Answer:
40 s
Explanation:
By Newton's 2nd law,
The rate of change of momentum is directly proportional to the unbalance force applied on the object,
By that you can get the equation,
F = ma ⇒ a = F/m
but a = (v-u)/t
So we get, t = (v-u)×m/F
where terms are in usual meaning
t =(3.2-0)×15000/1200 = 40 s