初中生体重大约为G=mg=50kg×10N/kg=500N，h=5cm=0.05m，∴做的总功为：W=Gh=500N×0.05m×180=4500J；∵t=1min=60s，∴跳绳时的功率为：P= W t = 4500J 60s =75W．故答案为：4500；75W．
