Linear momentum and velocity direction

  • #1
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.
 

Answers and Replies

  • #2
cjl
Science Advisor
1,898
468
Momentum is a vector quantity, so you absolutely do have to pay attention to the direction when solving problems with conservation of momentum.
 
  • #3
Doc Al
Mentor
45,248
1,596
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.
 
  • #4
Thank you very much guys :)
 

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