Which directions should be set to positive? Impulse/momentum

[Specter]

Homework Statement

In a physics lab, 0.30 kg puck A, moving at 5.0 m/s [W], undergoes a collision with 0.40 kg puck B, which is initially at rest. Puck A moves off at 4.2 m/s [W 30 degrees N]. Find the final velocity of puck B.

Homework Equations

The Attempt at a Solution

One thing I haven't understood is which directions to set positive. For example "Let north and east be positive". How do you know which direction to set positive? In this question, would I let north and west be positive? At the beginning of each example question in this unit, they set two directions to positive, but they don't explain why, or how to do it.

Thanks.

 

[kuruman]

You can choose any direction you wish as positive. The convention in two dimensions is "to the right" is positive and "up" is positive. Textbooks follow this convention to make things easier for beginners. With your specific problem, I would choose as positive the direction in which puck A is moving before the collision. The 30 degree angle is probably with respect to that direction.

 

[Specter]

You can choose any direction you wish as positive. The convention in two dimensions is "to the right" is positive and "up" is positive. Textbooks follow this convention to make things easier for beginners. With your specific problem, I would choose as positive the direction in which puck A is moving before the collision. The 30 degree angle is probably with respect to that direction.

Okay, thanks!

 

[esale]

You can choose. Most coordinate systems are arbitrary/relative/subjective. What matters the most is that you are consistent. So, in this instance, if you pick W as positive, then E better be negative. Same goes for N and S.

Also, it helps to understand the [W 30 degrees N] here and then Cartesian system becomes arbitrary. It means you have a vector pointing out straight west, pin the origin down, then rotate the end of the vector 30 degrees toward N. After you understand this, make your point of contact the origin, and you can solve from there.

-E