A ball is moving with speed 20m/s collides with a smooth surface. The magnitude of change in velocity of the ball will be :
A person standing on the edge of a cliff of height 40 m above the ground throws the first ball straight up with an initial velocity of 20 ms, and then throws the second ball straight down with the same initial velocity 20 ms. What are their respective velocities just before they hit the ground? (Take a = 10 m/s2)