# Relative velocity between a ball hitting a rod [closed]

A ball of mass $m$ travelling with a velocity $v_0$ collides elastically, and perpendicularly with a rod at a distance $d$ from the center of the rod. The rod has a mass $M$ and lies on a frictionless table.

Show that the relative velocity between the ball and the hitting point doesn't change because of the collision.

The rod length is not stated in the problem.

I tried to use the 2 following equations: $$m\vec{v_0}=m\vec{u_1}+M\vec{u_2} \\ md\vec{v_0}=md\vec{u_1}+\frac{1}{12}ML^2\cdot \vec{\omega}$$ where the origin is located at the rod's center, I added a new variable $L$, the rod's length, and $\vec{u_1}$ and $\vec{u_2}$ are the ball's and C.M of the rod velocities after the collision, respectivly. I am not sure whether I can use another eq., related to elastic collisions: $$\vec{v_0}+\vec{u_1}=\vec{u_2}$$ Using the whole 3, I wouldn't manage to show that $$\vec{u_0}=\vec{\omega }d$$ (Which is the condition I need to show).

Is there anything incorrect about my reasoning?

## closed as off-topic by ACuriousMind♦, Rob Jeffries, Kyle Oman, Brandon Enright, Kyle KanosJan 16 '15 at 0:57

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• Yes, it seems me incorrect. When the ball hits the bar, it transmits to the bar an angular momentum $m\vec v_0d$. This angular momentum may not pass integrally to the rod, a part $m\vec ud$ may be retained by the ball (it may be even in opposite direction than $m\vec v_0d$. I presume that as a result of the collision the bar will rotate around its center. The you have – Sofia Jan 16 '15 at 1:18