# Conservation of momentum in a system, direction changes?

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1. Mar 1, 2015

### x86

We all know that momentum is conserved in a system. But I am confused of one thing.

Suppose we have a system containing a wall with very large mass and a ball and use a classical x-y coordinate axis.

If I swing the ball at an angle of 45 degrees from the horizontal, upwards to the huge wall (which is at rest),

is it possible for the ball to move in a different direction that isn't collinear to its current direction after collision?

Say, after coliding with the wall, can the ball move in the +x direction?

Or will it always be bound to move on the on the 45 degree horizontal line?

I suspect that if we change the angle of colision, i.e. we rotate the wall, then we can make the ball move in any direction we want.

Is this true?

2. Mar 1, 2015

### mathman

I don't fully understand your question. However the incident direction and the reflected direction are in the same plane.

3. Mar 1, 2015

### Staff: Mentor

A real ball with friction and spin can do that.
No matter how exactly the ball will move afterwards, the wall takes the corresponding momentum change in the opposite direction (this can include a vertical component if you consider friction).

If you can rotate the wall, then you can get every direction even without friction, sure.