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F = G M*m/r^2 where G is the Gravitational Constant - M is the mass of

one object, m is the mass of the other and r is their separation.

So far so good.

Now imagine two point masses as above, separated by an initial distance 2r (it helps the algebra to make it 2r) and that they are initially at rest.

They are then 'let go' and allowed to move together by gravity alone. I could couch this in some fancy language about assuming the space is maximally symmetric blah blah blah - but the initial conditions are as stated above - it doesn't need GR or such.

**Now the question is:**

How long does it take them to collide?

A friend asked me this and I couldn't figure it out, can you?