Explanation:
Given that,
Mass of the cart, [tex]m_1=1\ kg[/tex]
Mass of the cart 2, [tex]m_2=3\ kg[/tex]
Final speed of cart 2, [tex]v_2=0.3\ m/s[/tex]
Final speed of cart 1 is 0 as it comes to rest.
Let us assume that the initial velocity of the cart 2 is 0. So using the conservation of linear momentum as :
[tex]m_1u_1+m_2u_2=m_1v_1+m_2v_2\\\\m_1u_1+0=0+m_2v_2\\\\m_1u_1=m_2v_2\\\\u_1=\dfrac{m_2v_2}{m_1}\\\\u_1=\dfrac{3\times 0.3}{1}\\\\u_1=0.9\ m/s[/tex]
So, the initial velocity of the 1.0-kg cart is 0.9 m/s.