# Homework Help: Doppler Effect; Two Moving Objects

1. Jan 25, 2012

### MAtkinson

1. The problem statement, all variables and given/known data

Two auto-mobiles are driving on the same road in opposite directions. The speed of the first auto-mobile is 90.0 km/h and that of the second is 60 km/h. The horns of both auto-mobiles emit tons of frequency 524 Hz. Calculate the frequency that the driver of each auto-mobile hears coming from the other auto-mobile.

2. Relevant equations

Doppler Effect

1. For a Moving Source and Stationary Object
f=fv/(v-vsource)

3. The attempt at a solution

Car one (90km/h) is moving 30 km/h toward the other.

f=524*340/(340-30)

2. Jan 26, 2012

### Staff: Mentor

How did you arrive at this?

Both cars are moving with respect to the road (and air) towards each other. You also need to revise your formula, since both observer and source are moving.

3. Jan 26, 2012

### MAtkinson

Wow never-mind that statement it would the cars would appear to be moving 150 km/h toward each other. But I'm still confused on how to setup the Doppler effect.

4. Jan 26, 2012

### Staff: Mentor

Don't worry about their relative speed, but their speeds with respect to the ground/air. Just like there's a Doppler formula for a moving source, there's one for a moving observer. Here both source and observer are moving, so you must combine both formulas.