A particle of mass m moves in a center of mass frame of reference with a momentum p=2m. The center of mass moves with a speed v along the axis in the frame of reference of the laboratory. What's the angle of the particle in the frame of reference of the laboratory. What i think I have to do is to calculate the speed of the particle in the laboratory frame of reference, than it is just a simple trigonometric calculation. But I don't really know how to do this.