Trouble understanding aspect of SHM

[PsychonautQQ]

Homework Statement

When deriving the basic equations for SHM, you get the
a_c = /omega^2 A
and then continue on to derive
a_x = -a_c cos/theta
I was wondering where the negative sign came from in the equation above. I don't see the need for it, the x component of the centripetal acceleration seems to point in the same direction as the acceleration "projected onto a diameter" already.

ps why aren't my latex commands for omega and theta not working?

 

[hjelmgart]

I think you need to provide some of the steps, if I am to be of any use. However, I am guessing it's from differentiating cosine twice?

 

[PsychonautQQ]

That is correct. However I was wondering if anybody knew offhand why the sign on the projected acceleration is the opposite sign of the centripetal acceleration

 

[hjelmgart]

I think it is because, it is described as a standing wave. If you look at the formula and imagine a spring going up and down. It will start by falling (since else it can't bounce up again), so of course the acceleration must be negative.

Then look at your cosine function, which gives a positive result until it reaches Pi/2, at that point it reaches the bottom, and it will start to bounce upwards again, and thus the entire expression gives a positive result, since the cosine becomes negative. It could be helpful to look at the cosine function, while reading this somewhat "bad" explanation, but I hope you get it :-)