📘Class 9 Science – Chapter: Force and Laws of Motion Notes

🌟 Introduction to Force

  • A force is a push or pull that changes or tries to change the state of motion of an object.

  • Force can change:

    • The speed of an object

    • The direction of motion

    • The shape or size of an object

Example: Kicking a football applies a force that sets it into motion.

🔹 Balanced and Unbalanced Forces

Type of Force Description Effect on Object Example
Balanced Force Equal forces in opposite directions No change in motion Tug of war (balanced)
Unbalanced Force Unequal forces on opposite directions Object accelerates or moves Kicking a football

Example: A ball lying on a table stays at rest due to balanced forces. When you kick it, unbalanced force moves it.

🔹 First Law of Motion (Law of Inertia)

  • An object remains at rest or in uniform motion in a straight line unless acted upon by an external force.

  • This law is also called the Law of Inertia.

Inertia: The tendency of an object to resist changes in its state of motion.

Example:

  • A book on a table stays at rest until pushed.

  • Passengers lurch forward when a moving bus suddenly stops.

🔹 Mass and Inertia

  • Inertia depends on the mass of the object.

  • Greater the mass, greater is the inertia.

Object Mass Inertia
Feather Low Low
Stone High High

Example: A heavy cricket ball resists motion more than a tennis ball due to greater inertia.

🔹 Second Law of Motion

  • The rate of change of momentum of an object is directly proportional to the applied force.

Formula:

  • Force (F) = Mass (m) × Acceleration (a)

  • F = m × a

Where:

  • F = force in newtons (N)

  • m = mass in kg

  • a = acceleration in m/s²

Example:

  • A 2 kg object accelerated at 3 m/s²:

    • F = 2 × 3 = 6 N

🔹 Momentum

  • The product of mass and velocity of an object.

  • Momentum (p) = Mass (m) × Velocity (v)

Unit: kg m/s

Example: A 5 kg object moving at 10 m/s:

  • p = 5 × 10 = 50 kg m/s

🔹 Third Law of Motion

  • For every action, there is an equal and opposite reaction.

Examples:

  • When you jump, your legs push down on the ground (action), and the ground pushes you upward (reaction).

  • Recoil of a gun when a bullet is fired.

📊 Table: Comparison of Newton’s Laws of Motion

Law Statement Keyword Example
First Law Object stays in rest/motion unless acted upon Inertia Book stays at rest
Second Law F = m × a Acceleration Pushing a trolley
Third Law Action = – Reaction Reaction Walking (feet push ground)

🎯 Numerical Example

A 4 kg object is accelerated at 2.5 m/s². Calculate the force applied.

Solution:

  • F = m × a = 4 × 2.5 = 10 N

Hand written Notes Of CELL: The Fundamental unit of Life