Types of Forces (Edexcel IGCSE Physics (Modular))

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

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