Speed of sound, frames of reference

[saccapella]

Homework Statement

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

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

Homework Equations

None.

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!

 

[gneill]

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?

 

[saccapella]

Slowly, right...?

 

[gneill]

Slowly, right...?

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

 

[saccapella]

300?

 

[gneill]

300?

Yup.

Now figure out what speed car C "sees".

 

[saccapella]

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

 

[gneill]

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

 

[saccapella]

Lol you're right on that :)