Force and Laws of Motion

An object at rest stays at rest; an object in motion continues in motion in a straight line at constant speed UNLESS acted upon by an unbalanced external force. Inertia = resistance to change. More mass = more inertia. Examples: Passengers jerk forward when bus brakes suddenly. Tablecloth trick — inertia keeps dishes in place.

Net force = rate of change of momentum = mass × acceleration. F = ma. Unit: Newton (N) = kg⋅m/s². 1 N = force that gives 1 kg mass an acceleration of 1 m/s². Larger mass needs larger force for same acceleration. Same force on smaller mass → larger acceleration.

For every action, there is an EQUAL and OPPOSITE reaction. Forces act on DIFFERENT objects. Examples: Walking (push ground back → ground pushes you forward). Rocket propulsion (hot gas ejected backward → rocket moves forward). Swimming (push water back → move forward). Recoil of gun.

Momentum p = mv (mass × velocity). Vector quantity. SI unit: kg⋅m/s. Law of Conservation of Momentum: In an isolated system, total momentum before collision = total momentum after collision. m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. No external force → momentum conserved.