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# Rates of change Q watch

1. Sphere of radius R melts such that the radius after time t is r and the rate of change is given by:

dr/dt = -r

If T is the time taken for the radius of the sphere to decrease to (1/4)R, show that T = ln4

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When I integrate I get

ln r = -t + lnA

but don't know what to do from there (if that's right)

Thanks.
2. You know that when t = 0 then r = R.

Use this to solve your constant of integration and you are home.
3. (Original post by Mr M)
You know that when t = 0 then r = R.

Use this to solve your constant of integration and you are home.
Thank you!

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Updated: March 9, 2013
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