Mr. Pradyuman's Physics Class Notes: Force and Laws of Motion

Introduction

• Class Ninth Spring Series, Chapter Number 9: Force and Laws 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 an external force, there is no change in velocity or position.
   • An object will remain in its state unless acted upon by an external force.

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)
   • Rate of change in momentum is directly proportional to the external unbalanced force.

3. Third Law: Action and Reaction

   • For every action, there is an equal and opposite reaction.
   • These forces act on two different bodies.
   • Examples of Action and Reaction:
     • Punching a wall
     • Jumping from a moving vehicle
     • Pushing a boat

Types of Forces

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

Effects of Force

• Force performs 6 major tasks:
  1. Brings an object from rest to motion
  2. Brings an object from motion to rest
  3. Increases speed
  4. Decreases speed
  5. Changes direction
  6. Changes 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 backward when a moving car suddenly brakes
• Examples of Inertia of Direction: Leaning sideways when a vehicle turns

Relationship between Acceleration and Force

• If the time is decreased, the force increases.
• F = m * a: As acceleration increases, force increases

Conclusion

• Importance of the Conservation of Momentum in the laws of momentum and action-reaction
• The chapter on Gravitation will be covered in the next class.