- #1
Obliv
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I'm trying to prove that kinetic energy is not conserved in inelastic collisions using the conservation of momentum. This is the set-up. An object A of momentum ##{m_1}{v_1}## collides inelastically with object B of momentum ##{m_2}{v_2}##
using momentum conservation ##P_i = P_f##
[tex] {m_1}{v_1} + {m_2}{v_2} = ({m_1}+{m_2}){v_f} [/tex] solving for ##v_f## we obtain
[tex] v_f = \frac{({m_1}{v_1}+{m_2}{v_2})}{({m_1}+{m_2})} [/tex] now the fun part
prove that [tex] KE_i \ne KE_f [/tex]
[tex] \frac{{m_1}{{v_1}^2}}{2} + \frac{{m_2}{{v_2}^2}}{2} \ne \frac{1}{2}({m_1} + {m_2})(\frac{{m_1}{v_1} + {m_2}{v_2}}{{m_1}+{m_2}})^2 [/tex]
I realize that the two quantities are no longer equal, especially if you try plugging in numbers. I was just wondering if this can be simplified to be seen more clearly.
I multiplied out the numerator and denominator and got some pretty nasty algebra.
[tex] \frac{{m_1}{{v_1}^2}}{2} + \frac{{m_2}{{v_2}^2}}{2} \ne \frac{1}{2}({m_1}+{m_2})\frac{({m_1}{v_1})^2 + 2{m_1}{v_1}{m_2}{v_2} + ({m_2}{v_2})^2}{{m_1}^2 + 2{m_1}{m_2} + {m_2}^2} [/tex]
I'll continue working on it later but if anyone has any shortcuts to simplifying this I would appreciate it.
using momentum conservation ##P_i = P_f##
[tex] {m_1}{v_1} + {m_2}{v_2} = ({m_1}+{m_2}){v_f} [/tex] solving for ##v_f## we obtain
[tex] v_f = \frac{({m_1}{v_1}+{m_2}{v_2})}{({m_1}+{m_2})} [/tex] now the fun part
prove that [tex] KE_i \ne KE_f [/tex]
[tex] \frac{{m_1}{{v_1}^2}}{2} + \frac{{m_2}{{v_2}^2}}{2} \ne \frac{1}{2}({m_1} + {m_2})(\frac{{m_1}{v_1} + {m_2}{v_2}}{{m_1}+{m_2}})^2 [/tex]
I realize that the two quantities are no longer equal, especially if you try plugging in numbers. I was just wondering if this can be simplified to be seen more clearly.
I multiplied out the numerator and denominator and got some pretty nasty algebra.
[tex] \frac{{m_1}{{v_1}^2}}{2} + \frac{{m_2}{{v_2}^2}}{2} \ne \frac{1}{2}({m_1}+{m_2})\frac{({m_1}{v_1})^2 + 2{m_1}{v_1}{m_2}{v_2} + ({m_2}{v_2})^2}{{m_1}^2 + 2{m_1}{m_2} + {m_2}^2} [/tex]
I'll continue working on it later but if anyone has any shortcuts to simplifying this I would appreciate it.