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# In one of the original Doppler experiments, a tuba was playedon a moving flat train car at a frequency of 75 Hz., and a secondidentical tuba played the same tone while at rest in the railwaystation. What beat frequency was heard if the train car approachedthe station at a speed of 10.0 m/s?

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asked 2021-03-11
In one of the original Doppler experiments, a tuba was playedon a moving flat train car at a frequency of 75 Hz., and a secondidentical tuba played the same tone while at rest in the railwaystation. What beat frequency was heard if the train car approachedthe station at a speed of 10.0 m/s?

## Answers (1)

2021-03-13
Beat freq is the difference between the two observedfrequencies. The sound heard from the stationary tuba is 75 Hz.From the moving tuba?
$$\displaystyle{f}_{{{o}{b}{s}}}={{f}_{{{s}{o}{u}{r}{c}{e}}}{\left(\frac{{v}}{{v}}-{v}_{{{s}{o}{u}{r}{c}{e}}}\right)}}={75}\cdot{\left(\frac{{343}}{{{343}-{10}}}\right)}={77.25}{H}{z}$$
So the beat freq was:77.25 - 75 = 2.25 Hz

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