Question

A girl of mass 50 kg jumps out of a rowing boat of mass 300kg on to a bank, with a horizontal velocity of 3m/s. With what velocity does the boat begin to move backwards?

Answer 1

Here law of conservation of momentum can be applied.
The boat and girl can be taken as an isolated system.

Initial momentum=Final momentum.

Here initial momentum of the system is zero because the boat and the girl as a system are at rest.
Let mass of girl=m1
Mass of boat=m2
Let velocity of boat=V

By law of conservation of momentum.
m1×30 + m2×v = 0
50×30 + 300×v =0
300×v =-150
v = -150/300
v = -0.5 m/s

Here the negative sign for velocity indicates that the boat moves in opposite direction to the girls motion.