# Mechanical Engineering - Automobile Engineering - Discussion

Discussion Forum : Automobile Engineering - Section 2 (Q.No. 8)
8.
The air resistance to a car at 20 kmph is R. The air resistance at 40 kmph will be
R
2 R
4R
4 R2
Explanation:
No answer description is available. Let's discuss.
Discussion:
7 comments Page 1 of 1.

Dhruvil said:   3 years ago
F is directly proportional to v^(3/2)

F= (1/2) (rho) (v^2) (Area) (skin friction coeff).
now, Skin fric coeff= 1.328/ (reynold num.)^(1/2),
and Reynold num = Rho*v*L/mu.

Now putting the value of skin friction Coeff and Reynold number in the main equation.

F= (v^2)/ (v^(1/2)),
F= k. V^(3/2).
So, the correct answer is 2.828R.
(1)

Anindam said:   4 years ago
@Nikunj_Patel.

Nikunj Patel said:   5 years ago
20kmph R resistance.
30kmph. 2R resistance.
40 kmph 4R resistance.
50 kmph 8R resistance.

Roshan Thakkar said:   5 years ago
The air resistance to a car at 20 kmph is R.
Multiply by 2 at both side,
20 = R.
20*2 = 2*R.
40 = 2R.

Ajeya said:   8 years ago
Can anyone tell me, any reference book for this problem?

Sibikrishna said:   9 years ago
Air Resistance Formula,

F = c x v^2.

For 20 Kmph, Given: F = R & v = 20 Kmph.

R = c x 400.
c = R/400.

For 40 kmph,

F = c x v^2.
F = (R/400) x 1600.
F = 4R.
(1)

DUGGU RATHORE said:   1 decade ago
R = K.A.V2 when vehicle speed is about 20km/hr.

Where K = Co-efficient of air resistance.
A = Projected area of vehicle.
V = Vehicle speed in km/hr.

When vehicle speed is about 40km/hr, speed is doubled so we can write V=2V,

Therefore, R = K.A.(2V)2.
R = K.A.4V2.
R = 4K.A.V2.
R= 4R (where, R = K.A.V2)
(2)