In what sense is the ball moving at 10 meters per second? "The speed of the ball relative to the ground is actually (10^2+8^2)1/2=12.8 metres per second," and if "it takes 0.4 seconds to travel the 5 metres between the players," then relative to the players the speed is 12.5 meters per second (5/0.4). Also, if the time it takes the ball to reach the other player is 0.4 seconds than its speed relative to the ground is (√(5^2 + (0.4(8))^2))/0.4 = 14.84. However, if it takes the ball 0.5 seconds to reach the other player than its speed relative to the players is 5/0.5 = 10 meters per second, and its speed relative to the ground is (√(5^2 + (0.5(8))^2))/0.5 = 12.8.