Question

According to Newton's second law of motion,force depends on rate of change of momentum. what is the force acting on a body which has motion y=ut+1/2at2?

Answer 1

Yes..U have written the equation of motion

Answer 2

According to Newton's Second Law,
$f = \frac{d(p)}{d(t)} = \frac{md(v - u)}{dt}$
Now in your equation of trajectory, the acceleration along the vertical direction is constant which is equal to a.
Therefore,
$f = m( \frac{v - u}{t} ) = ma$