Answer:
The answer is c). The alpha particle will have a speed of 3/5 v.
Explanation:
In a perfectly elastic collision the momentum and the kinetic energy are maintained.
The equation to describe the velocity of the particles is:
[tex]VA_f=(\frac{m_A-m_B}{m_A+m_B})VA_i+(\frac{2m_B}{m_A+m_B})VB_i[/tex]
From the problem you have:
[tex]VA_i=v\\VB_i=0[/tex]
Then you solve the equation using the information in the problem. In this case, A is the alpha particle and B is the proton.
[tex]VA_f=(\frac{4m-m}{4m+m})v\\VA_f=\frac{3m}{5m}v\\VA_f=\frac{3}{5}v[/tex]