Exercise 6.7
A sailor jumps out from a small boat to the shore (Fig. ). As the sailor jumps forward, will the boat move? If yes, in which direction and why.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Yes, the boat moves — backwards, i.e. away from the shore, opposite to the sailor's jump.
To jump forward the sailor's feet push backwards on the boat.
By Newton's third law, the boat simultaneously pushes the sailor forward with an equal and opposite force, which is what launches him towards the shore.
The two forces of this pair act on different objects — one on the boat, one on the sailor — so they do not cancel each other.
The backward force on the boat is a net force on it (water offers little resistance), so by \(\displaystyle F = ma \) the boat accelerates backwards.
Because the boat is small, its mass is not very large, so the backward push gives it a noticeable acceleration — which is why jumping from a small boat is tricky.