Question

When a body falls in air, the resistance of air depends to a great extent on the shape of the body, 3 different shapes are given. Identify the combination of air resistances that truly represents the physical situation. (The cross-sectional areas are the same).

A. 1 < 2 < 3
B. 2 < 3 < 1
C. 3 < 2 < 1
D. 3 < 1 < 2

Answer 1

Hint: Before going to solve this question let us understand the streamlined flow. The streamlined flow is also referred to as laminar flow. When there is no turbulence and fluctuations in velocity then it is called streamlined flow. It is a path traced by a particle in a flow.

Complete step by step solution:

Image: Three different shapes having the same area of cross-section.

Resistance experienced by the body when it is moving in an air medium depends upon its shape. So here we have three different shapes. Now we have to identify the correct option depending upon the shape whether it is in the increasing or in decreasing order of resistance.

Usually even in also and in airplanes has an aerodynamic shape which means it allows the streamlined flow of the fluids. By that time the air resistance will be less. So, if you observe the three diagrams. The first one is circular, the second is spherical, and the third is cigar (aerodynamic) in shape. Let us assume that the area of the cross-section remains the same.

As we can see here the weight is acting downwards and the resistance in an upward direction. During that time, the region available for that air resistance is less in the third diagram and comparatively more in the second one and it is still more in the first diagram. Therefore, in the third diagram, it is streamlined, so the air resistance of this will be the least. For the first diagram, the surface area is maximum so air resistance is minimum.

Hence, option C is the correct answer.

Note:In this question, it is important to remember that, when a body moving through a fluid has a streamlined shape (a bullet) has very little air resistance due to its inherent shape when compared to a flat plate moving through the same fluid with the phase of the plate in the direction of motion.