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A cannon ball is fired vertically upwards into the air. How fast would it have to be fired to take 1 second to land?
How fast would it have to be fired to take 10, 100, 1,000 or 1,000,000 seconds to land?
What would be the highest point of the ball in each case?
(Assume that gravity is a constant 10ms$^{-2}$ in your calculations.)
Extension problem: why not try the extension question Escape From Planet Earth?