sequence of work and ensure that you teach the correct knowledge for your year group. 

What do children need to know about forces?

Learning about forces should be as exploratory, hands-on and as pupil-led as possible. Some of the concepts can be quite abstract for pupils and purposeful practical demonstrations and enquiries can help to develop understanding and challenge misconceptions.  We have included some ideas for how to teach forces in part two of this topic guide.

Key scientific concepts: Pupils need to know ... 

• There are contact and non-contact forces that can push and/or pull objects.
• Magnetic forces act at a distance on magnetic materials.
• Friction is a contact force that affects how objects move.
• Unsupported objects will fall because of the force of the Earth’s gravity pulling on them.
• Air resistance and water resistance are friction forces that affect objects moving through air and water.
• Some simple machines allow a small force to have a greater effect.

1. There are contact and non-contact forces that can push and or pull objects.

Pupils need to know:

• Forces are pushes and pulls. We cannot see forces, just their effects.
• When an object changes its movement or shape, we know that a force is acting on it.
• To start something moving, something must push or pull it. We say a force has acted on the object.
• To slow down or stop a moving object a force must act on it.
• Forces come about when two objects interact with each other.
• If a force acts when two objects are touching, it is called a contact force.
• If a force can be felt at a distance, then it is called a non-contact force.
• Non-contact forces get weaker when the objects are further apart.
• The motion of objects that are heavier or moving faster are harder to change.

Background Science for Teachers

We can’t see forces, but we can see the effects they have on objects. This can make it quite an abstract topic for young scientists.

If an object starts to move, speeds up (accelerate), slows down (decelerate) or changes direction, the forces acting on it are unbalanced. If an object is stationary or moving at a constant speed in a fixed direction the forces acting on it are balanced. Unbalanced forces mean there is a resultant force acting on the object, the effects of which will be observed.

If an object’s shape is changed by being stretched or squashed, forces are acting on it. Pulling forces will stretch a material whereas pushing forces will squash or compress a material.

An extended elastic band and some squished modelling dough. 

Image credit: The Ogden Trust. Licensed only for use on Explorify.

Contact forces

Some types of forces only act when objects are touching, we call these contact forces and examples included friction, air resistance, water resistance and upthrust.

Note that water resistance and upthrust are not the same thing. Water resistance is a force that acts in the opposite direction to motion when an object is moving through water. Upthrust (or buoyancy) is the force that causes objects to float.

Floating and sinking

(Why some objects float or sink is covered in this next section which may be useful background for schools following the Scottish curriculum where the topic is covered in detail, For the other UK nations, the detail on floating and sinking is not covered in the curriculum.)

When any object is placed in water, the force of gravity (also known as weight) pulls it down. If the object floats, this is because the water exerts an upward force called upthrust (also known as buoyancy), which balances the weight (the downward force of gravity) of the floating object. (For more detail on weight,mass and gravity see section 4.) If the downward force of gravity of an object is greater than the upthrust it will sink.

The density of an object determines whether it floats or sinks. Density is the amount of mass in a given volume. An object which has a higher density than water (so a higher mass compared to the mass of water with the same volume as the object) it will sink. An object with a lower density than water (so a lower mass compared to the mass of water with the same volume as the object) it will float.

For example, if a piece of polystyrene that has a low density (low mass for its size) is placed in water, it displaces a volume of water, the mass of which is equal to the mass of the piece of polystyrene. As the density of polystyrene is lower than that of water the volume of water displaced will be less than the volume of the polystyrene so the upthrust from the water can balance the force of gravity from the polystyrene, so it floats. The more water that is displaced, the greater the upthrust force. Now imagine a solid piece of iron which is very dense (a large mass for its size). The volume of water with the same mass as the iron that would need to be displaced to create enough upthrust to make the iron float will be greater than the volume of the piece of iron. In this case the upthrust cannot balance the force of gravity of the piece of iron, which means it will sink.  Iron boats float because although they are made of iron the shape of the boat means it contains a lot of air, so the overall density of the boat is lower than water.

Non-contact forces

Sometimes forces between two objects can be felt over a distance, we call these non-contact forces and examples include the force due to gravity, magnetic forces and forces due to static electricity. Children can easily explore the electrostatic forces of attraction and repulsion that results from the build up of electric charge, using balloons. When a balloon is rubbed on your hair it will gain a negative charge which will then attract the hair which has become positively charged.

A bar magnet attracting iron filings through a container. Hair being pulled upwards by a balloon. 

Image credit: The Ogden Trust. Licensed only for use on Explorify.

Note that weight is a scientific term used to describe the force due to gravity acting on an object. Weight is different to mass -weight is the downwards force measured in Newtons (N), whereas mass is the amount of matter in an object measured in kilograms (kg).

2. Magnetic forces act at a distance on magnetic materials.

Pupils need to know:

• Some materials are magnets.
• A magnet attracts some materials which are called magnetic materials.
• Iron and nickel are magnetic materials, but most metals are not.
• The attraction between a magnet and a magnetic material can be felt across a gap and through other materials.
• Two magnets may attract or repel each other, depending on which parts of the magnets are closest together.
• A magnet has two areas, called poles, at which its magnetic force is strongest. Every magnet has two types of magnetic pole, a north pole, and a south pole. If two poles are opposite, they will attract, whereas poles that are the same will repel.

Background Science for Teachers

When you put a magnet near a magnetic material, the magnetic material behaves like a magnet, and they attract. When the magnet is taken away, the magnetic material no longer behaves like a magnet. This change can be made permanent and magnetic materials can be turned into magnets.

Many objects attracted to magnets that you find in the classroom will be made from steel such as paper clips, drawing pins, scissors and staples. Steel is a common magnetic material because it is made with iron. It is a common misconception that all metals are magnetic when in fact it is only a small proportion which includes iron, cobalt and nickel. You may have some rock samples at school that are magnetic such as magnetite or lodestone which contain iron ore.

Every magnet has a north pole and a south pole. If like poles, for example north and north, are brought together, they push away from each other – we say that they repel. If opposite poles are brought together, they pull together – we say that they attract. The closer the magnets are together, the stronger the force between them. There is a space around a magnet where magnetic materials and other magnets experience a force. We call this a magnetic field.

A diagram of the magnetic field around a bar magnet. 

Image credit: Amanda Poole. Licensed only for use on Explorify.

The inside of the Earth behaves like a giant bar magnet, with a north and south pole. The needle on a compass is a tiny magnet; one end will always point towards the Earth’s north pole. The Earth’s magnetic field is created by the constant movement of molten iron in the Earth’s core and protects us from radiation and charged particles emitted from the Sun. We observe auroras at the north and south pole when the Earth’s magnetic field pulls charged particles from the Sun towards them.

A drawing of Earth's magnetic field beside an image of the Northern Lights.

Image credit: Amanda Poole. Licensed only for use on Explorify.

Heating or dropping magnets can make them less strong as a magnet so they should be stored in a cool place and pupils should be encouraged to be careful when carrying them around the classroom.

3. Friction is a contact force that affects how objects move.

Pupils need to know:

• When an object slides, or is pushed or pulled across a surface, a force of friction acts on the object in the opposite direction to its movement.
• Friction is a force that always opposes motion.
• It is a contact force that acts when two objects slide over each other.
• The amount of friction depends upon the two surfaces. Smooth surfaces tend to have less friction than rough surfaces.

Background Science for Teachers

When two surfaces are in contact with each other there is a force acting on each surface that acts in a direction to stop them moving past each other. It is a contact force.

Smooth surfaces tend to have less friction than rough surfaces. Some liquids such as oil, when placed between two surfaces, have the effect of reducing friction – these liquids are called lubricants.

Friction forces can grip the surfaces to prevent them sliding across each other. Friction that helps us grip can be useful, for example when we walk friction between the surface of our shoes and the pavement prevents our shoes from slipping and allows us to push against the ground to push ourselves forwards. When the path is icy there is less grip force and our foot will slip, making it more difficult to walk. Children could carry out a comparative test to measure the forces need to move different types of shoes on different surfaces to find out what factors affect the size of friction forces.

A child measuring the force necessary to push a shoe across the ground. 

Image credit: The Ogden Trust. Licensed only for use on Explorify.

Friction forces are still there when two surfaces are slipping across each other, these slip forces are helpful in reducing motion and slowing moving objects down. You can investigate the friction on different strings and wires using balloon rockets, to see which travels furthest.

A balloon rocket.

Image credit: The Ogden Trust. Licensed only for use on Explorify.

To understand why this is going on we need to look at the surfaces on a microscopic level, then we see that no surface is completely smooth, instead they are covered in tiny bumps and ridges. These bumps and ridges catch on each other and act to prevent movement (grip) or reduce movement (slip).

4. Unsupported objects will fall because of the force of the Earth’s gravity pulling on them.

Pupils need to know:

• Unsupported objects fall towards the centre of the Earth, pulled down by the force of gravity.
• The downwards force on the object is called its weight and is measured in Newtons.

Background Science for Teachers

Gravity is a force that exists between all objects in the universe that have mass. The force of gravity is a non-contact force that acts at a distance. The Earth’s force of gravity pulls all objects with mass towards it, this makes the objects being attracted accelerate and fall towards the centre of the Earth.

When drawing arrows to represent the force of gravity acting on an object, it is helpful to always draw the arrow coming from the centre of the object that it is acting on and pointing straight towards the centre of the Earth (or the object which it is attracted to, for example a moon or planet).

The force of gravity due to the Earth extends well into space and does not stop, however it does get weaker the further you are from the centre of the Earth. A common misconception is that astronauts working in space appear to be floating around because there is no gravity, when in fact they feel weightless because they are in constant freefall. At the same time, they are moving forwards at very high speed, which keeps them in orbit, always falling towards Earth but never getting any closer. The Earth’s force of gravity is always acting on astronauts and their space craft. This effect can be demonstrated by putting a tennis ball in the foot of an old pair of tights, holding onto the other end of the tights and then swinging the ball round and round. As long as the ball is moving round fast enough it does not get any closer to the person swinging it and it does not fly away as long as the person holds onto the other end of the tights.

The Sun, planets and moons also pull objects with mass towards them with their force of gravity. The bigger the mass of the object, the bigger the gravitational pull it exerts on other objects. The Sun’s gravity is so strong that it attracts all the planets, moons, asteroids, comets and dwarf planets in the Solar system – keeping them all in orbit around it. Our Moon has a much smaller mass than planet Earth and so it has a smaller force of gravity pulling objects with mass towards it, this is why astronauts walking on the moon were able to jump higher than they could on Earth.

It is important to be clear on the difference between mass and weight. Mass can be thought of as the amount of matter in an object – the sum of all the atoms that make it up. Mass is measured in kilograms. The mass of an object is always the same, no matter where the object is. The weight of an object (the downward force acting on it due to the gravitational attraction of the Earth) is proportional to its mass. Near the Earth’s surface, the weight of a 1 kg object is roughly 10 N.

More generally, the gravitational force of attraction between any two objects is proportional to the masses of each of them and decreases with the distance between them. So, the weight of an object will get less with distance from the centre of the Earth and would be different on another planet or on the Moon.

5. Air resistance and water resistance are friction forces that affect objects moving through air and water.

Pupils need to know:

• When objects move through water or air, they must push the water or air out of the way to move.
• This friction between the moving object and the water is called water resistance, it acts in the opposite direction to the movement.
• The same effect happens in different liquids. Some liquids are much harder to move through than water and the resistance will be greater.
• The friction between the moving object and the air is called air resistance, it also acts in the opposite direction to the movement.
• If the moving object has a larger surface area it will have to push more air or water out of the way and so the air resistance or water resistance will be greater.
• If the moving object has a smooth streamlined shape, it will be easier for it to push the water or air out of the way so the air resistance or water resistance will be less.

Background Science for Teachers

When an object moves through a fluid (a liquid or a gas), a drag force acts on it in the direction opposite to its motion. If the driving force acting on an object stops, friction and/or drag forces will make it slow down until it stops. In water, this drag force is called water resistance and in air it is called air resistance.

When an object moves through water there is a special type of friction between the surface of the object and the particles in the water that we call water resistance. Water resistance opposes motion and will slow down a moving object. Objects with large surface areas experience a large water resistance force. Objects with a streamlined shape will experience less water resistance when they move through a liquid.

Some liquids are thicker – we say that they are more viscous. The more viscous a liquid, the more resistance to movement it provides.

When an object moves through air, the particles in the air collide with the surface of the object. The object surface experiences a force from the air and the air experiences a force from the surface. The force of the air on the object’s surface is called air resistance. Air resistance always acts in the opposite direction to the object’s movement. A vehicle with a large flat front such as a lorry will experience a greater air resistance force than a sports car with a streamlined shape.

6. Some simple machines allow a small force to have a greater effect.

Pupils need to know:

• Machines are devices that change the direction or size of a force. 
• There are different types of simple machines which can be found around school and the home such as wheels, levers, ramps, pulleys, screws, and gears.
• Most simple machines reduce the force you need to apply to lift/move a heavy object by making the distance over which you apply the force larger than the distance the object moves. For example, levers and pulleys.

Background Science for Teachers

Simple machines are mechanical devices that can either change the direction or size of a force to make it easier to do work. Very simply they are devices that are designed to reduce the effort force required to perform a simple task and this can be achieved by either the force acting over a greater distance or greater period of time allowing the same amount of work to be done with a smaller force. Screws, levers and inclined planes (ramps) are all designed to increase the distance over which the force acts so we can push with a much smaller effort force.

There are six types of simple machine:

1. Lever
2. Wedge
3. Wheel (including gears and axles)
4. Screw
5. Inclined plane (ramp)
6. Pulley
7. Pneumatic/Hydraulic (Only needed for the Northern Ireland curriculum)

There are three types of levers. The first has the load and effort on opposite sides of a pivot (or fulcrum), for example, a see-saw, crow bar, screwdriver or pliers. The photograph shows a lever comprised of a stiff beam (in this case, a wooden ruler) that turns about a fixed pivot or fulcrum (the pencil) located somewhere along the beam. Movement at one end of the beam results in movement at the other end in the opposite direction. The location of the fulcrum can magnify (or reduce) the force applied at one end by adjusting the distance over which the other end travels.

Two children measuring the movement of a small rocket, balanced upon a plank of wood being used as a lever.

Image credit: The Ogden Trust. Licensed only for use on Explorify.

The second type of lever has the load and effort on the same side as the pivot, for example a wheelbarrow. In the third, the effort is between the load and the pivot, so the effort is greater than the force applied to the load, as in the pair of tweezers or tongs.

A wedge is used to convert a force applied in the direction of the wedge’s movement to an outwards force that acts at right angles to the blade for example in an axe. It is often used to split, cut or lift heavy objects.

A wheel and axle consists of a wheel with a central fixed axle which makes sure that they rotate together. When a small force is applied at the edge of the wheel it is converted by rotation to a larger force at the smaller axle. This effect can be reversed by applying a large force to the smaller axle resulting in a smaller force at the edge of the larger wheel with much greater rotating speed. Gears are an example of a wheel and axle. Each wheel has teeth that can interlock with the teeth on another wheel to create a gear that turns on an axle. Gears are simple machines that can transmit forces from one place to another and spread force over a distance.

A child measuring the force required to move a bottle rocket using rollers.

Image credit: The Ogden Trust. Licensed only for use on Explorify.

A screw is a threaded shaft that when rotated, can be converted into movement in either direction (forwards or backwards) along the axis of rotation depending on the direction of its spiral thread. It changes a rotating movement into a linear movement.

An inclined plane is used to raise or lower heavy objects. The long movement of the object along the slope is converted into a smaller vertical movement. As long as the friction on the ramp is small, a reduced force is needed to raise a heavy object vertically although it must be moved a greater distance along the ramp to achieve this benefit.

A child measuring the force required to move a bottle rocket up an incline.

Image credit: The Ogden Trust. Licensed only for use on Explorify.

A pulley and attached cord allows for a change in the direction of the force applied to an object. A simple fixed pulley enables you to pull down while the load goes up (e.g. a pulley on top of a flagpole for hoisting a flag to a height). It requires the same force, but it is easier to pull down in the same direction as gravity than to pull up. Pulleys can also be used in combinations to increase the size of the effort force.

Complex machines involve a collection of simple machines working together – for example a bicycle or a wheelbarrow.

Hydraulic and pneumatic machines (please note this only needs to be taught in Northern Ireland) is the term used for simple machines which use pressurised liquids such as oil (hydraulic) or air (pneumatic) to create a force to do the work. A pump pressurises the liquid/air and the pressure is moved through the liquid/air through a series of hoses, valves, and cylinders. Pneumatic systems are usually less expensive, lighter in weight, more compact and easier to install and maintain than mechanical machines. Bulldozers, forklifts and hydraulic brakes in vehicles like planes use hydraulic systems to create a force.  They can exert a larger force than a pneumatic system, but they tend to be less precise.

Useful related resources      

     The Ogden Trust: Phizzi focus: Forces

This resource highlights the opportunities to connect the learning of forces with other subjects like mathematics, history and design technology and how to develop the abstract idea of forces.

Ideas to try with your class  

Now you've got the key scientific concepts under your belt, try our ideas to help you explore forces with your class in a way they will understand in part two of this topic guide! 

You can also take a look at the related topic guides for materials and Earth and space. 

Many thanks to the Ogden Trust, who have collaborated with Explorify to produce this 'Tackle the tricky bits' page.

Image credit: Details of wooden rigging pulley and ropes on sail ship by Riishede via Canva