 # Newton's Third Law of Motion

In the year 1687, Sir Isaac Newton announced his laws of motion, where he standardised how large objects move due to the effect of forces applied externally. Moreover, the third law of motion, also known as the law of action and reaction proved to be the most significant one.

### Law Of Action And Reaction or Newton's Third Law Of Motion Definition

This law says that every action has an equal and opposite reaction. For example, if a body A puts force Fa on body B, then B at the same time exerts force Fb on body A. Moreover, both the forces acting on each body have the same magnitude and in reverse direction: Fa = - Fb.

Furthermore, in some instances, direction and magnitude are decided entirely by one body between these two. For example, consider object A is putting force on object B. Then, the force acting on object B is "action", and this opposite force on object A is "reaction." As said before, this law is also referred to as action-reaction pair law where Fa and Fb are action and reaction, respectively.

However, in some cases, both bodies together determine direction and magnitude. In this scenario, it is not relevant to point out which force is "action” and which one is "reaction." Moreover, action and reaction occur concurrently, and both of them belong to a sole interaction, and none of them occurs without each other.

Additionally, action-reaction pair law is seen in everyday life. Take a look at some Newton's third law of motion examples below:

• A person Walking on the Road

When a man or woman is out on a stroll, he/she exerts force in a backwards direction on the ground. On the other hand, the ground puts a forward directional force to the man/woman that makes him/her move forward.

• Swimming

Another Newton's 3rd law example is when an individual is swimming, he/she pushes the pool water backwards. In return, the water pushes the swimmer in a forward direction, which makes him/her proceed.

• Flying Bird

Agreeing with Newton's third law, a bird pushes air downwards with the help of its wings. Conversely the air helps the bird to move in the upward direction.

### Solved Numerical

(i) A rabbit which has a mass of 4 kg jumps upon a table which has a mass of 25 kg. As this rabbit walks across this table, what is the average force which is applied  by the table on the rabbit? Given g = 10 m/s²

Solution. The applying force between this given table and a rabbit is entirely dependent on the mass of this rabbit. As a result, the mass of this table is unimportant. Moreover, this force exerted by the rabbit is nothing but its own weight. According to the law of action reaction definition, this table also applies a same amount of force. Hence, force applied by this table is:

Fg = FN = ma

Therefore, (4 X 10) kg m/s² = 40N.

### Do It Yourself

1. Newton's third law mentions that any action will have _______ and ______ reaction.

(a) Equal, similar

(b) Similar, different

(c) Equal, opposite

(d) Greater, opposite

2. Say, you use a stick to hit a wall. Here, a equal but opposite reaction is _____

(a) Wall pushes against you

(b) Wall pushes against this stick

(c) Stick pushes against you

(d) You push against this stick

3. Friction and gravity, both are examples of a force.

(a) True (b) False

Besides these above-mentioned 3rd law of motion examples, there are plenty of other instances where the law of Physics action reaction is applicable. Download our Vedantu app today and access not just detailed study material on this topic, but also to access online interactive classes.

1. What are Three Examples of Action Reaction Force Pairs in day to day Life?

Ans. Three examples of Newton's 3 law of motion in day to day life are that of hitting a brick wall, jumping, walking.

2. What is the Other Term for Newton's Third Law of Motion?

Ans. The other name for Newton's third law of motion is law of action and reaction.

3. Why is Newton's Third Law Essential in Creation of Thrust?

Ans. With regards to third law – every action has an opposite and equal reaction, and it demonstrates a rocket engine's creation of thrust. Hence, it has great importance in aerospace engineering of launching a rocket, which also serves as one of the best Newton’s third law of motion examples.