#### Question

# Physics Help...A train moving with a speed of 32.8m/s sounds a 141 Hz horn.?

what frequency is heard by an observer standing near the tracks as the train approaches?

#### Answers

Taking speed of sound at 340 m/s

141 x 340/(340- 32.8) = 156.05 Hz

observer (vo) = 0 at rest at origin say

source (engine) vs = + 32.8 m/s > moving in + x direction

sound v = + 343 m/s > sound also reach observer + x direction

n' = heard frequency = n (real freq.) [v - vo] / [v - vs]

n' = 141 [343 - 0] / [343 - 32.8] = 155.91 Hz

***** you can use this formula always. keep all (v, vo, vs) in the same direction. if anyone is moving in opposite direction make it positive by using negative sign in formula for that. eg if sound is travellingbackwards, taken v = - 343

practice with this composite formula.

I will take 343m/sec. as speed of sound in air.

343 + 32.8 = apparent sound velocity of 375.8m/sec.

375.8/343 = 1.0956:1 ratio.

141 x 1.0956 = 154.48Hz.