# If the Earth were flat...

**Electricity and Magnetism**Level 4

There was a time, long ago, when people thought that the Earth was flat. Let us suppose that the Earth is a flat disk with density \(\rho=5.5\times 10^{3}~\text{kg}/\text{m}^3\). The acceleration due to gravity, right at the surface of the planet, and close to center of the disk is \(g=9.8~\text{m}/\text{s}^2\). Determine the thickness \(h\) **in kilometers** of the disk. You may assume that \(h\ll R\), where \(R\) is the radius of the disk.

**Details and assumptions**

The universal gravitational constant is \[ G=6.67\times 10^{-11}~\text{m}^{3}\text{kg}^{-1} \text{s}^{-2} \]