Sir Isaac Newton gave three laws of motion in his book Principia (1687). These laws describe the relationship between a body and the forces acting on it. They are basic to physics and are asked in nearly every science exam.
First Law (Law of Inertia)
- A body stays at rest or in uniform motion unless an external force acts on it.
- This property is called inertia.
- Example: passengers jerk forward when a moving bus suddenly stops.
Second Law (Law of Force)
- The rate of change of momentum equals the applied force.
- Formula: Force = mass x acceleration (F = ma).
- SI unit of force is the newton (N).
- It explains why heavier bodies need more force to accelerate.
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Third Law (Action-Reaction)
- To every action there is an equal and opposite reaction.
- Example: a rocket moves up by pushing gases down.
- Example: a swimmer pushes water back to move forward.
Momentum and Conservation
- Momentum = mass x velocity.
- In the absence of external force, total momentum is conserved.
- This is the law of conservation of momentum.
Quick Revision Points
- Newton's laws given in Principia (1687).
- First law is the law of inertia.
- Second law: F = ma.
- SI unit of force is the newton.
- Third law: action = reaction.
- A rocket works on Newton's third law.
- Momentum = mass x velocity.