Help me this homework.

Question

The question is..

 

A train is travelling towards a railroad crossing. It blows its whistle which has a frequency of 2.50 x 10^3 Hz. You hear a frequency of 2.75 x 10^3 Hz as you wait at the railroad crossing. The speed of sound in air is 343 m/s. What is the speed of the train?

please help me..

 

plus, The amount of friction is proportional to the normal force, is that true?

Answer 1

Frequency of the train, f =  2.50 x 10^3 Hz

Hearing frequency,f ' =2.75 x 10^3 Hz

Speed of the air , v = 343 m/s

Speed of the man ,vd =0

 

Calculate the speed of the train by using the Doppler shift equation:

Train cross the man.

f ' = f (v-vd) / (v-vs)

2.75 x 10^3 =2.50 x 10^3(343) / (343-vs)

(343) / (343-vs) =(2.75 x 10^3)/(2.50 x 10^3)

(343) / (343-vs) =(2.75 /(2.50)

343 / (343-vs) =1.1

343-vs =311.82

vs=1.1v-377.3

vs =31.2 m/s

 

Therefore, speed of the train is 31.2 m/s

 

yes frictional force is proportional to the normal force.

Comments

How did you find "1.1"? I am sorry but I don't get it.. help me!

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Hi please check 2.75*10^3 is divided 2.50*10^3   is 1.1