(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A hole is drilled from the North to South pole. THe air is evacuated. An object dropped from the surface would accelerate at a=-gs/R(e), where s is the distance of the object from the center of the Earth. Find the magnitude of velocity at the middle of the earth and the time in seconds required for the object to fall to the middle.

R(e)=radius of the Earth given as 6370 km.

g=9.81m/s^2

2. The attempt at a solution

I have figured out the first part.

a(s)=dv/ds * ds/dt = v * dv/ds

Using this, since acceleration is given in terms of position,

a(s) ds = v dv

Integrating with the left side from s0=6370000 meters to s1=0 AND v0=0 and v1=v,

I found the magnitude of the velocity at the middle of the Earth to be 7905 m/s

Then comes the problem. I can't seem to figure out the time it takes. This is what I have so far.

From the above integral, I found velocity as a function of position.

Using v(s) = ds/dt, I put ds/v(s) = dt.

I tried integrating this which would be

ds/SquareRoot(g*s^2/R(e)) = dt.

I get t=12630 seconds. The answer is 1266 seconds. Where did I go wrong?

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# Homework Help: Figuring out time to center of the Earth via a vacuum hole

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