[填空题]
A coffee cup on the df qlt g ( +iwunmi/kh0)n y95ortpp1+-ashboard of n6ive u nni:i9j.kq03 by.+lva car slides forward on the dash when the driver decelerates from nj 6 9nyneuk.qi:+vv0b.ii 3l 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?
