# Homework Help: 2D Conservation of Momentum

1. Dec 11, 2009

### syntax_error

1. The problem statement, all variables and given/known data

Two masses, m1 = 4 kg and m2 = 12 kg, have initial velocities of v1i = 28 m/s [+x 35 +y] and v2i = [-y 60 +x]. If the first mass has a final velocity of v1f = 18 m/s [-y 40 +x], find:

a). The final velocity of m2.
b). Whether or not the collision can be said to be elastic.
c). If a thirst mass, m3 = 8 kg with an initial velocity of v3i = 45 m/s [+x 75 +y], were to strike the 1st mass 5 sec after the initial collision took place, will the first 2 masses ever collide again, and if so, where? Justify your answer. (Assume that this collision is elastic).

2. Relevant equations

P = mv
Pi = Pf
Eki = Ekf

3. The attempt at a solution

a). Using Pi = Pf, I got the velocity of m2 to be 16 m/s [+x 11 +y].

b). The answer I got was that collision is inelastic because,
Eki != Ekf
L.S = 2744 J
R.S = 2184 J
Since L.S is not equal to R.S the collision isn't elastic.

c). Here's the problem. They haven't given any information as to what happens after the collision so I don't know how to use Pi = Pf or Eki = Ekf here. Anybody have any pointers on how to solve this?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 12, 2009

### Staff: Mentor

I don't understand your notation. Can you explain what [+x 35 +y] and [-y 60 +x] mean?

First you'll need the velocity of the first mass after its collision with the third mass. To find that, you'll need to use both momentum and energy conservation.

3. Dec 12, 2009

### syntax_error

[+x 35 +y] is basically the same as [East 35 North] and [-y 60 +x] is [South 60 East].

I found the velocity of the first mass using this equation,

V1f = (m1-m3/m1+m3)v1i + (2*m3/m1+m3)v2i

The answer came up to be 9 m/s [East 80 North]

What next?

4. Dec 13, 2009

### Staff: Mentor

Figure out where m2 is at this time (5 seconds after the first collision), then see if their paths intersect.