Help with understanding this momentum problem

[Kolika28]

Homework Statement

I understand the task and what I supposed to do. The only thing I don't understand is why I need a minus sign when calculating $v_x$

Relevant Equations

$v_x=v*cos(\theta)$

Solution on the link: https://www.slader.com/textbook/9780321675460-university-physics-13th-edition/269/exercises/12/

 

[Lnewqban]

The initial x velocity of the ball is considered to be positive; therefore, the post impact x velocity is negative (opposite direction).

 

[Delta2]

Here we have the typical sign convention used in physics when we have movement along a line: we consider one direction (lets say to the right) positive while the opposite direction (to left) negative. So velocity is considered to be positive when it travels towards the right and becomes negative when it travels towards the left.

 

[Kolika28]

Sorry, for not answering before now. My internet has been down the last couple of days. I understand the minus sign now. Thank you so much for your answers, I really appreciate it!

 

[Lnewqban]

You are welcome