Newton’s First Law of Motion

Newton’s First Law of Motion, also known as the Law of Inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. This law explains the natural tendency of objects to resist changes in their state of motion. It forms the foundation of classical mechanics and helps in understanding the behavior of objects in motion or at rest.

1.0Statement of Newton’s First Law

• If a body is at rest, then it remains at rest and if it is moving with constant velocity then it continues to move with constant velocity until or unless it is acted upon by an external force. (or)
• Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.

To change the state of motion of a body an external force is necessary. There are two states of motion of a body.

(1) State of rest (v = 0, a = 0)

(2) State of uniform motion (v≠  0, a = 0)

If the vector sum of all the forces acting on a particle is zero, then and only then the particle remains

unaccelerated (i.e., remains at rest or moves with constant velocity).

2.0Galileo's Inclined Plane

Galileo’s key insight came from observing a ball rolling down one incline and up another. He noticed that:

• The ball always tried to reach the same height.
• When the second incline was shallower, the ball traveled further.

 Galileo deduced that on a flat surface with no friction, the ball would roll forever — the core idea Newton later formalized.

3.0Inertia

The first law is also known as the law of inertia.

Inertia: The resistance of a particle to change its state of rest or of uniform motion along a straight line.

It could be of three types:

1.  Inertia of rest
2.  Inertia of motion
3.  Inertia of direction

• Inertia is property of a body by virtue of which it opposes any change in its state of rest or state of motion.Mass of a body is a quantitative or numerical measure of a body's inertia.

Inertia ∝ mass

• The larger the inertia of a body, more will be its mass.

4.0Inertia of Rest

It is the inability of a body to change its state of rest by itself.

Example:

• When we shake a branch of a mango tree, the mangoes fall down.
• When a bus or train starts suddenly, the passengers sitting inside tend to fall backwards.
• When a horse starts off suddenly, its rider falls backwards.
• A coin is placed on cardboard and this cardboard is placed over a tumbler such that the coin is above the mouth of a tumbler. Now if the cardboard is removed with a sudden jerk, then the coin falls into the tumbler.
• The dust particles in a blanket fall off when it is beaten with a stick.

5.0Inertia of Motion

It is the inability of a body to change its state of uniform motion by itself.

Example:

• When a bus or train stops suddenly, the passengers sitting inside lean forward.
• A person who jumps out of a moving train may fall forward.
•  A bowler runs with the ball before throwing it, so that his speed of running gets added to the speed of the ball at the time of throw.
•  An athlete runs through a certain distance before taking a long jump because the velocity acquired during the running gets added to the velocity of the athlete at the time of jump and hence he can jump over a longer distance.
•  A ball is thrown in the upward direction by a passenger sitting inside a moving train then the ball will fall: -
•  back to the hands of the passenger, if the train is moving with constant velocity.
•  ahead of the passenger, if the train is retarding (slowing down)
•  behind the passenger, if the train is accelerating (speeding up)

6.0Inertia of Direction

It is the inability of a body to change its direction of motion by itself.

Example:

•  When a straight running car turns sharply, the person sitting inside feels a force radially outwards.
•  Rotating wheels of the vehicle throw out mud, mudguards fitted over the wheels prevent this mud from spreading.
•  When a knife is pressed against a grinding stone, the sparks produced move in the tangential direction.

7.0Frames of Reference

Newton’s First Law is valid only in an inertial frame of reference—a frame that is not accelerating.

Inertial Frame:

• A frame either at rest or moving with constant velocity.
• Newton’s First Law applies without correction.

Non-Inertial Frame:

• A frame undergoing acceleration.
• Pseudo-forces (like centrifugal force) must be introduced to apply Newton’s laws.

Example:In a car making a sharp turn (non-inertial frame), you feel like you are being “thrown” sideways. This isn’t due to a real force acting on you, but rather your inertia — your body tries to continue in a straight line while the car turns beneath you.

8.0Role of Friction and Air Resistance

While Newton's First Law holds ideally in the absence of external forces, in everyday situations friction and air resistance play significant roles:

• Friction between surfaces slows moving objects.
• Air resistance opposes motion through the atmosphere.

9.0Key Points Newton's First Law of Motion

Note:

1. Force is the cause of changes in motion.

Force does not cause motion. We can have motion in the absence of force, as described in Newton’s first law. Force is the cause of change in motion as measured by acceleration.

2. ma is not a force

Equation 2 does not say that the product ma is a force. All forces on an object are added vectorially to generate the net force on the left side of the equation. This net force is then equated to the product of the mass of the object and the acceleration that results from the net force.