Simple question about choosing coordinates

[krn93]

Hi, i was just confused on whether certain things will be positive or negative depending on my coordinate system. Let's say I am standing on a cliff, and i make down my positive and up my negative. Would any distance below me be positive, and any distance above me be negative? Also the reference point to determine if a height is negative or positive is from where you set the origin right? Thank you.

 

[gsal]

Any distance below you be positive? Well, not quite...any position will be positive.

A coordinate system indicates positions, not distances. If you are at the cliff, as you say, to which you had climbed without your back pack and now you define the zero of your coordinate axis right where you are and now you start pulling your back pack with a rope...your back pack will be traveling a negative distance as it reaches you even though it has been moving along the positive portion of your coordinates axis.

 

[krn93]

what if i was using the d=volt+1/2at^2 equation for something, and the d was the height of the cliff which I am on top off. gravity would be positive, and would my d be positive also?

 

[gsal]

If the zero of your coordinate system starts right at the cliff and increases down, then, yes, as gravity pulls down, 'a' would be positive and your 'd' would be positive; but 'd' is not the height of the cliff...it is the distance at which an object dropped from the cliff with initial velocity Vo would be at time 't'