# HW 10 Question 6

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So I am having problems solving for the height of Block M2 after the collision.  I have gotten the height of block one through the conservation of energy and momentum methods, and tried to re-substitute values and it was incorrect. Help?

For this question, you first use energy conservation to find the velocities of each of the blocks before the collision. The collision is elastic, so both kinetic energy and momentum are conserved. The calculations are complicated so you can use the elastic collision equations directly to find their final velocities.

$$v_{1f}=\frac {m_1-m_2}{m_1+m_2}v_{1i}+\frac {2m_2}{m_1+m_2}v_{2i} \\ v_{2f}=\frac {2m_1}{m_1+m_2}v_{1i}+\frac {m_2-m_1}{m_1+m_2}v_{2i}$$

Then use energy conservation to find the heights that each of the blocks goes to. Double check your calculation.
answered Oct 25, 2013 by (21,750 points)
In this question, v1i=v2i (because v=sqrt(2gh), it doesnt matter the mass. So from equation 1, we get v1f=v1i. Similarly, we get v2f=v2i, where v2i=v1i, so the heights (for both blocks) after collision are exactly the same as the heights the blocks released. However, Webassign says thats wrong. Thank you for your help.