80-m aluminum bar is held with its length parallel to the east-west direction and dropped from a bridge. Now before the bar hits the river below, its speed is 22 m/s, and the emf induced across its length is 6.5 X 10^-4 V. Assuming the horizontal component of the earth's magnetic field at the location of the bar points directly north, a) Decide the magnitude of the horizontal component of the earth's magnetic field, and b) state whether the east end or the west end of the bar is positive.