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NCERT Solutions for Class 9th Science Chapter 6 .6 Newton's Third Law of Motion — Pause and Ponder

Book page 1106 Updated on2026-09-08

Q9.
Why does a fireperson sometimes struggle when holding the pipe issuing water?
Answer

Because the water pushes the hose backwards with a force equal to the force the hose uses to push the water forwards — and that backward force can be very large.

Hose pushes water forwards at high speed, with force F
Newton's third law → water pushes hose backwards with force F
The fireperson must supply a forward force of F just to keep the hose still
Fast-moving water = large change of momentum every second = large F
Why the force is so big: a fire hose throws out several kilograms of water every second at a very high speed. Producing that large change of velocity in the water in so short a time needs a large force (F = ma), and the reaction on the hose is equally large. If the fireperson does not brace hard against it, the net force on hose-plus-person is backwards and they are pushed back — which is why firefighters often hold the nozzle in pairs and lean into it.
Tip: a rocket, a canoe paddle and a fire hose are the same physics: throw mass one way, get pushed the other way.
Q10.
Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.
Answer

By firing its engine and expelling gas — the only way to get a force when there is nothing outside to push against.

Engine expels exhaust gas in some direction with force F
By Newton's third law, the gas pushes the spacecraft with force F in the opposite direction
That is the net force, since gravity is negligible
a = F ÷ m → the velocity changes along the direction opposite to the exhaust
  • To speed up: fire the exhaust backwards, opposite to the direction of motion.
  • To slow down: fire the exhaust forwards, along the direction of motion.
  • To turn: fire small side thrusters, so the reaction force acts sideways and changes the direction of the velocity.
Why nothing else would work: with negligible gravity and no air, no external agent is touching the spacecraft. Newton's first law then guarantees its velocity cannot change on its own. The craft must therefore carry its own reaction mass — the fuel — and throw it away to get a force.
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