An athlete sprints to the finish line at the speed of 10m/s, and then immediately slows down the run-up for 20 meters and then stops. Suppose it takes xs * * time from the sprint to the deceleration run-up to 15m?

A:

Acceleration: v 2-VO 2 = 2ax

V=0, v0= 10, x=20, and a=-5/2 is obtained by substitution.

s = v0t+ 1/2at 2，

Get15 =1/2 * (5/2) 2 * t+10t, get t = …