Newton's Laws
Physics · न्यूटन के नियम
📋Quick Overview
Sir Isaac Newton gave 3 laws of motion in his book 'Principia Mathematica' (1687). These 3 laws explain WHY objects move the way they do. They are the foundation of classical mechanics. Almost every competitive exam asks at least 1 question on Newton's laws — especially examples from daily life.
📖Newton's First Law — Law of Inertia
"An object at rest stays at rest, and an object in motion stays in motion at constant velocity, UNLESS acted upon by an external force." In simple words: Things don't change their state on their own — you need a force to start, stop, or change direction of motion. This tendency to resist change is called INERTIA.
- •When a bus suddenly starts, passengers fall BACKWARD (body was at rest, wants to stay at rest)
- •When a bus suddenly stops, passengers fall FORWARD (body was in motion, wants to stay in motion)
- •Shaking a tree makes fruits fall (tree stops, fruits continue motion)
- •A carpet is beaten to remove dust (carpet stops, dust continues)
- •Seat belts in cars protect us because of inertia
- •Inertia depends on MASS — heavier objects have more inertia (harder to push a truck than a bicycle)
📖Newton's Second Law — F = ma
"The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force." Simply: Force = Mass × Acceleration (F = ma). More force = more acceleration. More mass = less acceleration for same force. This law gives us the UNIT of force: Newton (N) = kg × m/s².
- •A cricket ball hit hard goes farther (more force = more acceleration)
- •Kicking a football vs kicking a stone of same size — stone hurts more (more mass, more force needed)
- •Momentum (p) = mass × velocity (p = mv). Force = rate of change of momentum = dp/dt
- •Catching a cricket ball: fielder moves hands back to increase time → reduces force (F = mv/t)
📖Newton's Third Law — Action-Reaction
"For every action, there is an equal and opposite reaction." Simply: When you push something, it pushes you back with the same force but in the opposite direction. Action and reaction act on DIFFERENT bodies (not the same body).
- •Walking: You push the ground backward, ground pushes you forward
- •Rocket: Hot gases push downward, rocket moves upward
- •Swimming: You push water backward, water pushes you forward
- •Gun recoil: Bullet goes forward, gun kicks backward
- •When you sit on a chair, you push the chair down (your weight) and the chair pushes you up (normal force)
📝Conservation of Momentum
- •Law: Total momentum before collision = Total momentum after collision (if no external force)
- •This comes directly from Newton's Third Law
- •Example: When a gun fires, the bullet gains forward momentum and the gun gains backward momentum. Total = zero (same as before firing)
- •Rockets work on conservation of momentum — mass of gases goes down, rocket goes up
📖Summary Table — 3 Laws at a Glance
| Law | Also Called | Key Idea | Formula | Daily Life Example |
|---|---|---|---|---|
| 1st Law | Law of Inertia | Objects resist change in state | — | Passengers fall when bus starts/stops |
| 2nd Law | Law of Force | Force causes acceleration | F = ma | Harder you hit cricket ball, farther it goes |
| 3rd Law | Action-Reaction | Every action has equal opposite reaction | F₁ = -F₂ | Rocket, swimming, walking, gun recoil |