[填空题]
When you are running and wanb3nob s1x8dt5t+3a7hbklo*v i to stop quickly, you must decelerate quickly. (a) What is the origin of the force that causes you to stop? (b) Estimabk *v7+h 3ailote (using your own experience) the maximum rate of deceleration of a person running at top speed to come to rest. m/s$^2$
