According to the conservation of momentum, when two objects collide, their net momentum before collision & after collision is equal.

Imagine two object of masses m_1 & m_2 in motion with initial velocities of u_1 & u_2 respectively. After their collision, they have velocities of v_1 & v_2 (mass is constant). Applying the conservation of momentum, we get,

m_1u_1+m_2u_2=m_1v_1+m_2v_2…(1)

Likewise we can form such an equation for the conservation of kinetic energy that says,

\dfrac{1}{2}m_1u_1^2+\dfrac{1}{2}m_2u_2^2=\dfrac{1}{2}m_1v_1^2+\dfrac{1}{2}m_2v_2^2…(2)

(We know that E_k=\dfrac{1}{2}mv^2)