Astronomy 1F03 Problem Set 2 Wednesday Oct 12th, 2011 Please write your answers on a separate sheet and show your working. 1. Your weight is the downward force you feel due to the action of gravity on your body when you are resting on a surface. Assume you are standing on the surface of the Earth and the full Moon is directly overhead. DeFne your normal weight as that due to the force of gravity of the Earth alone. Estimate the percentage change in your normal weight due to the combined action of the tidal forces of the Sun and the Moon. 2. a) A typical comet contains around 10 13 kg of water ice. Estimate how many comets would have to strike the Earth to account for the roughly 1 . 4 × 10 21 kg of water currently found on the Earth as oceans (with an average depth of 2-3 km). If this water accumulated over a period of 500 million years, estimate how frequently Earth was hit by comets in that period. (±or comparison such impacts occur on a time scale of millions of years now). b)
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