You are correct that the final velocity is needed to calculate the acceleration in the equation I provided.
To solve this problem, we can use a different equation that relates displacement, initial velocity, acceleration, and final velocity:
s = ut + (1/2)at^2
where: s is the displacement (800 m), u is the initial velocity (0 m/s), a is the acceleration (4 m/s^2), t is the time.
We can rearrange the equation to solve for t:
800 = 0 + (1/2)(4)t^2 800 = 2t^2 t^2 = 400 t = √400 t = 20 s
Now that we have the time it takes for the body to travel 800 m, we can calculate the final velocity using the equation:
v = u + at
v = 0 + 4(20) v = 80 m/s
Therefore, the final velocity of the body is 80 m/s.