Suppose that one day all 330 million people in the US climb up on tables. At time t=0, we all jump off. The resulting shock, as we hit the Earth's surface, will start the entire Earth vibrating in such a way that its surface first moves down from its normal position and then moves up an equal distance above its normal position. The displacement y of the surface is a sinusoidal function of time with a period of about 54 minutes. Assume that the amplitude is 50 meters. (a) At what time will the first maximum (the greatest distance above the normal position) occur? (b) Write an equation expressing displacement in terms of time lapsed since the people jumped. (c) Predict the displacement when t=21. (d) What are the first three times at which the displacement is -37 meters?