# Homework Help: Speed of sound, frames of reference

1. Jun 23, 2011

### saccapella

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

[PLAIN]http://www.antonine-education.co.uk/Physics%20A%20level/Options/Module_8/Topic_6/cars_4.gif [Broken]

What is the speed of sound, relative to:
A -
B -
C -

2. Relevant equations

None.

3. The attempt at a solution

For B, since B is a stationary reference point, the speed of sound relative to B should 330ms^-1

For A, I thought it should be 360, since A is a moving reference in the same direction as the speed of sound. Meaning A = 330+30=360

For C, I thought it should be 300, since the moving reference is moving in the opposite direction to the speed of sound, making B=330-30=300

My answers for A and C are wrong, however. Help!

Last edited by a moderator: May 5, 2017
2. Jun 23, 2011

### Staff: Mentor

Imagine that the sound is represented by a runner going at some velocity v. From A's point of view, will the runner be moving more quickly or more slowly?

3. Jun 23, 2011

### saccapella

Slowly, right...?

4. Jun 23, 2011

### Staff: Mentor

Right... so... what will be the speed of sound as "seen" by car A?

5. Jun 23, 2011

### saccapella

300?

6. Jun 23, 2011

### Staff: Mentor

Yup.

Now figure out what speed car C "sees".

7. Jun 23, 2011

### saccapella

360...? Because the sound is towards C so the speed will 'seem' faster?

8. Jun 23, 2011

### Staff: Mentor

I think you understand it well enough now to answer your own question

9. Jun 23, 2011

### saccapella

Lol you're right on that :)