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NEWTON’S LAW OF MOTION- Physics

Newton’s laws of motion are three fundamental laws formulated by Sir Isaac Newton to explain the motion of objects and the effects of forces acting on them. These laws describe the relationship between force, mass, and acceleration. They form the foundation of classical mechanics and help explain many everyday phenomena, such as walking, running, and the movement of vehicles.

NEWTON’S FIRST LAW OF MOTION (Law of Inertia)

An object remains in a state of rest or uniform motion in a straight line unless acted upon by an external, unbalanced force.

Inertia is the inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion in a straight line unless acted upon by an external force.

Types of Inertia & Common Exam Examples:

Inertia of Rest 

  • When a stationary bus or train suddenly starts, passengers lurch backward.
  • Beating a hanging carpet with a stick causes dust particles to fall off.
  • Shaking the branches of a tree causes ripe fruits to detach and fall.

Inertia of Motion 

  • When a speeding bus applies brakes suddenly, passengers lean forward.
  • A long-jumper runs a certain distance before taking off to build momentum.
  • Switching off a ceiling fan does not stop the blades immediately.

NEWTON’S SECOND LAW OF MOTION

 Second Law of Motion (Law of Momentum)

  • Definition: The rate of change of linear momentum of a body is directly proportional to the applied force and takes place in the direction of the force.
  • Key Concept: Gives the quantitative definition / measurement of force.
  • Formula:

F = m×a  

Where:

  • F = Force
  • m = Mass
  • a = Acceleration (a = v-ut)
  • Linear Momentum (p): Product of mass and velocity (p = m×v).
  • SI Unit: (kg × m)/s
  • CGS Unit: (g × cm)/s

Key Applications:

  • Catching a Cricket Ball: A fielder pulls their hands backward while taking a catch to increase the impact time (t), which decreases the rate of change of momentum, thereby reducing the impulsive force on their hands.
  • Shock Absorbers: Vehicles use springs and shock absorbers to increase impact time over bumps, minimizing the jerk experienced.

NEWTON’S THIRD LAW OF MOTION

To every action, there is always an equal and opposite reaction.

Key Concept:

  • Forces always occur in pairs.
  • Action and reaction never act on the same body; they act simultaneously on two different bodies. Therefore, they never cancel each other out.

Real-Life Applications of Newton’s Third Law:

  • Recoil of a Gun: When a bullet is fired forward, the gun exerts an equal force backward (also explained by the Law of Conservation of Linear Momentum).
  • Walking: A person pushes the ground backward (Action), and the ground pushes the person forward (Reaction).
  • Rocket Propulsion: Burning gases rush downward with high velocity, driving the rocket upward.
  • Swimming / Rowing a Boat: Water is pushed backward to propel the swimmer or boat forward.