✔ 最佳答案
from begining to the maximum point :
v=u+at =>0=10+(-10)t =>t=1 s
so the sand bag reach the maximum after 1 s
and the sand bag back to the starting point after 1 s ( total 2 s )
also , now the bag is 4 10 ms^-1
s=ut+(1/2)at^2=(10)(2)+(1/2)(10)(2)^2=20+20=40m//