Sir Isaac Newton is famous for his three laws of motion. “Newtons’ law of gravitation” and “Newton’s law of cooling” are also given by him, but people know more about the three Newton’s laws of motion. Although they seem very basic, their application is vast. These three laws of motion form the basis of the whole of mechanics. Rest everything is just math.

Newton’s laws of motion list

  1. Law of inertia or Newton’s first law of motion
  2. F=ma or Newton’s second law of motion
  3. Newton’s third law of motion or the law of equal and opposite reactions

Newton’s first law of motion

Statement: It states that a body remains at rest or in constant motion if the net external force acting on the body is zero. So basically, a body maintains its motion or non-motion provided it is not disturbed by an external agent. It is Newton’s law 1.

Formula: If net external F=0, then a=0. (Here F= force, a= acceleration)

Newton’s first law examples: 

1A. On a funnier note, a sleeping person wants to keep sleeping unless someone wakes him up, and a person looking at their mobile phone continues to look at their mobile phone unless someone stops them from doing so. An external agent is needed to change the course of inaction or action. (Jokes apart, this is not an example of Newton’s first law of motion, but it is very close to it)

1B. Now, let us take another example to understand it better. If you are sitting in the car and the driver presses the brake padel suddenly, your upper body keeps going forward even though your lower body which is almost fixed on the seat, stops. So the whole body tends to keep moving forward as it was initially moving forward, but the sudden braking stops the car, and hence the seat attached to the car and your lower body which is almost fixed on the car seat, also comes to rest.

1C. Continuing with the example above, let us consider that you are sitting in the car, initially at rest. The driver presses the accelerator firmly; hence the car+seat+your lower body starts moving forward along with the car. Your upper body, though, still wants to be at rest, and therefore it appears to us that the upper body moves backward. 

1D. You might have seen a 100 m sprint. After they reach the finishing line, the athletes keep moving forward even though the race is over and they can stop abruptly. But they can not do it because of the inertia (or the momentum), and they can not stop suddenly and can only slow down over a distance. 


Newton's laws of motion cartoon image courtsey


Newton’s second law of motion

Statement: The acceleration of the body times its mass is equal to the net external force applied on it, and it is Newton’s law 2.

Formula: Net external F=ma ( F= force, m=mass, a=acceleration)

Newton’s second law examples: 

2A. Let the force applied be F= 10 N, the mass of the body=m=2 kg. What will be the acceleration?

Answer: Since F=ma as per Newton’s 2nd law of motion, so 10=2a, that means a=10/2=5 m/s^2

2B. A racing car starts from rest and reaches a velocity of 10 m/s uniformly in 2 seconds. If the car’s mass is 2000 kg, find the net force acting on the car.

Answer: Using V=u+at ( kinematics formula for constant acceleration), we get 10=0+a*2

That gives us a=5 m/s^2. So F=ma=2000*5=10000 N

Newton’s third law of motion  

Statement: A force can never act in isolation. Forces always exist in pairs. If a force F acts on a body by an external agent, then the same force F acts on the external agent by the body. The directions of these two forces are exactly opposite to each other. It is called Newton’s law 3.

Formula: F12=-F21

Here F12 is the force acting on body 1 due to another body 2. 

F21 is the force acting on body 2 due to body 1.

Even though their magnitudes are the same, their directions are opposite to each other. That is why the negative sign!

Newton’s third law examples:

3A. If Natasha hits her younger brother Ali with a force of 10 N, then Ali also hits back to Natasha with the same force of 10 N. Ali did not do that intentionally when Natasha hit him, but it is automatic. When her hands touched Ali’s body, in response, Ali’s body exerted the same 10 N force BACK on Natasha. There is no way Natasha can hit Ali with 10 N force without facing the same 10 N force back from Ali, and it is “impossible!”

3B. When a gun is fired, the bullet moves forward very high speed. Wherefrom the bullet got the force? It got the force from the gun. So when the Buller got a forward force from the gun, the gun got a backward force from the bullet. It is called recoil. This recoil is so strong that when a novice fires a bullet, he falls back if he is not alert. See this YouTube video to know about recoil.

So this was all about Newton’s laws of motion. Thanks for reading this informative blog post. 

Mechanics is taught in the first semester of your college education, and Newton’s 3 laws are the backbone of the Mechanics portion of Physics.

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