Introduction
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:
A person Walking on the Road
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.
Swimming
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.
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
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
Newton's third law mentions that any action will have _______ and ______ reaction.
Equal, similar
Similar, different
Equal, opposite
Greater, opposite
Say, you use a stick to hit a wall. Here, an equal but opposite reaction is _____
Wall pushes against you
Wall pushes against this stick
Stick pushes against you
You push against this stick
Friction and gravity, both are examples of a force.
True
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.
FAQs on Newton's Third Law of Motion
1. What are three examples of action-reaction force pairs in day-to-day life?
Three examples of Newton's 3 laws 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?
The other name for Newton's third law of motion is the law of action and reaction.
3. Why is Newton's Third Law essential in the creation of thrust?
With regards to the 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.
4. How does Newton's third law apply to other phenomena?
The third law of Newton is evident in numerous other situations. She exerts a force backward on the floor as she paces in front of the whiteboard. This reaction force causes the professor to accelerate forward due to the effect of the floor.
In a similar fashion, a car accelerates when its drive wheels are pushed backward by the ground on which they are mounted. A gravel road and spinning tires can cause the wheels to push back as they throw rocks backward.
Rockets, for instance, propel themselves forward by exhaling backward at high velocity. Therefore, the rocket in the rocket combustion chamber exerts a large backward force on the gas, which, in turn, exerts a large reaction force forward on the rocket. The thrust is the result of that force. Rockets propel themselves by pushing against the ground or air behind them, which is a common misconception. Vacuum is actually a better place for them to operate since combustion gases can more readily escape.
5. The third law of motion can be applied to lying in bed, but not to lie in bed?
You push on your bed and the bed pushes on you when you lie on your bed. Moreover, both the earth's gravity and your gravitational force are pulling on each other simultaneously. The 3rd law is at work here, too.
There must always be the same type of third law pair. You and your bed must always have the same type of third law pair. Earth's gravitational pull and your planetary gravity.
We can ask another question now: why are you stationary while lying on your bed. That's because the force exerted by the bed on you matches your weight. In this case, there are two types of forces - both contact forces and gravitational forces. Since the sum of these forces is zero, your acceleration is zero. The formula F = ma is Newton's second law.
