Let assume the time of the collision of two particle be t The equation of the motion is given by, s=ut+
1
2
at2 For particle A the above equation is written as 30=(0)t+
acos60∘t2
2
30=
0.4
2
×
1
2
×t2 t=√300s This implies that the both particle travel equal distance in horizontal direction in t=√300s The speed at the time of collision is calculated as u=