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# Velocity Semi Amplitude

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### #1 Ninjametry

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Posted 22 January 2023 - 08:10 PM

What is the meaning of velocity semiamplitude ? Can you please state the formula for it?

I found this in this exam -

(Question 7 part c solution)

Edited by Ninjametry, 22 January 2023 - 08:10 PM.

### #2 Tony Flanders

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Posted 23 January 2023 - 06:43 AM

The amplitude of a periodic function is the difference between its maximum value and its minimum value. The word "semi" is Latin for half, so the semi-amplitude is half the amplitude. It's particularly relevant for a function like this one that's symmetric with respect to the X axis, in which case the semi-amplitude represent the deviation from the mean.

The curve in question plots radial velocity against time, hence the "velocity" in "velocity semi amplitude."

A good way to answer this kind of question is with a web search. If you had typed "semi amplitude" into Google, you would have found this excellent article, which would have answered your question at greater length, but with more precision, than I did above.

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### #3 Ninjametry

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Posted 23 January 2023 - 09:08 AM

Thank you! i understood what that meant but-

In the graph given in the problem , the maximum and the minimum  velocities  are not equal

The velocity max is 438 km/s( Given as negative)

The velocity min is 350  km/s(given in positive form)

The question they are asking is how fast the binary is moving towards us

my method would be like (438-350)/2 as i thought it would eliminate the rotational speeds along the line of sights

evaluating gives 44km/s

The answer given is 21 km/s

should we divide by 4 or something or am i fundamentally flawed?

Thank you

### #4 Ninjametry

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Posted 24 January 2023 - 08:02 AM

Bump...?

### #5 DeepSky Di

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Posted 24 January 2023 - 09:51 PM

Moved to Science! forum for a better fit to forum rules. Hope you get more replies here, and welcome to Cloudy Nights.

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### #6 Ninjametry

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Posted 25 January 2023 - 05:45 AM

Thank you so much !

Hopefully someone will help me out!

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