NCERT Solutions for Class 9th Science Chapter 6 Chapter opener — Think It Over
Book page 94 Updated on2026-09-08
Q1.
Why does a canoe move forward when the canoeist pushes water backwards with their paddle and why does it move faster when they push harder?
Answer
Because the water pushes back. This is Newton's third law of motion at work — the paddle and the water form an action–reaction pair.
Paddle pushes water backwards with force F
Water pushes paddle forwards with the same force F
The two forces act on different objects — water and paddle — so they do not cancel
The forward force on the paddle is passed on through the canoeist's arms to the canoe, and the canoe accelerates forward.
Why pushing harder makes it faster: a harder push means a larger F on the water, so by the third law the water pushes the paddle forward with a larger F too. By the second law, a = F/m — with the mass of the canoe fixed, a larger forward force gives a larger acceleration, so the canoe gains speed more quickly and reaches a higher velocity in each stroke.
The action–reaction pair that drives a canoe. The two arrows are equal in size but act on different bodies.
Q2.
Suppose the same canoeist uses the same paddle force in two different canoes, one empty and one carrying another passenger. In which case will the canoe move faster?
Answer
The empty canoe moves faster.
a = F ÷ m (Newton's second law)
Same paddle force F in both cases
Empty canoe: smaller m → larger a
Loaded canoe: larger m → smaller a
Why it happens: acceleration is inversely proportional to mass. Adding a passenger increases the total mass of the canoe-plus-load, so the same force produces a smaller acceleration. In the same stroke time the empty canoe therefore gains more velocity.
Check it yourself: if the empty canoe plus canoeist is 100 kg and the passenger adds 50 kg, a 300 N paddle force gives a = 300 N ÷ 100 kg = 3 m s–2 for the empty canoe and a = 300 N ÷ 150 kg = 2 m s–2 for the loaded one — two-thirds as much.