Forces...tackle the tricky bits

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?

Forces (including magnetism) is a tricky topic because of its abstract nature. Although the effect of forces on objects is observable, the forces themselves are invisible and therefore open to misunderstandings and misconceptions. Lots of play and hands-on investigation is the best approach, which begins with younger children exploring how toys and other objects move and can be manipulated. We have included some ideas for how to teach forces in part two of this topic guide. 

Key scientific concepts: Pupils need to...

1. Recognise and explain the effects of forces acting between moving surfaces: air resistance, water resistance and friction
2. Know that magnetic forces can act at a distance and identify which materials are magnetic
3. Describe magnets as having two poles and predict whether two magnets will attract or repel each other, depending on which poles are facing.
4. Explain that gravity is a force acting between the Earth and all objects
5. Understand how some mechanisms, including levers, pulleys and gears, allow a smaller force to have a greater effect.

1. The effects of forces acting between moving surfaces

Pupils need to know:

• Objects move differently on different surfaces, depending on the forces acting upon them and the materials they are made from
• Some forces need contact between two objects (and some act at a distance)
• Air resistance, water resistance and friction are contact forces that act on moving objects
• Forces can change the speed, direction and shape of an object

Background science

A force is a push or pull on an object that causes a change in its speed, direction or shape. Forces can be divided into:

• contact forces (e.g., friction and air resistance) when two objects (or substances, e.g., air and water) are touching 
•  non-contact forces (e.g., gravity and magnetism) where objects don’t need to touch 

You will see below in Section X, that the force of gravity acts on all objects. A stationary object, such as a book on a table, has the force of gravity acting on it.

This force acts vertically downwards.  The table pushes upwards on the book with a force of equal size.

A moving object, such as a car on a racetrack, has additional forces at work, including:

• The movement of the car’s tyres driven by the engine is pushing it forward.
• The contact between the ground and the tyres is causing friction to slow the car down.
• Air resistance is pushing against its direction of travel, also slowing the car down

The size of each of these forces determines what happens to the car. The car can only start to move forward if the push force (generated by the car’s engine) is greater than the forces of friction. The faster the car travels, the greater the air resistance, which is why race cars are designed to reduce the effect of air resistance by being aerodynamic. 

Like air resistance, water resistance is the force of friction acting on a moving object, but in this case, it is involves water particles rather than air particles. If you go swimming, the friction between your skin and the water particles creates water resistance. The more streamlined the shape of an object (e.g., a torpedo or a seal), the less water resistance is created and the faster it can travel.

2. Magnetic forces act at a distance

Pupils need to know:

• Magnetism does not require direct contact to act on an object
• Materials that are attracted to magnets are called magnetic
• Magnets have two poles (north and south) and opposite poles are attracted, like poles repel each other

Background science

Magnetism, perhaps more than the other forces, is the most difficult for children to fully understand because it occurs ‘at a distance’ without contact. It is important that children recognise the effects of magnetism, but they do not need to explain what it is. 

We can observe the magnetic force between metals such as iron, nickel and cobalt. Children may well come across steel and stainless steel in the classroom. These ferrous metals contain iron and so they are magnetic too.

Not all metals are magnetic. Aluminium, for example, which is used for drinks cans, cannot be magnetised.

Every magnet has two poles, which we call north and south. When the north pole of one magnet is close to the north pole of another magnet, they repel each other because like poles repel. However, if a north pole of one magnet is near the south pole of another magnet, they are drawn together as opposite poles attract.

2. Magnetic forces act at a distance

Pupils need to know:

• Magnetism does not require direct contact to act on an object
• Materials that are attracted to magnets are called magnetic
• Magnets have two poles (north and south) and opposite poles are attracted, like poles repel each other

Background science

Magnetism, perhaps more than the other forces, is the most difficult for children to fully understand because it occurs ‘at a distance’ without contact. It is important that children recognise the effects of magnetism, but they do not need to explain what it is. 

We can observe the magnetic force between metals such as iron, nickel and cobalt. Children may well come across steel and stainless steel in the classroom. These ferrous metals contain iron and so they are magnetic too.

Not all metals are magnetic. Aluminium, for example, which is used for drinks cans, cannot be magnetised.

Every magnet has two poles, which we call north and south. When the north pole of one magnet is close to the north pole of another magnet, they repel each other because like poles repel. However, if a north pole of one magnet is near the south pole of another magnet, they are drawn together as opposite poles attract.

Image credit: GraphicsRF.com via Shutterstock SL 

Some magnets are permanent meaning their magnetism is there all the time and cannot be turned on or off. Other metals can become temporarily magnetic when they are placed close to a magnet (within its magnetic field).

3. Gravity acts on all objects

Pupils need to know:

• Unsupported objects fall to the ground because of the force of gravity

Background science

Definitions:

Mass is the amount of ‘stuff’ or ‘matter’ something is made up of and is measured in Kg.

Gravity is a pulling force that acts between objects.

Weight or gravitational force is the technical termwe use to describe the force of gravityand is measured in Newtons.

The work of Galileo Galilei in the 16th century and Isaac Newton a hundred years later helped to develop the theory of gravitation. Gravity is the force by which all things with mass (including planets, stars, galaxies and even light) are brought toward (or 'gravitate' toward) one another. 

Anything that has mass will experience a gravitational force if it is near another mass (just as a magnet will feel a force if it is placed near another magnet). Objects with more mass such as the Sun in our solar system have a greater gravitational pull and this force does get weaker as you move further way from the mass.

This force of gravity from our Sun keeps all the planets in orbit. Earth's gravitational pull is what keeps you on the ground and what makes things fall. Earth's gravity comes from all its mass, which exerts a pull on all the mass in your body. That's what gives you weight and means that you land back on the ground when you jump.

We could not live on Earth without gravity. The sun's gravity keeps Earth in orbit around it, at a distance that allows us to safely enjoy the sun's light and warmth. It holds the atmosphere and the air we need to breathe around the Earth. Gravity also keeps the moon in orbit around Earth, which uses its own gravitational pull to pull the oceans towards it, creating our tides. 

4. Some mechanisms can cause a smaller force to have a greater effect

Pupils need to know:

• Simple machines use mechanisms (e.g., levers, pulleys and gears) that make it possible for a smaller force to have a greater effect

Background science

Machines, of all shapes and sizes, are mechanical structures designed to help humans perform specific tasks. Most simple machines reduce the force you need to apply to lift or move a heavy object by making the distance over which you apply the force larger than the distance the object moves. Mechanisms like levers, pulleys and gears are common in many simple machines. They can all be used to make certain tasks (e.g., lifting, pushing, pulling etc.) easier by making a smaller force have a greater effect.

• Levers: Made of a bar and a pivot point (called a fulcrum), a lever increases the force applied by the person pushing the lever enabling them to lift a heavy load. Levers allow for a smaller force to be applied over a longer distance. For example, to open a tin of paint you could use a coin or a screwdriver as a levers. If you use the coin, you will need to apply more force than if you were to use a screwdriver because the screwdriver is longer and therefore a more effective lever.
• Pulleys: A collection of one or more wheels over which you loop a rope to make it easier to lift things by multiplying the force being applied. If you have a single wheel and a rope, a pulley helps you reverse the direction of your lifting force. So, you pull the rope down to lift the weight up. The more wheels you have, and the more times you loop the rope around them, the more you can lift. If you have two wheels, for example, you will be able to lift twice as much mass still using the same amount of effort
• Gears: Gears use cogs with teeth to increase the size of the force and also transmit it from one part of a machine to another. The teeth of the cogs interlock. As one gear is turned, it transmits a force to the gear it is locked to, causing it to turn in the opposite direction. If two connected gears are of different sizes, they can increase the size of the turning force. A smaller gear will turn rapidly with less force, whereas a larger gear will turn more slowly with a greater force. For example, if you apply force to a gear with 10 teeth which is connected to a gear with 5 teeth, the second, smaller gear will make two complete revolutions for every revolution that the larger gear makes. A simple example is on a mountain bike where the two sets of gears are connected by a chain. The gears are of different sizes and a cyclist can shift between different combinations of these gears (cog sizes) depending on whether they want pedalling to be easier or harder.

Ideas to try with your class

Now you've got the tricky 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 & space.

Image credit: Three pulleys on the side of a ship by ranjatm via Pixabay