Thrust force

Question

Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on 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______

 

a) upward upward

b) upward downward

c)downward upward 

d)downward downward

Answer 1

It must produce an upward thrust to cancel the net downward force due to its having a higher density than water.  So it must eject water downward to produce upward thrust.  The correct answer is (c).

Answer 2

This is an application of Newton's third law.  The scallop uses internal energy to exert a force on the air downward, and the propelled air exerts an equal force in the opposite direction, upward, on the scallop.