A boy jumps to the shore from a stationary boat weighing 80 kg. The boy’s weight is 40 kg

A boy jumps to the shore from a stationary boat weighing 80 kg. The boy’s weight is 40 kg, his speed when jumping is 2 meters per second. What speed has the boat gained?

Given:
m1 = 80 kg – boat weight
V1 = 0 – at the initial moment of time the boat is motionless
m2 = 40 kg – boy’s weight
u2 = 2 m / s – the boy’s speed at the moment of the jump
u1 -?
we use the formula for the conservation of momentum m1 * V1 + m2 * V2 = m1 * u1 + m2 * u2
the left side of the equality is the sum of the impulses of the two bodies before the jump, the right side is the sum of the impulses after the jump
before the jump, the speed of the boy and the boat was 0, therefore
0 = m1 * u1 + m2 * u2
m1 * u1 = – m2 * u2
u1 = – m2 * u2 / m1
u1 = – 40 * 2/80 = – 1 m / s