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Newton’s First Law

Aug 22, 2022

Key Concepts

  • Inertia
  • Mass


In this session we are going to learn about inertia and mass and differentiate between them. 


Aristotle’s fallacy: 

Aristotelian law of motion states that “an external force is required to keep a body in motion”. 

Example: A toy cart is at rest. When a boy pulls the handle attached to the cart then the cart comes into motion and when the boy releases the handle the cart comes at rest. 

The fallacy of Aristotelian law of motion:  

It was proved wrong by Galileo Galilei and Isaac Newton.  


According to Aristotelian law, a force is required to keep a body in uniform motion. But no force is required to move a body in uniform motion. 

Galileo’s Observation: 

Galileo Observed that a smooth ball rolling on a smooth surface would roll to the height from which it was rolled, regardless of the angle of incline of the ramp. He reasoned that it must behave this way because whatever caused it to accelerate down the slope also caused it to decelerate up the ramp. 

Galileo's observation 

He concluded that: 

  • The state of rest and the state of uniform linear motion are equivalent. No net force is acting in both cases.  
  • Therefore, there is no need to apply an external force to keep a body in uniform motion.  
  • An external force is applied to encounter frictional or any other force so that the forces are balanced.  

Newton’s first law of motion 

Everybody continues to be in its state of rest or in uniform motion in a straight line unless and until compelled by an external force to change its state of rest or of uniform motion. 

In simple words, if the net external force on a body is zero, its acceleration is zero. Or acceleration can be non-zero only if there is a net external force on the body. 

This is also known as the law of inertia. 



An object at rest prefers to stay at rest and an object in motion prefers to stay in motion. 

The tendency of an object to stay in the original state of motion it is in is known as Inertia. 

It is the natural tendency of an object to resist any change in its state of motion. 

Tendency to stay at rest 
 Tendency to stay in motion 


The measure of inertia is mass. 

The heavier the object is, the greater is its inertia or resistance to change to its state of motion. 

Difference between inertia and mass

Questions and answers 

Question 1: The bucket filled with sand is more difficult to push. Why?  

Question 1 figure 


The bucket filled with sand has more mass as compared to the empty bucket. As the mass increases the inertia also increases and thus the resistance to the state of rest. This is the reason why it is more difficult to lift the bucket filled with sand.  

Question 2: What is the relationship between mass and inertia? 


As the mass increases, inertia increases.  

As the mass decreases, inertia decreases. 

Question 3: List the difference between inertia and mass. 


Answer 2 figure 


  • The tendency of an object to stay in the state of motion it is in is known as Inertia.
  • The measure of inertia is mass.
  • As the mass increases, the inertia increases.
  • Newton’s first law is known as law of inertia.


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