Newton’s First Law of Motion

Key Concepts

• Aristotle’s fallacy
• Galileo’s observation
• Newton’s First Law of Motion

Introduction: 

Luggage is tied to a bus that is moving on a road so that it does not fall. When a person is taking a turn towards the right he will tilt towards his left. In this session, we are going to learn about why this happens. 

Explanation: 

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. However, no force is required to keep a body in a 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. 

He concluded that: 

• The state of rest and the state of uniform linear motion are equivalent. No net force is acting on the body 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 the 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 uniform motion in a straight line unless and until compelled by an external force to change its state of rest or 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. 

For example: 

1. When a bus or a train starts suddenly, the passenger standing inside it, tends to fall backward. 

2. The dust particles fly off when we beat a carpet with a stick. 

3. The passengers fall forward when a fast-moving bus stops suddenly. 

4. Some luggage is usually tied with a rope on the roof of buses. 

5. When a car moving on a road takes a sharp turn towards the right, the passenger tends to fall towards the left. 

Questions and answers 

Question 1: Explain Newton’s first law of motion when a bus or a train starts suddenly. 

Answer: 

According to Newton’s first law of motion, Everybody continues to be in its state of rest or of uniform motion in a straight line unless and until compelled by an external force to change its state of rest or uniform motion.  

When a bus or a train starts suddenly, the passenger standing inside it tends to fall backward. 

Question 2: Explain Newton’s first law of motion when we beat a carpet with a stick. 

Answer: 

According to Newton’s first law of motion, Everybody continues to be in its state of rest or of uniform motion in a straight line unless and until compelled by an external force to change its state of rest or uniform motion  

The dust particles fly off when we beat a carpet with a stick. 

Question 3: Explain Newton’s first law of motion when a bus or a train stops suddenly. 

Answer: 

According to Newton’s first law of motion, Every body continues to be in its state of rest or of uniform motion in a straight line unless and until compelled by an external force to change its state of rest or uniform motion  

The passengers fall forward when a fast-moving bus stops suddenly. 

Question 4: Explain Newton’s first law of motion when a bus or a train takes a sharp right turn. 

Answer: 

According to Newton’s first law of motion, Every body continues to be in its state of rest or of uniform motion in a straight line unless and until compelled by an external force to change its state of rest or uniform motion  

When a car moving on a road takes a sharp turn towards the right, the passenger tends to fall towards the left. 

Summary

1. Aristotelian law of motion states that “an external force is required to keep a body in motion”. Galileo proved him wrong,
2. Galileo concluded that ‘the state of rest and the state of uniform linear motion are equivalent”. No net force is acting in both cases.

3. Newton’s first law of motion- “Everybody continues to be in its state of rest or of uniform motion in a straight line unless and until compelled by an external force to change its state of rest or of uniform motion.”