A running back start 10 yards beind the line of scrimmage (xi=-10 yds i). He runs forward at +5 yds/s until reaches a point 25 yards past the line.

Question

A running back starts 10 yards behind the line of scrimmage (xi=-10 yds i). He runs forward at +5 yds/s until reaches a point 25 yards past the line. What is the running back's displacement and how long does it take him to reach his final position?

Answer 1

How far?

displacement: d = x – x_i

So, if x = 25 yd and x_i = –10 yd, then x – x_i = ?

How long?

d = v × t which means that t = d/v = (x – x_i)/v

If v = +5 yd/s, then t = (x – x_i)/v = ?

Answer 2

Displacement =x_f-x_i= 25 -(-10)=35 yards

Displacement = speed x time

therefore

time=Displacement / speed=35/5=7 seconds