Drag force is a function of:

(i) projected area of the body

(ii) mass density of the fluid

(iii) velocity of the body

The correct answer is

This question was previously asked in

HPPSC AE Civil 2016 (HPPTCL) Official Paper

Option 4 : (i), (ii) and (iii)

__Concept__:-

When a body is immersed in the fluid under motion, it is subjected to Drag and lift forces, this is due to pressure and viscosity.

**Drag Force:**

It is the horizontal component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It has two components pressure drag and friction drag. It is given by –

\({F_D} = \frac{1}{2}{C_D}\rho A{V^2}\)

- Friction Drag is also known as Skin Friction Drag.
- The drag on a body resulting from viscous shearing stresses over its contact surface.
**It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity. also depend upon the mass density of the fluid**- The drag of a very streamlined shape such as a thin, flat plate is frequently expressed in terms of skin friction drag.

**Lift force:**

It is the vertical component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It is given by –

\({F_L} = \frac{1}{2}{C_L}\rho A{V^2}\)

- Lift forces acts due to the pressure difference between the top and bottom surface of the plate or body.
- If the axis of the body is parallel to the direction of fluid flow the lift force is zero and only drag force acts.

Here,

CL and CD – Coefficient of lift and drag force respectively.

A – a maximum projected area of the body (m2)

ρ – density of fluid (kg/m3)

V – average velocity of stream (m/sec)

__Note__: - If fluid is assumed to be ideal (Non viscous) and body is symmetrical such as sphere or cylinder, Both drag and lift force will be zero.