Scallops use jet propulsion to move from one place to another. their shells make them denser than water, so they normally rest on the ocean floor. if a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water so that the thrust force on the scallop is

Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on the ocean floor. If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water downwards so that the thrust force on the scallop is upwards.

Explanation:

For every action, there is a reaction with the same magnitude but in the opposite way (third law of Newton), so when the scallop ejects water downwards, another force appears that pushes the scallop upwards.