Sign Convention for Projectile Motion on an Inclined Plane

[Simon Hall]

Hi, I have a quick question about projectiles on an inclined plane. Specifically, when the projectile is launched down an inclined plane. Is it not possible to use your choice of sign convention? If the projectile is moving from left to right like so:

V
...V
.....V
....__ V__

Could I not choose the x-direction as increasing from left to right just as easily as increasing from right to left?

In the first instance, increasing left to right, the initial velocity and acceleration due to gravity in the x-direction would both be positive, and in the second instance they would both be negative.

The trouble is: it only seems to work (I only get the "right" answer) when it's the former, Ux and Ax are +. Why is this the case, does anybody know?

 

[DrClaude]

It works whichever direction you choose for x. Write down your calculations and we can help you point out where it goes wrong.

 

[BvU]

Hello Simon,

Please post in the homework forum and use the template. Usually x is from left to right; no problem if you are consistent. But gravity often is only in the vertical direction, which we are used to call y !

 

[Simon Hall]

Hi, thanks for the responses. I had thought so. I now realize that I was changing the sign convention without (correspondingly) altering the landing angle formula! Thanks.