I was reading some text on coefficient of restitution when I encountered this ,
In the following you all would notice that in Impulse of deformation for particle $m_2$ we can see that change in momentum would be $m_2v - m_2u_2 = m_2(v-u_2)$ but what about particle $m_1$. When we write change in momentum for $m_1$ it should have been $m_1v-m_1u_1$ ie: $m_1(v-u_1)$ but then why is it written $m_1(u_1-v)$ . The same thing is happening in the Impulse of reformation part. I can't seem to identify where I am going wrong in my explanation. So can somebody please give me the right explanation.