When Mary is 22 m from the finish line, she has a speed of 4.0 m/s and is 5.0 m behind Sally, who has a speed of 5.0 m/s. Sally thinks she has an easy win and so, during the remaining portion of the race, decelerates at a constant rate of 0.51 m/s^2 to the finish line.

Question

What constant acceleration does Mary now need during the remaining portion of the race, if she wishes to cross the finish line side-by-side with Sally?

Answer 1

So they cross at the same time.

Sally:
d=Vi*t+1/2 at^2 where a=-.51m/s^2 d=22-7
solve for time t.

Mary:
d=22=Vi*t-1/2 a t^2 solve for a, using t above.

Answer 2

d=22-5
not d=22-7