# Homework Help: Magnitude of momentum of an object to ensure the combined masses will be stationary?

1. Jan 11, 2012

### mirandab17

Three objects travel. Object P (4 kg) travels downward at 8.3 m/s. Object Q (2 kg) travels to the east at 7.2 m/s. Another object (R) is traveling at them from the south east corner.

What is the magnitude of the momentum of object R so that the combined masses remain
stationary after they collide?

I figured out that you simply create a triangle of the momentums, finding that 36 is the magnitude of the momentum of object R as the right answer. However, I don't really get what I did completely. Is it because this triangle represents conservation of momentum? Isn't the object R coming from the south east a seperate factor? As in, as the hypotenuse of the triangle, doesn't it represent final momentum....?

Little confused here.

2. Jan 11, 2012

### Staff: Mentor

Re: Magnitude of momentum of an object to ensure the combined masses will be stationa

Momentum is a vector, and the rules for adding and subtracting vectors will be helpful here. A vector pointing to the southeast can rewritten as the sum of a vector pointing to the south and another vector pointing to the east.

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