Posted by **Cooper-Nichol** on Sunday, September 30, 2007 at 10:35pm.

Can someone please show me how to derive this...I am going crazy I have tried everyhthing that I can think of with this equation..even the Range Equation and that doesnt work! So please! I am dying and aahaha! How does this work in terms of mass and L? I think L has to be radius though, and you use something to do with centripetal force? HELP

a= gtan?

- Physics Derivation URGENT!! -
**Cooper-Nichol**, Sunday, September 30, 2007 at 10:51pm
that should be a= gtan? (theta, not ?)

- Physics Derivation URGENT!! -
**drwls**, Sunday, September 30, 2007 at 11:28pm
What is the physical situation that this formula is supposed to apply to? What is theta? One can't derive a formula of this type without knowing the problem

- Physics Derivation URGENT!! -
**Cooper-Nichol**, Sunday, September 30, 2007 at 11:35pm
Thats my thoughts exaclty! It is this rubber stopper of mass m is in a car and then when the car accelerates it makes an angle with the veritcal which is theta. We are supposed to find a=gtantheta in terms of mass and L, which must be radius, cause the rubber stopper (which is on a string on a rear view mirror) is spinning, so something to do with centripetal motion...this is all I can gather...please try and help me with this, I know its a lot to ask, but Im drowning with this, its the only question I cant even begin to guess!

- Physics Derivation URGENT!! -
**drwls**, Sunday, September 30, 2007 at 11:42pm
Forget about the rubber stopper spinning and that it has a radius. Just treat it as a mass at the end of a string. The string exerts a tension force T on the stopper. When the car accelerates, the string deviates from a vertical line by an agle theta. Write down the force balance on the stopper.

T cos theta = m a

T sin theta = m g

Divide the first equation by the second:

(sin theta)/(cos theta) = tan theta

= a/g

Note that the mass cancels out.

- Physics Derivation URGENT!! -
**drwls**, Monday, October 1, 2007 at 2:24am
You are right; I got my sines and cosines mixed up, and then I mixed up the equaitons to cancel out my mistake. I was in too big a hurry. Nice work catching my error(s)!

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