Unit 3.2 – Newton’s Laws of Motion

1. Newton’s First Law of Motion (Law of Inertia)

What Does It Say?

Key Idea: Inertia

• Inertia is the property of an object to resist changes in its state of motion.
• The more massive an object, the greater its inertia.

Examples in Daily Life

1. Inertia of Rest:
   • When a bus suddenly starts moving, passengers inside tend to fall backward. This happens because their bodies want to remain at rest (inertia of rest).
2. Inertia of Motion:
   • When a moving bus stops suddenly, passengers lurch forward. Their bodies want to keep moving (inertia of motion).
3. Inertia of Direction:
   • When a car takes a sharp turn, passengers feel pushed to the side. Their bodies want to continue in the original direction (inertia of direction).

2. Newton’s Second Law of Motion (Law of Acceleration)

What Does It Say?

Simplified Formula

• $F$ = Force applied.
• $m$ = Mass of the object.
• $a$ = Acceleration produced.

Key Idea: Momentum

• Momentum ($p$) is the product of mass and velocity:

• A greater force is needed to change the momentum of a heavier or faster-moving object.

Examples in Daily Life

1. Pushing a heavy box requires more force than pushing a lighter one because the heavier box has more mass.
2. A cricket ball accelerates faster when hit with a greater force by the batsman.

3. Newton’s Third Law of Motion (Action-Reaction Law)

What Does It Say?

Key Idea: Action-Reaction Pairs

• Forces always come in pairs.
• The action and reaction forces act on different objects, so they don’t cancel each other out.

Examples in Daily Life

1. Firing a Gun:
   • When a bullet is fired from a gun, the gun recoils backward.
   • Action: Force exerted by the gun on the bullet.
   • Reaction: Force exerted by the bullet on the gun.
2. Jumping Off a Boat:
   • When you jump from a boat to the shore, the boat moves backward.
   • Action: Force you apply on the boat.
   • Reaction: Force the boat applies on you.
3. Swimming:
   • When you push water backward with your hands, the water pushes you forward.
   • Action: Force applied on the water.
   • Reaction: Force applied by the water on you.

Special Cases and Applications

1. Projectile Motion

• A projectile (e.g., a ball thrown into the air) follows a curved path called a parabola.
• Examples:
  • A bomb dropped from an airplane.
  • A javelin thrown by an athlete.

2. Free Fall

• When an object falls freely under gravity, its acceleration is $g=9.8{\text{m/s}}^2$.
• Example: A stone dropped from a height accelerates downward due to gravity.

3. Retardation (Deceleration)

• When an object slows down, its acceleration is negative, called retardation.
• Example: A car braking to stop experiences retardation.

Equations of Motion (Uniformly Accelerated Motion)

1. First Equation:

• $v$ = Final velocity.
• $u$ = Initial velocity.
• $a$ = Acceleration.
• $t$ = Time.

2. Second Equation:

• $s$ = Displacement.

3. Third Equation:

Real-Life Applications of Newton’s Laws

1. Vehicles and Shock Absorbers:
   • Vehicles are fitted with springs and shock absorbers to reduce jerks while driving on uneven roads.
2. Catching a Ball:
   • A cricket player lowers their hand while catching a ball to increase the time of contact, reducing the force exerted on their hand.
3. Soft Ground in Sports:
   • Wrestling and high jump events use soft ground to reduce injuries caused by falling.
4. Rocket Propulsion:
   • Rockets expel gases downward (action), and the reaction force propels them upward.

Quick Recap

1. Newton’s First Law: Objects resist changes in motion (inertia).
2. Newton’s Second Law: $F=m\cdot a$; force causes acceleration.
3. Newton’s Third Law: Every action has an equal and opposite reaction.
4. Applications: Projectile motion, free fall, retardation, sports, vehicles, etc.