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:
Two protons and two neutrons bound together tightly in an atomic nucleus with glowing energy lines
Strong Force: very strong but short range (10-15 meters, that's only 0.000000000000001 of a meter).
Holds an atom's nucleus together.
A neutron decaying into a proton, emitting an electron and an antineutrino
Weak Force: weak and short range (10-17 meters).
Responsible for radioactive decay and neutrino interactions.

And these two we experience every day:
An electron orbiting a proton with magnetic field lines and electric charges shown
Electromagnetic Force: electricity and magnetism.
Acts over unlimited distances and can be attractive or repulsive.
It holds atoms and molecules together, and also pushes them apart when charges are the same.
The Earth orbiting the Sun, showing a gravitational pull vector arrow pointing toward the Sun
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.

Let's learn more about the last two.

Two wrestlers pushing and pulling against each other in a match

Electromagnetic Forces

Contact forces, friction and air resistance are all due to electromagnetic interactions between atoms!

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

Microscopic view of two surfaces with negative electron clouds repelling each other

Try this right now: Press your hands together as hard as you can. Why don't they pass right through each other?

It is because the electrons in the atoms of your left hand are repelling those in your right hand! You are feeling the electromagnetic force in action.

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

Two jagged surfaces rubbing against each other, showing interlocking microscopic bumps
Two rough surfaces trying to slide past each other.

Without friction we couldn't go anywhere!

A person slipping on an icy sidewalk next to a person walking safely on dry pavement
A bit more friction, please!

But friction also causes wear and tear on mechanical equipment, so it isn't always helpful.

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

Electromagnetic force includes both:

Gravitational Force

An apple falling from a tree branch with a downward acceleration vector of 9.8 m/s²

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

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

An apple with a downward arrow representing the gravitational force acting on it

Imagine you hold the apple. Gravity pulls the apple downward. The gravitational force on the apple is called its weight.

The apple pushes down on your hand, and your hand pushes back up on the apple with an equal-sized force.

Talking About Force

We use special words when talking about force:

A finger pressing against a stone, showing equal and opposite force arrows

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.

Those two forces are equal in size and opposite in direction, and they act on different objects.

Example: a car pushes down on the road, and the road pushes up on the car.

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

See Newton's 3rd Law.

Force can be a push or pull:

A hand pushing a yellow ball forward, with an arrow pointing in the direction of motion
Push

A hand pulling a yellow ball with a string, with an arrow pointing toward the hand
Pull

An object can be in tension or compression:

A yellow ball being pulled from opposite sides, showing outward tension arrows
Tension

A yellow ball being squeezed from opposite sides, showing inward compression arrows
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):

A yellow ball with a curved arrow showing a rotational force or torque applied to it
Moment or Torque

An object twisted by forces is in torsion:

A cylindrical bar with twisting arrows at both ends showing torsion
Torsion (twisting)

Force Physics Index