Question

two objects of masses m and 2m are moving on a straight line but in opposite direction with same speed u. they stick to each other after collision . the velocity of the combined system after collision

Answer 1

now use the formula for conservation of momentum
mu+mu=mv+mv
mu+2mu=mv+2mv
3mu=3mv
u=v
now the velocity will be u that is the initial velocity ok

Answer 2

Hello friend!!!

Mass of 1st object = m
and mass of 2nd object = 2m

Initial velocity of 1st object = u
and initial velocity of 2nd object = u

After collision, the total mass = u + 2u = 3u

According to conservation of momentum the sum total of the momentum before the collision is equal to the sum total of momentum after the collision.
Mathematically, m1u1 + m2u2 = m3v
where m1 and m2 are the mass of the objects before collision and m3 is the mass of the system after collision.

u1 and u2 are the initial velocities before collision and v is the final velocity after collision.

Thus,
mu + 2mu = 3mv
3mu =3mv
u=v

Therefore, the velocity after collision is same as velocity before the collision.

HOPE THIS HELPS..... :-)