Question

When we jump out of a boat standing in water it moves _______.
A) To our sides
B) Opposite to our direction
C) In our direction
D) Doesn’t move at all

Answer 1

Hint: To determine the direction in which the boat moves, we can use Newton’s third law. Newton’s third law tells us that every action has an equal and opposite reaction.

Complete step by step answer
As mentioned in the hint, every action has an equal reaction and the reaction will have a direction opposite to the direction of the action. When we jump from a boat that is standing in water, we have to apply force on the boat to jump away from it.
When we propel ourselves forward, we apply force on the boat. According to Newton’s third law, the boat will experience an equal amount of reaction force however the direction of the reaction force will be in the direction opposite to the direction in which we jump.

Newton’s third law is a consequence of the law of conservation of momentum which states that the net momentum of a system is zero if no external force is applied to it. Considering us and the boat as a system, if we propel ourselves forwards and gain forward momentum, to maintain zero net momentum the boat will gain a negative momentum so that the net momentum of the system will remain to be zero.

Hence the correct choice is option (B).

Note
Assuming that the boat does not move vertically too much in the water, the horizontal component of our momentum will be compensated by the movement of the boat in the horizontal direction. But the direction of the boat will always be in the direction opposite to the direction in which we jump.