Compute the all parts of given problem:
Problem: Dan is gliding on his skateboard at 3.76m/s . He suddenly jumps backward off the skateboard, kicking the skateboard forward at 8.02m/s (as measured by an observer on the ground). Dan's mass is 44.1 kg and the skateboard's mass is 5.09 kg.
Part I: How fast is Dan going as his feet hit the ground?
Part II: If the jump requires 0.112s to complete, what average force did Dan exert on the skateboard?
Please use great detail in explaining the solution. Thanks.