A Proton of Mass M Is at Rest $\vec{U}^{j}$

Multiple Choice

A proton of mass m is at rest when it is suddenly struck head-on by an alpha particle (which consists of 2 protons and 2 neutrons) moving at speed $\vec{U}^{j}$ If the collision is perfectly elastic, wha speed will the alpha particle have after the collision? (Assume the neutron's mass is equal to th proton's mass.)

A) zero
B) 3v/5
C) $2 v / 3$
D) $5 v / 3$