A ball of mass 0,15 kg is thrown vertically downwards from the top of a building to a concrete floor below. The ball bounces off the floor. The velocity versus time graph below shows the motion of the ball. ignore the effects of air friction. TAKE DOWNWARD MOTION AS POSITIVE. 3.1 velocity (m-s*¹) 20 -15 time (s) From the graph, write down the magnitude of the velocity at which the ball bounces off the floor.​