First Law (Law of Inertia)

A body remains at rest or in uniform motion in a straight line unless acted upon by an external force.

• Concept: Objects resist change in motion → this resistance is called inertia.

• Example:

  • A stationary book stays at rest until you push it.

  • A moving car continues until brakes are applied.

Key Point:

• Force is not needed to keep moving; it is needed to change motion.

2️⃣ Second Law (Law of Acceleration)

The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass.

F⃗=ma⃗\vec{F} = m \vec{a}F=ma

Where:

• F⃗\vec{F}F = net external force

• mmm = mass of the body

• a⃗\vec{a}a = acceleration

Key Points:

• Force → causes acceleration

• Heavier objects → less acceleration for same force

• Vector quantities: direction of force = direction of acceleration

Example:

• Pushing a car vs a bicycle: bicycle accelerates more for the same push.

3️⃣ Third Law (Action–Reaction)

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

• Forces always come in pairs.

• If object A exerts force on B → B exerts equal force on A in opposite direction.

Example:

• Rocket moves forward → exhaust gases push backward.

• Jumping → feet push ground down → ground pushes you up.

🔹 Important Notes