# 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 beDiscussion:

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.

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.

How? Explain please.

How? Explain please.

Nikunj Patel said:
5 years ago

20kmph R resistance.

30kmph. 2R resistance.

40 kmph 4R resistance.

50 kmph 8R 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.

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.

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)

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)

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