[Physics Class Notes] on Newton's Third Law of Motion Pdf for Exam

In the year 1687, Sir Isaac Newton announced his laws of motion, where he standardized 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 body A puts force [F_{a}] on body B, then B at the same time exerts force [F_{b}] on body A. Moreover, both the forces acting on each body have the same magnitude and are in the reverse direction: [F_{a} = – F_{b}].

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 [F_{a}] and [F_{b}] 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.

Your Earth pulls down on you, as you probably already know. The Earth is also pulling upon you, but you don’t realize it. A gravity force of 500 N is exerted by the Earth on you, similarly, you exert a gravitational force of 500 N on the Earth. The third law of motion explains this remarkable fact.

As Newton argued in his third law, if an object exerts a force on another, it must exert a force of the same magnitude and opposite direction on the object it repels.

The law of symmetry in nature says that force always occurs in pairs, and one cannot exert force on another body without experiencing it. This law is sometimes loosely referred to as action-reaction, where the force exerted is the action, whilst the force experienced is the reaction.

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

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

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

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

1. A rabbit that has a mass of 4 kg jumps upon a table that 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 = kg m/s^{2} ]

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 the same amount of force.

 

Hence, the force applied by this table is:

[F_{g} = F_{N} = ma]

Therefore, [(4 times 10) kg m/s^{2} = 40N.]

Do It Yourself

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

1. Equal, similar

2. Similar, different

3. Equal, opposite

4. Greater, opposite

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

1. Wall pushes against you

2. Wall pushes against this stick

3. Stick pushes against you

4. You push against this stick

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

1. True 

2. 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 app today and access not just detailed study material on this topic, but also to access online interactive classes.