The final velocity is 1.64 m/s
Explanation:
We can solve this problem by using the law of conservation of momentum: in fact, in absence of external forces, the total momentum of the two girls must be conserved before and after the collision.
Therefore we can write:
[tex]p_i = p_f\\m_1 u_1 + m_2 u_2 = (m_1+m_2)v[/tex]
where:
[tex]m_1 = 17.5 kg[/tex] is the mass of Olivia
[tex]u_1 = 8 m/s[/tex] is the initial velocity of Olivia
[tex]m_2 = 68 kg[/tex] is the mass of Elizabeth
[tex]u_2 = 0[/tex] is the initial velocity of Elizabeth
[tex]v[/tex] is the final combined velocity of the two girls
Re-arranging the equation and substituting the values, we find the final velocity:
[tex]v=\frac{m_1 u_1 + m_2 u_2}{m_1+m_2}=\frac{(17.5)(8)+0}{17.5+68}=1.64 m/s[/tex]
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