Types of Forces (Edexcel IGCSE Science (Double Award)): Revision Note
Types of forces
A force is a push or pull that arises from the interaction between objects
There are lots of different types of forces that occur when objects interact
What are the different types of forces?
Gravitational force (weight)
There is a gravitational force of attraction between all objects with mass
The more massive the object, the greater the gravitational force exerted by it
When a football is thrown (or kicked), the gravitational pull of the Earth on the football pulls it toward the (centre of the) Earth
Reaction force
When an object rests on a surface, the surface exerts a push force on the object
This reaction force acts at right angles (perpendicular) to the surface
When a football rests on the horizontal surface of the grass, the grass exerts a push force (reaction force) vertically upwards on the football
Reaction force acting on a football

The push force exerted on an object by a surface is called the reaction force
Friction
Frictional forces always oppose the motion of an object, causing it to slow down
Friction occurs when two surfaces move over one another
When a box is pushed across a carpet, the carpet exerts a frictional force on the box, slowing its motion
Drag force
Drag force is a type of frictional force that occurs when an object moves through a fluid (a gas or a liquid)
The particles in the fluid collide with the object moving through it and slow its motion
When a pebble is thrown into water, the water molecules flow against its solid surface, causing it to slow down
Air resistance
Air resistance is a specific type of drag force and is therefore also a frictional force
Air resistance occurs when particles of air collide with an object moving through it and slows its motion
When a skydiver opens their parachute, air resistance opposes their motion and reduces their speed so it is safe to land
Air resistance acting on a skydiver

A skydiver uses air resistance to reduce their speed so that they can land safely
Thrust
Thrust is a force produced by an engine that speeds up the motion of an object
The engine of a car exerts a thrust force and increases its speed
Upthrust
When an object is fully or partially submerged in a fluid, the fluid exerts an upward-acting push force on the object
A boat floats on a lake due to the upthrust exerted by the water on the boat
A ball held underwater will shoot upwards when released due to the upthrust exerted by the water pushing it back to the surface
Upthrust acting on a boat

The water pushes up on the boat, this is the force of upthrust
Electrostatic force
There is an electrostatic force between two objects with charge
Like charges repel one another, and opposite charges attract one another
When an electron gets close to a positively charged ion, the ion exerts a pull force on the electron (attraction)
When an electron gets close to another electron, the electrons experience a push force from one another (repulsion)
Electrostatic forces acting on charged particles

Like charges experience a repulsive push force and opposite charges experience an attractive pull force
Magnetic force
There is a magnetic force between objects with magnetic poles
Like poles repel one another, opposite poles attract one another
When a north pole gets close to a south pole, they experience a pull force from one another (attraction)
When a north pole gets close to a north pole, they experience a push force from one another (repulsion)
Magnetic forces acting on magnets

Opposite poles experience an attractive pull force, and like poles experience a repulsive push force
Tension
Tension occurs in an object (like a rope or spring) that is stretched
When a pull force is exerted on each end of an object, tension acts across the length of the object
When two people pull a rope in opposite directions, tension acts along the rope and pulls back on each person
Tension force in a rope

Two people exert a pull force on the rope from each end, creating tension in the rope
Examiner Tips and Tricks
The force of gravitational attraction on an object is called its weight. Remember not to refer to this force as simply 'gravity', as this term can mean several different things, and examiners will probably mark it as wrong.
Similarly, when referring to air resistance, avoid using terms like 'wind resistance' (there is no such thing!) or 'air pressure', which is a different concept. Drag is an acceptable alternative to the force of air resistance because air resistance is a special type of drag.
Effects of forces
When a force acts on an object, the force can affect the object in a variety of ways
The object could:
change speed
change direction
change shape
The effects of forces on an object often depend on the type of force acting
The push force (thrust) of an engine can cause a car to speed up, whilst the force exerted by the brakes (friction) can cause it to slow down
The gravitational pull of the Sun on a comet causes the comet to change direction
When two opposing forces push on each end of a spring, the spring changes shape (it compresses)
The effects of different forces on objects

Forces can change the speed or direction of motion of an object, or even change its shape
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