Linear momentum and velocity direction

AI Thread Summary
In discussions about conservation of linear momentum, it is essential to consider the direction of velocity since momentum is a vector quantity. The initial velocities of the blocks must be correctly represented, with one potentially being negative if they are moving in opposite directions. The correct equations for momentum conservation should reflect these directional considerations. Participants emphasized that assumptions about post-collision velocities should be avoided, allowing for variables v1 and v2 to be solved independently. Proper attention to direction is crucial for accurate momentum calculations.
metallica007
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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
t9870n.jpg

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.
 
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Momentum is a vector quantity, so you absolutely do have to pay attention to the direction when solving problems with conservation of momentum.
 
metallica007 said:
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.
 
Thank you very much guys :)
 

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