[填空题]
A coffee cup on the dax)+nsg ,)5scrvzl ,o/s ksdr2shboard of a car slides forward ;x-m .qmvp u,mt.jev.xe 6y1a on the dash when the driver deceleratq x,pt.6.v ymxe1j;m emau-v.es 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?
