Newton's third law

Newton's third law (N3) tells us about the interaction between two bodies. A casual statement of this law is "all forces have an equal and opposite reaction". However, this throws up a couple of confusing ideas:

Why do objects ever move?
Is a normal force a reaction to an object's weight?

Key Concepts

Newton's Third Law

If body A exerts a force on body B, body B exerts an equal and opposite force on body A.

It is important to note that Newton's third law only applies in the following circumstances:

The forces act on different bodies.
The forces are the same type.
The forces are the same size.
The forces act in opposite directions.

Therefore, body A is perfectly capable of accelerating, as the reaction force does not act on A. And the Newton's Third Law pair to an object's weight is the gravitational force on the Earth. We will consider these misconceptions in more detail below.

Contact forces

The man pushes the block so, according to N3, the block must push the man. For the man to move forwards there must be another force acting, this is the friction exerted by the ground on the feet.

NB: There would also be a reaction force exerted by the man on the ground. However, the Earth has such a large mass that its spin is unlikely to be affected!

Recoil

For the bullet to come out of the gun, there must be an unbalanced force acting on it. If the gun provided this force then there must be an equal and opposite force on the gun.

Because these forces are a Newton's third law pair, they are the same size. Since F=ma:

The small mass of the bullet means that its acceleration is high
The large mass of the gun means that its acceleration is low

The forces are acting on different bodies in opposite directions. This means that momentum is conserved:

Initial momentum = zero
Bullet has momentum to right = gun momentum to left
Final momentum = zero

Non-contact forces

It is a common misconception that the weight and normal force are action and reaction but they aren't. The reaction to weight is the attraction of the Earth to the object.

Essentials

We know from conservation of momentum that an increase in mass must lead to a reduction in speed (assuming no external forces act). If a trucks collects rocks, the rocks gain momentum but truck loses it.

In terms of forces, this occurs because the truck exerts a forward force on the rocks, accelerating them forward. The rocks exert a backwards force on the truck due to N3, slowing it down.