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## Homework Statement

A stationary boulder explodes into four pieces. After the explosion, three of the pieces have masses and velocities as follows:

M1: 2.5kg, V: 9.0 m/s due east

M2: 3.5kg, V: 27 m/s due west

M3: 2.0kg, V: 15 m/s due north

M4: 3.8kg, V: ?

## Homework Equations

p=mv

## The Attempt at a Solution

p_i=0

therefore p_f=0

therefore m1v1+m2v2+m3v3+m4v4 = 0

Here is where I get confused: does the momentum for Mass 1 and 2 "cancel out" since they're in opposite directions? I tried plugging everything in as positive numbers and solving for v4, but that's obviously wrong ):

Thanks for your time and patience!