What is the pattern of circular wavefronts produced by a moving source?

[Peter G.]

Hi,

The diagram below shows wavefronts produced by a stationary wave source, S. The spacing of the wavefronts is equal to the wavelength of the waves. The wavefronts travel with a speed V.

a) The source S now moves to the right with speed 1/2 V. In the space below, draw four successive wavefronts to show the pattern of waves produced by the moving source.

Ok, so I am in doubt: Should my all my four wavefronts to the right have half the wavelength of the original diagram? And then, to the left, the distance between the wavefronts should be twice the original wavelength?

I am asking this because in my original answer I couldn't check off from the marking scheme:

approximately correct spacing of wavefronts in front and behind source.

Furthermore, what is meant by circular wavefronts originating from four successive source positions? How do I show four different source positions? I have the source as a point, supported by an arrow showing the rightward movement)

Any help appreciated,

Thanks,
Peter G.

 

[Delphi51]

You will need a compass and ruler.
Mark a dot on the right side of your paper for the LAST position of the source. Imagine a circle of radius zero around it - the source has just emitted a circular wave at time 4 periods. Go in the past 1 period. The source travels one half wavelength in that time. Say the wavelength is 1 cm. Then in the past 1 period, it will be one half wavelength to the left - mark a dot. Since one period has elapsed since that was the position, make a circle around it one wavelength in radius. Continue another period in the past and draw a circle another wavelength larger. And so on.