Contact and non-contact forces

In a nutshell

A force is a pull or push that acts on an object when it interacts with another object. Forces are measured in newtons ( $N$ ). There are two types of forces, contact forces and non-contact forces.

Contact forces

A contact force acts between two objects that are in physical contact, the objects must be touching each other for this type of force to occur. There are many examples of contact forces in day-to-day life.

Type of contact force	Description
Friction	This force acts between two objects that are sliding against each other. It acts along the touching surface between the objects and prevents them from moving against each other.
Air resistance	This force acts on an object that is moving through the air. It acts in the opposite direction to that in which the object is moving, so it slows down the object.
Tension	This force is a pulling force that acts on an object with a string, tube, or rope attached. If an object is hanging by a string or rope, then the tension acts in the opposite direction of the weight of the object.
Normal reaction force	If an object is pushing on a surface, the surface pushes back with a balancing force called the normal reaction force. This force acts on the object, in a direction that is perpendicular to the surface that the object is in contact with.
Thrust	This force is a driving force that causes objects, like cars, to move. In the case of a car, the thrust is exerted by the engine.
Upthrust	This force acts upwards on an object in a liquid or a gas, it pushes the object up. This is why boats stay afloat, and why airplanes can stay in the sky.

Non-contact forces

A non-contact force acts between two objects that are not in physical contact. In other words, the objects do not need to be touching for them to interact with one another.

Type of non-contact force	Description
Gravitational force	This force always pulls two objects closer together, it never pushes them apart. The force is greater when the objects are closer together and if the objects have more mass. A mass will only experience a gravitational force in a gravitational field.
Magnetic force	This force pulls two magnets closer together or pushes them apart. The north pole of a magnet is attracted to the south pole of another magnet, so the magnetic force pulls them together. The two north poles, or two south poles, from a pair of magnets repel one another so the magnetic force pushes them apart. A magnet will only experience a magnetic force when it is in a magnetic field.
Electrostatic force	This force pulls two charges closer together or pushes them apart. Positive and negative charges are attracted to one another so the electrostatic force pulls them together. Two charges that are both positive or both negative repel one another and the electrostatic force pushes them apart. A charge will only experience an electrostatic force when it is in an electric field.