# Help with another momentum question

1. May 2, 2004

### wikidrox

An explosion blows a rock into 3 parts. Two peices go off at right angles to eachother, a 1.0 kg piece at 12 m/s and a 2.0 kg piece at 8 m/s. The third piece flies off at 40 m/s.

a) Draw a vector diagram to show the direction in which the third piece goes.

I don't know how I can determine which direction it goes

b) What is its mass?

I don't know how I can get this if I am unable to determine the total momentum.

2. May 2, 2004

### HallsofIvy

Staff Emeritus
Set up a coordinate system. Assume the 1 kg piece goes off in the positive x-direction, at 12 m/s, and that the 2 kg piece goes of in the positive y-direction (so the motions are at right angles) at 8 m/s. It should be east to calculate the total momentum of the two pieces. Since the mass was originally at rest and there was no external force (the explosion is an internal force) the total momentum of all three pieces must also be 0: the momentum of the last piece must be in exactly the opposite direction to the total momentum of the first two and of exactly the same length. Divide that length by the speed, 40 m/s, to find the mass of the third piece (sounds like it's going to be small).