What is the 3rd of Newton's laws?

When two objects interact, the forces they exert on each other are equal and opposite.

When two objects interact with one another, they apply a force to each other that is of equal magnitude but opposite direction

The third of Newton's law states that action and reaction are equal but opposite

Newton's 3rd law is as follows: All forces come in pairs. These forces are:

1. the same type of force (e.g. both are gravitational)
2. have the same size (magnitude)
3. act in opposite directions to one another
4. act at the same time,
5. act on different objects

The 'action = reaction' referred to is this pair of forces that must fit all of these 5 points to be considered an action-reaction pair.

A common mistake is thinking that a book lying on a table has weight and normal reaction as its action-reaction pairs. This is incorrect because a) both forces act on the same object (the book) and b) these are not the same type of force - normal reaction is electromagnetic whereas the weight of the book is gravitational. The fact that both of these forces have the same magnitude and act in opposite directions is an example of equilibrium instead.

But all forces come in pairs so what are the other 2 forces that do make up the action-reaction pairs? Well the first is the gravitational pull from the Earth, this forms the action-reaction pair with the weight of the book. Secondly, the normal reaction of the book onto the table with the normal reaction of the table onto the book forms the other action-reaction pair.

For every action there is equal and opposite reaction

The 3rd law of Newton states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The third law is also known as the law of action and reaction.

When two objects interact, they exert forces on each other which are equal in magnitude and opposite in direction

For every action (force) there is an equal and opposite reaction (force). That means equal in size/magnitude and opposite in direction. This is the simplest way to remember. For example, if a jet engine pushes exhaust gas backwards with a force of 5,000 N then the exhaust gas will push the engine forwards with a force of 5,000 N.

However there are rules (certainly at A level) that need to be applied to find a Third Law pair of forces:

1. Act on the same body
2. Same type of force
3. Same magnitude
4. Acting in the same plane
5. Opposite in direction

As such the reaction force of a book on a table is not a third law pair with the weight of the book. There are a few forces but one pair would be the gravitational force of the Earth on the book (weight of the book) is paired with the gravitational pull of the book on the Earth.

For every action, there is an equal and opposite reaction.

Some people get this confused, these two forces are not both acting on the same object, your weight force and the reaction force from the ground do not count as an equal and opposite pair.

Sometimes the reaction force from the ground might be smaller or greater than your weight force, for example when you are in a lift.

An example of an equal and opposite pair is the reaction force the ground exerts on you, and the force you exert on the ground.

Newton's third law refers to the forces two different objects exert on each other.

For every action, there is an equal and opposite reaction

The third law is known as the law of action and reaction. It states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.

Every action has an equal and opposite reaction.

i.e if object A exerts a force on object B, then object B must exert a force of equal magnitude and in the opposite direction to object A.

Hi Reginald,

Newton's 3rd Law of Motion states that for every force there is an equal and opposite and force in nature.

This means that if there are to objects, object A and object B, and object A exerts a force of 60N on the right onto object B, then object B must exert an equal force of 60N in the opposite direction.

Every force has an equal and opposite reaction. (Easier way might be to think momentum is always conserved)

Formally known as; When Body A exerts a force upon Body B, Body B will exert an equal and opposite force upon Body A

A more commonly known phrase is; "every action has an equal and opposite reaction"