A boat heads into shallow water and gets stuck in sand. The rower stands up and uses an oar to push the boat off the sandbank. The resulting change of momentum involves which of the following: the boat, the water, the earth, the rower
The rower and boat both change velocity after the push, and as $P(momentum)=m(mass)*v(velocity)$, the momentum changes.
A force is applied to the earth. $F(force)=m(mass)*a(acceleration)$. So there is an acceleration. $V=u+at$, so if there is an acceleration, the velocity changes. $P=m*v$, so the momentum of the earth changes.
But why not the water?
Surely when the boat moves through the water there is a force being applied on the water by the front of the boat. So wouldn't the momentum of the water change too?
Answer in my textbook is:
While there may be more friction if the boat was on the sand and out of the water entirely, the momentum cange does not involve the water but occurs on the rower and the boast as a result of a force along the earth. Answers are the rower, the boat and the earth only.