# Types of Force

Force is a push or pull.

## 4 Fundamental Forces

There are only four fundamental forces in the Universe.

 These two happen at the atomic level: Strong Force: very strong but short range (10-15 meters, that is only 0.000000000000001 of a meter). Holds an atom's nucleus together. Weak Force: weak and short range (10-17 meters). Responsible for radioactive decay and neutrino interactions. And these two we experience every day: Electromagnetic Force: electricity and magnetism. Unlimited range. Can be attractive or repulsive. Holds atoms and molecules together, and keeps them apart from each other. Gravitational Force: the attraction of all matter and energy. Unlimited range. Is very weak, but always attractive, so it adds up on a grand scale and makes the structure of our Universe.

## Electromagnetic Forces

Contact forces, friction and air resistance are all due to this force!

Electrons on one surface repel electrons on the other surface. This leads to contact force:

Friction is the force that pushes back when things try to slide past each other

Two rough surfaces trying to slide past each other.

Without friction we couldn't go anywhere!

But friction also causes wear and tear on mechanical equipment, so it is not always helpful.

Air resistance (also called drag) is a type of friction. It is greater at higher speeds.

Electromagnetic force also includes magnetism, which acts at a distance: like charges repel each other and unlike charges attract.

## Gravitational Force

Gravity is the attraction of objects with mass or energy towards each other.

It can, for example, make an apple fall to the ground:

Imagine you hold an apple against gravity: the force from the apple is called weight.

We use special words when talking about force:

Reaction Force: the force that pushes back when another force is applied.

Pushing on this stone gets a reaction force in return.

An Action Force gets a Reaction Force.

The weight of a car gets a reaction force from the road.

Force can be a push or pull:

Push

Pull

An object can be in tension or compression:

Tension

Compression

Tension or compression can be important! For example concrete is very strong in compression but breaks easily in tension, so they often put steel inside concrete to give it strength in tension.

A turning force (which is force times distance) is called a torque (or moment):

Moment or Torque

An object twisted by forces is in torsion:

Torsion (twisting)