Dork Scratchings

Article in the Telegraph... And now, the punchline...

How fast would you have to fire a cannonball for it to never hit the ground? Newton's very first ideas about gravitational orbits are said to have come about from a thought experiment. A cannonball was known to lose 5 metres of altitude a second after being fired horizontally, but the Earth - being round - curves away from the cannonball as it flies forward. So it occurred to Newton to ask: How fast would the cannonball have to be fired for the curvature to completely compensate for the vertical loss? If a cannonball was fired at this speed it would never lose any altitude, and end up orbiting the Earth. The diagram above shows that the answer can be found using simply trigonometry and comes to $$ \begin{align} v &= \sqrt{gr} \\ &= \sqrt{9.81 ms^{-2} \times 6.371\times 10^6 m} \\ &= 7868\space ms^{-1} \\ &= 17603\space mph \end{align} $$ In general, (non-relativistic) orbits are elliptical The next stage was to look at more general orbit