Advanced Doppler effect questions

[Frozen_Mind]

1. The problem statement
1) A train whistle has a frequency of 1000 Hz. If the train is speeding
at a velocity of 60 km/h past a stationary railroad crossing
attendant, what is the apparent frequency a) as the train approaches him, b) as it moves away from him?
Assume speed of sound to be 350 m/s.

2) A second train is approaching the first train (above) with a speed of
60 km/h. Obtain the frequency of the whistle of the first as heard by
the engineer of the second train.

3) Sound is reflected by a screen moving with a speed 20 km/h toward
the source and observer. If the frequency of the source is 450 khz,
what is the apparent frequency of the reflected sound?

Homework Equations

f2 = f1Vs/(Vs + or - Vo), f2 = f1(1 + or - Vo/Vs)

meh I'm really not that quite sure :S

The Attempt at a Solution

I could calculate number 1 using the doppler effect equation f2 = f1Vs/(Vs + or - Vo), but I'm not quite sure how to solve for number 2 and 3... help please ?

 

[Pheo1986]

i think (not 100% sure) for part 2 and 3 you can use the same method as part 1 except the velocity will be the sum of the 2. so for part 2 the total velocity will be 60 + 60 = 120 km/h

 

[Doc Al]

Homework Equations

f2 = f1Vs/(Vs + or - Vo), f2 = f1(1 + or - Vo/Vs)

What do Vs and Vo mean?

Look up the Doppler formula for sound. There's a formula for when the source is moving and another for when the observer is moving. Of course you can combine them into a single formula when both are moving.

i think (not 100% sure) for part 2 and 3 you can use the same method as part 1 except the velocity will be the sum of the 2. so for part 2 the total velocity will be 60 + 60 = 120 km/h

No, that's not the right way to solve part 2. Instead you must combine the effect of a moving source (Vs) and a moving observer (Vo) using the complete Doppler formula.

For part 3, the reflecting screen becomes the source. Apply the Doppler formula twice.