1st Law of Motion

Inertia and Newton's first law of motion

Inertia

Inertia is the property of a body to resist any change in its state of motion or rest.

It depends on the mass of the body. The greater the mass, the greater the inertia. This means heavier objects are harder to start or stop moving than lighter ones.

Types of inertia

1. Inertia of rest: The tendency of a body to remain at rest unless acted upon by an external force. Example: A book on a table remains at rest until pushed.
2. Inertia of motion: The tendency of a moving body to continue moving with the same speed and in the same direction unless a force changes that state. Example: A passenger continues to move forward when a car stops suddenly.
3. Inertia of direction: The tendency of a body to keep moving in the same direction unless a force causes a change. Example: Water in a bucket moving in a circular path continues in a straight line if the bucket is suddenly stopped.

Newton's first law of motion

Statement: "An object will remain at rest or move with uniform velocity in a straight line unless acted upon by a net external force."

This law is also called the Law of Inertia.

Mathematical implication

According to Newton's first law, when the net external force $F = 0$, then:

$a = \frac{F}{m} = 0 \Rightarrow \text{velocity remains constant}$

Where:

$a$ = acceleration

$F$ = net external force

$m$ = mass of the body

Thus, in the absence of an unbalanced force, the acceleration is zero, and the object remains in uniform motion or at rest.

Examples to support the law

1. A person standing in a bus falls backward when the bus suddenly moves forward. This is due to the inertia of rest.
2. When a moving car stops suddenly, passengers tend to lurch forward. This is due to inertia of motion.
3. Water droplets fly tangentially off a spinning umbrella when it is stopped suddenly. This is due to inertia of direction.

Newton's First Law defines the natural tendency of objects to resist changes in their state of motion, a concept which is known as inertia. An external unbalanced force is always required to change the motion of an object.

The two parts are summarized in the following diagram.

Applications of Newton's first law of motion

Also known as the Law of Inertia.

i. Seat belts in cars

Application: The seat belt in a car keeps passengers in place when the car suddenly stops.

Explanation: When a car suddenly halts, the body of the passenger tends to continue moving forward due to inertia. The seat belt exerts an external force to stop the passenger from moving, ensuring safety.

Example: In a collision, the car stops, but without a seat belt, the passenger's body would continue moving forward until something (like the dashboard) stops it.

ii. Space travel

Application: Objects in space move at a constant velocity unless acted upon by a force.

Explanation: In the vacuum of space, there is no friction or air resistance to slow down an object, so spacecraft continue moving at constant speeds unless a force, such as a rocket's propulsion, changes their motion.

Example: Once a spaceship is launched, it will keep moving in the same direction at the same speed unless acted upon by an external force like gravity or a fuel burn.

iii. A book on a table

Application: An object at rest stays at rest unless acted upon by an external force.

Explanation: A book sitting on a table will not move unless someone applies a force to it, such as pushing it.

Example: If you place a book on a table, it will stay there until you push it or apply some force to make it move.

iv. Passenger in a bus

Application: A passenger inside a moving bus tends to stay in motion when the bus suddenly stops.

Explanation: The passenger's body wants to maintain its motion (inertia), so when the bus halts abruptly, the passenger continues moving forward until restrained by the seat or another object.

Example: If the bus stops suddenly, a standing passenger will fall forward due to their body's inertia.

v. Driving a car at constant speed

Application: When a car drives at a constant speed on a smooth road, no external force is needed to maintain that speed.

Explanation: According to Newton's First Law, the car will continue moving at the same speed in the same direction unless forces like friction or air resistance act upon it.

Example: A car coasts along a flat road at a steady speed if no forces like friction or resistance are acting on it.

vi. Sports: a ball at rest

Application: A ball that is resting on the ground will remain at rest until a force is applied to it.

Explanation: The ball will not move unless acted upon by a force, such as being kicked or thrown.

Example: A soccer ball sitting still on the ground will remain stationary until a player kicks it, applying an external force.

vii. Objects in a moving train

Application: If a train is moving at constant speed and a ball is rolled inside, the ball will move in the direction of the train's motion.

Explanation: Inside the moving train, the ball will keep moving in the direction of the train's motion due to inertia, unless acted upon by an external force like the train's walls.

Example: If a ball is rolled on the floor of a moving train, it will continue moving with the same velocity as the train unless friction or the train's walls stop it.