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rock.freak667

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**[SOLVED] Coefficient of restitution help**

## Homework Statement

A light elastic string AB, of natural length 0.5m and modulus of elasticity 12N, has a particle of mass 0.3 kg attached to B. The end A is attached to a fixed point at a distance of 0.7m above a horizontal table. The coefficient of restitution between the particle and the table is e. The particle is released from rest at A, hits the table and rebounds to a height of 0.22m above the table. Find the value of e,

assuming that there is no air resistance.

## Homework Equations

[itex]e= \frac{\Delta v}{\Delta u}[/itex]

KE +PE= constant

## The Attempt at a Solution

So using the law of conservation of mechanical energy:

When the mass is moving down the total energy is

[tex]E=\frac{1}{2} \frac{\lambda}{a} x^2 +mgh+ \frac{1}{2}m(u-0)^2[/tex]

[tex]E=\frac{1}{2}\frac{12}{0.5}(0.2)^2 +(0.3)(9.81)(0.7)+ \frac{1}{2}(0.3)u^2[/tex]

When the mass hits the table and comes back up

[tex]E=\frac{1}{2}\frac{12}{0.5}(0.2)^2 + (0.3)(9.81)(0.22) + \frac{1}{2}(0.3)(eu)^2[/tex]

and those two are equal and they should give me e, but I have an unknown,u, as well. What did I forget to do?