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Newton's laws of motion

Understand and apply Newton's first, second, and third laws to analyze forces, motion, and interactions in various physical situations.

Why this shows up in the exam

NEET frequently tests your ability to identify and use Newton's laws in both straightforward and complex systems.

How NEET tests this

Numerical · 4 QsApplication · 4 QsDirect recall

Learn the idea

Newton's laws tell how forces determine motion: (1) a body at rest or uniform motion stays so unless a net external force acts; (2) the net external force equals mass times acceleration (F=ma); (3) forces always occur in equal‑and‑opposite pairs on two different bodies. The key insight is that the *net* external force on a single object decides its acceleration – direction and magnitude follow directly from F=ma.

🧠 Memory hook: IAR – Inertia, Acceleration, Reaction – the three pillars of motion

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • First law (law of inertia): zero net external force → zero acceleration
  • Second law: net external force vector = mass × acceleration vector (F=ma)
  • Third law: for every action there is an equal and opposite reaction; the two forces act on different bodies
  • Weight w = mass × g acts vertically downward
  • Air resistance is a force opposite to the direction of motion and can vary with speed

How to approach it

  1. 1Read the question and list all forces acting on the body of interest
  2. 2Draw a clear free‑body diagram showing magnitude and direction of each force
  3. 3Apply the appropriate Newton law – use the first law if the net force is zero, otherwise use F=ma to find acceleration or net force direction
  4. 4If interaction forces are asked, invoke the third law on the pair of bodies

Worked example — watch it click

A parachutist is in free fall before opening her parachute. The net force on her has a magnitude F and is directed downwards. This net force is somewhat less than, her weight w because of air resistance. Then, she opens her parachute. At the instant after her parachute fully inflates, the net force on her would be:

  • A)Greater than F and still directed downwards
  • B)Less than F and still directed downwards
  • C)Zero
  • ✅Directed upwards, but could be more or less than F

The concept behind this problem

The parachute problem checks whether you apply Newton's second law when the resistive force suddenly becomes larger than the weight, so the net force reverses direction and may be larger or smaller than the original net force F.

Step by step

  1. 1When parachute inflates, air resistance increases dramatically, exceeding weight w.
  2. 2Net force = air resistance - w is directed upward.
  3. 3The magnitude depends on parachute size and could be greater or less than the original downward net force F.

Watch out

Students often think the net force becomes zero after the parachute opens, forgetting that the increased air resistance can make the net force upward and of any magnitude.

Common slip-ups that cost marks

  • •Treating the action‑reaction pair as acting on the same body – they cancel only on different bodies
  • •Ignoring the sign (direction) of the net force when converting to acceleration
  • •Assuming frictionless or weight‑only situations when the problem mentions air resistance or other forces

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 9NEET 2019

A particle moving with velocity v is acted by three forces shown by the vector triangle PQR. The velocity of the particle will

Push further

More challenging

12 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 12

A block of mass 2 kg is placed on a frictionless inclined plane making an angle of 30° with the horizontal. The entire system (inclined plane + block) is placed inside a lift accelerating upwards at 5 m/s². What is the acceleration of the block relative to the inclined plane? (Take g = 10 m/s²)

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