Aball with a mass of 160 g which contains 4.40×108 excess electrons is dropped into a vertical shaft with a height of 115 m . at the bottom of the shaft, the ball suddenly enters a uniform horizontal magnetic field that has a magnitude of 0.200 t and direction from east to west. if air resistance is negligibly small, find the magnitude of the force that this magnetic field exerts on the ball just as it enters the field. use 1.602×10−19 c for the magnitude of the charge on an electron.