Force and Laws of Motion | Science Grade 9 CBSE

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Six ideas behind the laws

From a simple push to a rocket leaving the ground — it all rests on six ideas. Tap each term to see what it means and how it links to Newton’s laws.

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The three big ideas

Force — a push or a pull. It can change an object’s speed, direction or shape. Unit: newton (N).
Balanced forces add up to zero net force → no change in the state of rest or motion. Unbalanced (net) force changes speed or direction.
Inertia — the tendency to resist a change in the state of rest or motion. It depends on mass: more mass means more inertia.
Newton’s first law (law of inertia): an object stays at rest, or keeps moving uniformly in a straight line, unless a net force acts on it.

Common mistake: thinking a heavier object has less inertia. The opposite is true — a heavier object has MORE inertia, so it is harder to start moving and harder to stop.

Momentum p = m × v — "mass in motion". A vector; unit kg·m/s. A heavy, fast object has large momentum.
Newton’s second law: the net force equals the rate of change of momentum, which gives F = m × a.
More force → more acceleration; more mass → less acceleration for the same force.

Worked example. What net force gives a 2 kg trolley an acceleration of 3 m/s²?

F = m × a = 2 kg × 3 m/s² = 6 N.

Newton’s third law: for every action there is an equal and opposite reaction. The two forces act on different bodies, so they do not cancel.
Conservation of momentum: when no external force acts, the total momentum before = total momentum after an interaction (e.g. a collision or a recoil).
Walking, swimming, rowing and rocket flight all work by pushing one way so the surface or gas pushes you the other way.

Where you'll meet it

Newton’s laws, all around you

Seat belts & head-rests

In a crash the car stops suddenly but your body keeps moving forward by inertia. The seat belt holds you back and the head-rest stops your head snapping backward — both work against inertia to protect you.

Rocket launch

A rocket burns fuel and pushes hot gas downward at high speed. By Newton’s third law the gas pushes the rocket upward with an equal and opposite force — that reaction lifts it off the ground.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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