Dropping a brick

[Martin]

I guess everybody should know how to calculate the falling time at approximately uniform acceleration.

 

Here's a problem. Say Milky masses a trillion (10^12) solar and there is nothing else around and you release a brick at a distance of 3 million lightyears. How long does it take to fall 0.5 million light years?

 

It's elementary. Type this into google:

sqrt(9 million (light year)^3/(G*mass of sun))

 

I've canceled 10^12 in the numerator and in the denominator. Otherwise it is just (approx.) uniform acceleration GM/r^2

and distance fallen = (1/2)a t^2.

 

the point is that google calculator is a remarkable tool because it handles all the units for you. It knows the mass of the sun. It knows what a light year is. It knows Newton's constant G.

 

All you need is simple algebra, solve for the time t.

 

It will even tell you the answer in years. Nice calculator.