# Doppler physics homework

1. Nov 28, 2005

### dustybray

I'm having trouble solving this one:

Two motorcycles are traveling in opposite directions at the same speed, when one of the cyclists blasts her horn, which has a frequency of 544 Hz. The other cyclist hears the frequency as 563 Hz. If the speed of sound in air is 344 m/s, what is the speed of the motorcycles? Ans. 5.88 m/s

If I use f[o] = f * ( ( v + v[o] ) / ( v – v ) )

Then I guess I could make v = -v[o]; so,
( f[o] / f ) = ( ( v + v[o] ) / ( v + v[o] ) )
or
( f[o] / f ) = ( ( v - v[o] ) / ( v - v[o] ) )

But how do I solve for v[o]??

Thanks,

dusty.......

2. Nov 28, 2005

### Staff: Mentor

Starting with f[o] = f * ( ( v + v[o] ) / ( v – v ) ),

then f[o]/f = ( ( v + v[o] ) / ( v – v ) ), and then

f[o]/f * ( v – v ) = ( ( v + v[o] ), and then let v = v[o].

But one may be confusing v's.

The equation should include the speed of sound, and in the initial equation, v would be the speed of sound, and on then solves for v,v[o], both being equal.