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bkraabel
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Homework Statement
Which head-on collision between a small car and a large truck
causes a larger conversion of kinetic energy: one in which their
initial momenta are the same magnitude, or one in which their
initial kinetic energies are the same? Assume the same kinetic
energy for the two-vehicle system in both cases.
Homework Equations
Notation:
[itex]v_{1,i}^{(1)}[/itex] is the initial speed of small car.
[itex]v_{1,f}^{(1)}[/itex] is the final speed of small car.
[itex]v_{2,i}^{(2)}[/itex] is the initial speed of large truck.
[itex]v_{2,f}^{(2)}[/itex] is the final speed of large truck.
Initial kinetic energy [itex]K_i[/itex] is the same in both cases,which gives
[itex]\frac{1}{2}m_1(v_{1,i}^{(1)})^2\approx m_1(v_{1,i}^{(2)}) \equiv m_2(v_{2,i}^{(2)})[/itex]
and
[itex]v_{1,i}^{(1)}\approx\sqrt{2}v_{1,i}^{(2)}[/itex]
The Attempt at a Solution
Assume same coefficient of restitution in both cases. Because [itex]m_{1} << m_{2}[/itex], this leads to [itex]\frac{v_{1,f}^{(1)}}{v_{1,i}^{(1)}}\approx\frac{v_{1,f}^{(2)}}{v_{1,i}^{(2)}}[/itex]
For initial momentum the same (case 1), I get
[itex]K_{f}^{(1)}\approx\frac{1}{2}m_{1}({v_{1,f}^{(1)}})^2\approx \frac12m\left[\frac{v_{1,f}^{(2)}}{v_{1,i}^{(2)}}v_{1,i}^{(1)}\right]^2= K_i\left[\frac{v_{1,f}^{(1)}}{v_{1,i}^{(1)}}\right]^2[/itex]
For initial kinetic energy the same (case 2), I get
[itex]K_{f}^{(2)}\approx\frac12m_1(v_{1,f}^{(2)})^2+\frac12K_i[/itex]
Calculating the amount of kinetic energy converted into other forms of energy for both cases and taking the difference gives
[itex]\Delta K^{(1)}-\Delta K^{(2)}=\frac12m_1(v_{1,f}^{(2)})^2[\sqrt{2}-1]-\frac12K_i[/itex]
But I can't determine whether the second term is less than or greater than the first term. Another approach is to note that the [itex]K_f^{(2)}[/itex] is greater than one half the initial kinetic energy, so if we could show that
[itex]\left[\frac{v_{1,f}^{(1)}}{v_{1,i}^{(1)}}\right]^2<\frac12[/itex]
then we would know that [itex]K_f^{(1)}< \frac12K_i[/itex]
Any ideas? Other approaches? Mistakes in my algebra?
thanks!