Question

If a boat and its riders have a mass of 900 kg and the boat drifts in at 1.4 m/s how much work does sam do to stop it?

Answer 1

The initial kinetic energy of the boat and its rider is
$K_i = \frac{1}{2} mv_i^2 = \frac{1}{2}(900 kg)(1.4 m/s)^2=882 J$

After Sam stops it, the final kinetic energy of the boat+rider is
$K_f = 0 J$
because its final velocity is zero.

For the law of conservation of energy, the work done by Sam is the variation of kinetic energy of the system:
$W=K_f-K_i =0-882 J=-882 J$
where the negative sign is due to the fact that the force Sam is applying goes against the direction of motion of the boat.