Explain why objects moving in fluids must have special shapes.

Hint: In general, when a body moves, the body feels some opposing force. When a body moves in fluid, then also it feels some drag force. These drag forces can be pressure drag or frictional drag etc. to reduce the opposing forces we can use some methods.

Complete step-by-step answer:
When an object moves in fluid, the motion of the object is opposed by the frictional force. The velocity of the object in fluids depends on the shape of the body because the frictional drag of the fluid on a body depends on the shape of the body. For some shape the frictional drag is low and for some shape the frictional drag is high. So, we must have special shapes of objects to make it move easily in water.
A body moving through a fluid always experiences a drag force. This drag can be due to pressure drag or frictional drag. The drag force depends on the shape of the body. To minimise these drag forces on the body moving in the fluid, we must have special shapes.

Note: Streamline body is a body for which the drag force on objects moving in fluid is minimum. Streamline body represents a geometry for which the friction with fluid is low. It can be simply defined as the geometry of a fish. The two ends of the body are pointed and the middle part is broad. The submarines are also made keeping this geometry so that the drag forces acting on it will be very low.