The runner is moving by uniformly accelerated motion, starting from rest (so, his initial velocity is zero). The law of motion of the runner is
[tex]x(t) = \frac{1}{2} at^2[/tex]
where x(t) is the distance covered after time t, and a is the acceleration of the runner. By re-arranging the formula, we get
[tex] a= \frac{2S}{t^2} [/tex]
We know the runner has covered a distance of S=12m in t=4.0 s, and if we plug these numbers into the equation, we find the acceleration of the runner:
[tex]a= \frac{2S}{t^2} = \frac{(2*12m)}{(4s)^2} =1.5 m/s^2[/tex]
