Question

Consider an ambulance traveling north toward an accident scene at 29.6 m/s and emitting a frequency heard within the ambulance as 1420 Hz. A variety of observers hear the ambulance. Determine the frequency that each hears. a) An accident victim lying motionless in the street ahead of the ambulance. b) A helpful witness running south toward the ambulance at 5.4 m/s. c) The motorist who struck the victim, fleeing the scene to the north at 34.8 m/s. d) An ambulance chaser following the ambulance at 29.6 m/s. e) Another motorist south of the ambulance and traveling south at 16.2 m/s.

Answer 1

Using Doppler's effect in each cases:

A.

when Source is moving towards stationary observaer

f1 = f0*V/(V - Vs)

V = speed of sound = 343 m/sec

Vs = speed of source

f1 = 1420*343/(343 - 29.6)

f1 = 1554.12 Hz

B.

when source and observer are moving towards each other

f1 = f0*(V + Vo)/(V - Vs)

f1 = 1420*(343 + 5.4)/(343 - 29.6)

f1 = 1578.58 Hz

C.

When the Source is approaching the Stationary observer and observer moving away from it

f1 = f0*(V - Vo)/(V - Vs)

f1 = 1420*(343 - 34.8)/(343 - 29.6)

f1 = 1396.44 Hz

D.

When the Observer is approaching the Stationary source and source moving away from it

f1 = f0*(V + Vo)/(V + Vs)

f1 = 1420*(343 + 29.6)/(343 + 29.6)

f1 = 1420 Hz

E.

When Source and object are moving away from each other

f1 = f0*(V - Vo)/(V + Vs)

f1 = 1420*(343 - 16.2)/(343 + 29.6)

f1 = 1245.45 Hz