[填空题]
A coffee cup on the dashboa4eh2p v -nll/ord of a car slides forward on the dak qv p n6hj/k95bcb)ctx1y7:ysh when the driver decelerate5y)kcnb tqb6h vpj:9yk/1c 7 xs from 45 $\mathrm{~km} / \mathrm{h}$ to rest in 3.5 s or less, but not if he decelerates in a longer time. What is the coefficient of static friction between the cup and the dash?
