Question

Objects moving in fluids must have special shapes. Why?

Answer 1

Streamline shape

1. There have some particular shapes by which we can reduce the frictional force on the moving objects in fluids, these particular shapes are called streamlined shapes.
2. Speedboat is one type of streamlined shape.

Resistive force due to fluid

1. When an object moves in a fluid, it experiences a frictional force, which will reduce its motion in the fluid.
2. We know that frictional force depends on the contact surfaces of the bodies. When a body travels in a fluid, the fluid molecules will apply a resistive force on the object's surface. Streamlined shapes are the shapes that minimize the frictional (resistive) force due to liquid.

Diagram

Hence, objects moving in fluids must have special shapes, those shapes are called streamline shapes.