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So far, all the homework problems I've solved involving a ball colliding (elastically or not) with an immovable wall at some angle $\theta$ with the normal assume that momentum is conserved horizontally, and only the vertical components of momentum are affected by the collision (that is, only the vertical components can be used in Newton's experimental law for finding the value of e).

Here, $$ucos(\theta) = vcos(\alpha)$$ while

$$-e = \frac{vsin(\alpha)}{-usin(\theta)}$$

Diagram showing velocities before and after collision.

Why is this so? I'm not able to understand why colliding with a surface would not decrease the horizontal component of velocity.

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The wall can only execute a force perpendicular to it, so (without friction) only the vertical component can change.

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