The Big Idea
Forces are pushing and pulling at everything in our Universe. Even as we sit in our classroom, the walls
and the ceiling are pushing and pulling at each other, while gravity and friction hold us in our seats. Let's
find out more about forces!
Explaining The Theme
The Big Picture
A force is a ‘push’ or a ‘pull’ or a combination of them. These pushes and pulls create movement and
changes of speed or direction; they can also stop movement. We cannot actually see a force, but we can
experience its effects or results. The greater the force, the greater effect it has. Forces act in opposition
to one another. If one force is greater than another, an object will move in that direction. Forces include
gravity, friction, magnetic and electrical forces. Forces can be measured in ‘Newtons’ (N) using a force
meter. Newton devised three laws of motion which form the basis of theories of movement.
Newton’s First Law - An object at rest will remain at rest unless an outside force acts on it. If an
object is moving, it will continue to move in a straight line at the same speed until an outside force
acts on it.
Newton’s Second Law - A force acting on an object causes it to accelerate in the direction of the
force; the amount of acceleration depends upon the size of the force and the mass of the object.
Newton’s Third Law - For every action there is an equal and opposite reaction.
Meaning that for every force there is a reaction force that is equal in size, but opposite in direction.
Whenever an object pushes another object it gets pushed back in the opposite direction equally hard.
Gravity results from an attraction between masses, which causes them to pull towards one another. The
effect of this, because of the relative sizes of the Earth and objects on it, is that the objects are pulled
towards the centre of the Earth. They are stopped by the Earth’s surface. In space, the force of gravity is
less. This is experienced as ‘weightlessness’. Weight results from the force of gravity acting on a mass
(the volume and material of which an object is made).The mass of an object will not affect the speed that
the object falls as it is determined by the pull of gravity. Two objects of different masses will fall at the
Friction is the force that slows down moving objects. When friction is high, surfaces cannot move easily
over one another. Friction acts in the direction opposite to that in which the object is moving. Drag is a
type of frictional force. When an object pushes through a liquid or a gas, that liquid or gas pushes back on
the object. This is called drag. Streamlining reduces drag.
Magnetism results from the attraction that metals have to one another. Iron, nickel and cobalt are
naturally magnetic. Magnets can make materials they attract magnetic. Magnets have two ends or poles.
One of these ends will be attracted to the North Pole of the Earth, while the opposite end of the magnet
is attracted to the South Pole. Electricity can be used to create temporary magnets.
Two magnets with different poles will pull (attract) each other.
Flight and ‘uplift’
Most things that can fly have wings. Wings are aerodynamically designed to cut through the air. The air
flowing around the wing creates an upward force called ‘lift’ which pushes the wing upwards. Aircraft
wings have an arched shape called an ‘aerofoil’. Air moving over the aerofoil creates uplift – the faster
the aircraft moves, the greater the uplift.
The faster moving air above the wing has a lower pressure than the slower moving air below the wing.
The higher air pressure forces the wing upwards. A kite works in a similar way to a wing, i.e. the air
underneath the kite pushes it upwards.
Aircraft speed along the runway until the lift they produce exceeds their weight and they can fly. Birds
flap their wings to produce lift; once up in the air they can glide by holding out their wings. When birds
fly level their weight equals the lift they have achieved. They descend to the ground when they slow and
their lift decreases.
Gliders don’t have engines so they are towed until they are travelling fast enough to lift off the ground.
Then the pilot flies the glider at a downward angle to produce enough speed for the wings to lift.
Floating and ‘upthrust’
When the ancient Greek scientist Archimedes got into a bath of water, it overflowed. It was this
experience that helped him to explain why some objects float and others sink. That is, if the weight of an
object is more than the upward force (‘upthrust’) of the water displaced then an object will sink. If it is
less, it will float.
Heavy rocks, stones and solid pieces of metal will sink but hollowed-out metal objects such as ships and
boats will float. Submarines have tanks that can be filled with water or air to vary their weight. When the
tanks are filled with water a submarine will dive and sink but when filled with air it will float back up to
Aerodynamics – how an object moves through the air
Buoyancy – a force that pushes an object up when it is in a liquid or a gas
Drag – a force that acts on an object as it travels through a liquid or a gas
Force – a push or a pull; when an object begins to move a force must have started it
Friction – a force that slows objects when they meet each other; a smooth surface has little friction
Gravity – a force that attracts all objects to each other; on Earth gravity pulls objects towards the ground
Lift – an upward force acting on an aircraft wing when it moves through the air
Magnetic force – a force that attracts or repels an object
Mass – on Earth, this is the weight of an object in grams, kilograms or tonnes; objects in space have mass
but are weightless
Upthrust – the force of water pushing up against floating objects
Weightlessness – objects are weightless in an atmosphere that has no gravity