# Momentum and collisions question.

• Apr 23rd 2011, 09:21 AM
silvercats
Momentum and collisions question.
2 objects called A and B are weighted.3KG and 2KG.A has 2m/s velocity and B has 6m/s before the collision and they collide face to face.after the collision ,object A moves to opposite direction with a speed of 4m/s.What is the velocity of B after the collision?

answer has to be 15m/s.But I never get it

here how i did it
put m1u1+m2u2=m1v2+m1v2 to the right

3kg * 2m/s + 2kg *( -6m/s) = 3kg * (-4m/s)+2v
=6-12=-12+2v
v=3m/s not 15
• Apr 23rd 2011, 09:29 AM
abhishekkgp
Quote:

Originally Posted by silvercats
2 objects called A and B are weighted.3KG and 2KG.A has 2m/s velocity and B has 6m/s before the collision and they collide face to face.after the collision ,object A moves to opposite direction with a speed of 4m/s.What is the velocity of B after the collision?

answer has to be 15m/s.But I never get it

here how i did it
put m1u1+m2u2=m1v2+m1v2 to the right

3kg * 2m/s + 2kg *( -6m/s) = 3kg * (-4m/s)+2v
=6-12=-12+2v
v=3m/s not 15

looks right... you sure that 4m/s is not relative velocity of A w.r.t B after the collision.
• Apr 23rd 2011, 04:49 PM
silvercats
Quote:

Originally Posted by abhishekkgp
looks right... you sure that 4m/s is not relative velocity of A w.r.t B after the collision.

yup.
• Apr 23rd 2011, 09:04 PM
abhishekkgp
Quote:

Originally Posted by silvercats
yup.

well then i can't find any mistakes if the collision is one dimensional.
• Apr 24th 2011, 12:06 AM
silvercats
I think books's answer must be wrong.I got the same thing too .thanx!