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# C3 differentiation 'rate' question watch

1. The force F newtons,between two magnetic poles is given by the formula

F= 1/(500r^2)

where r(m) is the distance apart.

Find the rate of change when the poles are 0.2m apart and the distance between them is increasing at a rate of 0.03 ms^-1.

_________________

So, I think I want to find
dF/dt

Using the chain rule, that would be

dF/dr * dr/dt

dF/dr = -1000r^-3

dr/dt = 0.03

Multiplying them after substituting r for 0.2 always gives the wrong answer. HELP!
(REally want helP, GIVE help kiNdly)
2. (Original post by Hippysnake)

dF/dr = -1000r^-3
Check this, I think.
3. (Original post by Hathlan)
Check this, I think.
Yeah, stupid me. Thanks. (Will rep, including PM guy)

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Updated: July 13, 2009
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