At a waterpark, sleds with riders are sent along a slippery, horizontal surface by the release of a large compressed spring. the spring with force constant k = 40.0 n/cm and negligible mass rests on the frictionless horizontal surface. one end is in contact with a stationary wall. a sled and rider with total mass 70.0 kg are pushed against the other end, compressing the spring 0.375 m. the sled is then released with zero initial velocity. part a what is the sled’s speed when the spring returns to its uncompressed length?