Physics Class Notes by Padyuman Sir: Force and Law of Motion

Introduction

• Class Ninth Spring Series, Chapter Number 9: Force and Law of Motion
• In this lecture, we will mainly discuss Newton's three Laws of Motion.

Newton's Laws of Motion

1. First Law: Inertia:

   • If a body is at rest, it will remain at rest, and if a body is in motion, it will remain in motion unless acted upon by an external force.
   • Without external force, there is no change in velocity and position.
   • An object will remain in its state until an external force is applied.

2. Second Law: Momentum

   • 'Force = Mass x Acceleration (F = ma)':
     • Direct relation between force and acceleration.
   • Momentum: Product of mass and velocity (P = m * v)
     • Change in momentum = Mass x Change in velocity (∆P = m ∆v)
   • The rate of change in momentum is directly proportional to the external unbalanced force.

3. Third Law: Action and Reaction

   • Every action has an equal and opposite reaction.
   • These work on two different bodies.
   • Examples of action and reaction:
     • Punching a wall
     • Jumping out of a moving vehicle
     • Rowing a boat*

Types of Forces

• Balanced Force:
  • Total force is zero (Net force = 0)
  • The body remains at rest or moves in uniform motion.
• Unbalanced Force:
  • Total force is non-zero (Net force ≠ 0)
  • The velocity of the body changes (acceleration occurs).

Effects of Force

• Force does six main things:
  1. Bringing an object from rest to motion
  2. Bringing an object from motion to rest
  3. Increasing speed
  4. Decreasing speed
  5. Changing direction
  6. Changing the shape of an object

Types of Inertia

1. Inertia of Rest: Object remains at rest
2. Inertia of Motion: Object remains in motion
3. Inertia of Direction: Object remains in its direction

Examples

• Examples of Inertia of Rest: Leaning forward when a vehicle suddenly stops
• Examples of Inertia of Motion: Leaning back when a moving car suddenly brakes
• Examples of Inertia of Direction: Leaning when a vehicle takes a turn

Relation between Acceleration and Force

• If time is reduced, force increases.
• F = m * a: Force increases with the increase in acceleration*

Conclusion

• The importance of the conservation of momentum in the law of momentum and action-reaction
• The Gravitation chapter will be covered in the next class.