A railroad car of mass m is moving at speed v when it collides with a second railroad car of mass M which is at rest. The two cars lock together instantaneously and move along the track. What is the kinetic energy of the cars immediately after the collision? a. 1/2mv^2
b. 1/2(M + m)(Mv/M)^2
c. 1/2(M+m)(mv/m)^2
d. 1/2(M+m)(mv/(m+M))^2