A body of mass 4 kg is accelerated upon by a constant force, travels a distance of 5 m in the first second and a distance of 3m in the third second. The force acting on the body is

The force is constant, i.e ,the acceleration is constant.

For 1^st second 
$5=u.1+\frac{1}{2}a{\left(1\right)}^2$ 

For 3^rd second

$3=u.1+\frac{1}{2}a(3^2-2^2)$

Solving the above equations gives

$a=-1m/s^2$

Therefore, constant force$=ma=4\times 1=4N.$