f=ma
一个重2000kg的车匀速行驶,速度为40m/s,踩下刹车后10秒停止（假设匀减速）
1,刹车的force对车所做的force (what is the breaking force applied to the car?)
2 ,刹车踩下时车又行驶了多少?（how far does the truck travel while the breaks are apllied?)