If the Earth became a black hole, how big would it be?

You don't need to know much about general relativity to get the right answer up to a factor of 2. You just need to know it involves only the Earth's mass, the gravitational constant G and the speed of light.

(1/n)

If you just build a distance from the Earth's mass m, G and the speed of light c, you get Gm/c². This answer will be half the correct answer - not bad!

A bit of Newtonian physics actually gives the right answer. Laplace came up with the idea of a black hole in 1796!

(2/n)

So, if you look up G, c and the Earth's mass and use a calculator, you can figure out how big the Earth would become if it were squashed down to a black hole!

At worst you'll be off by a factor of 2.

I enjoy calculations calculations like this... easy and fun.

(3/n, n = 3)