# 2d Inellastic collisions

hhh79bigo
Hi all,

I was wondering if someone can help me, its been a while since I did classical mechanics, I have a question I would appreciate a little help from anyone!

2 objects with the same mass m and same velocity v have an inelastic collision . After the collision the 2 object system of mass 2*m moves with speed v/2. What is the angle "Theta" between the final line of motion and either of the initial velocities?

All I need is a way to get started if anyone can help!!

Thanks alot guys

hhh79bigo Homework Helper
How about using conservation of momentum?

hhh79bigo
So should I use

2mv=2m(v/2)cos(x) where x is the angle described as theta.

in which case theta is cos-1(2) which cannot be done

or can it Im not sure

Thnx

hhh79bigo :surprised

Homework Helper
HOW are to objects with the same VELOCITY going to collide? If they have the same velocity then they are both going at the same speed in the same direction and will always have a constant distance between them! Did you mean the same speed?

"What is the angle "Theta" between the final line of motion and either of the initial velocities?"
You need to make it clear which of the initial velocities theta is measured with respect to- and the angle between the initial velocities has to come into this somewhere. I would be inclined to set up a coordinate system with the x-axis along the initial line of motion of one of the bodies.

hhh79bigo
The initial velocities are the same, the problem I have been given has said so. The only thought in my mind is that the 2 bodies collide from an angle but the question doesn't specify that.

The first question was copied word for word from my script. The only other thought is that the 1st body is set off at an earlier time than that of the 2nd body. But again my question doesnt specify.

Homework Helper
The two ball are moving with velocities of same magnitude v. This must be the sense of the question. The total momentum of the two balls will not be 2mv (in magnitude) then.

hhh79bigo
hhhhhm i dont fully understand if the masses are the same and the velocity is the same and they are moving linearly, then surely the total momentum before the collision is 2mv

correct me if Im wrong

Im just trying to understand, because this part of mechanics has always confused me

Homework Helper
They are not moving linearly. If you and your friend driving with same velocities on a straight track how one can catch the other, the distance between you two will remain constant, as Hallsofivy has already posted. For collision if they have same speed they must move at an angle.

hhh79bigo
Ok its just that it doesnt specify that in my question from my proffessor it says that the "Velocities" are the same in which case both the direction and magnitude maybe its a type error. I appreciate your help though. I understand that the velocities cant be the same Im just confused :)

Homework Helper
and with the symmetry we can immediatly guess that the final line of motion must bisect the angle between the initial velocities. Is it??????

hhh79bigo
I presume so however there is no specifications to the initial angles.

thanks