Linear momentum and velocity direction

[metallica007]

Hi everyone
When using the concept of the conservation of the linear momentum ΣPi = ΣPf to solve a problem, should I consider the the direction of the velocity? For Example, the following problem

which one of the following equations is correct?
m1v0+m2v0=m1v1+m2v2
or
m1v0-m2v0=-m1v1+m2v2
note: both of the blocks have the same intial velocity (v0) because the surface is frictionless.

 

[cjl]

Momentum is a vector quantity, so you absolutely do have to pay attention to the direction when solving problems with conservation of momentum.

 

[Doc Al]

When using the concept of the conservation of the linear momentum ΣPi = ΣPf to solve a problem, should I consider the the direction of the velocity?

As cjl already stated, you must take direction into account.

which one of the following equations is correct?
m1v0+m2v0=m1v1+m2v2
or
m1v0-m2v0=-m1v1+m2v2
note: both of the blocks have the same intial velocity (v0) because the surface is frictionless.

Careful here. You're really using v0 as the initial speed; the initial velocities are +v0 and -v0. After the collision I would not make any assumptions about the directions; just let v1 and v2 be the velocities, which you'll solve for.

 

[metallica007]

Thank you very much guys :)