38. (( David is swinging his 0.5 m long sling and exerting a centripetal force of l.250 N. Soon he will hurl the 250 g rock in the sling at Goliath, What is the velocity of the rock as it leaves the sling? ( John is swinging a 100 g mass on a string around his head. On one end of the string is a spring scale that says he is exerting 5 N of centripetal force. Attached to the mass is a speedometer that tells him the mass is moving at 4 m/s. How long is the string