Calculating gravity - is your down plus or minus?

[Femme_physics]

Calculating gravity -- is your "down" plus or minus?

I wonder what do most physicists get used to. I prefer to think of down as minus. What about you?

 

[Pengwuino]

I bet most physicists will say -9.8 because your radial basis vector points outwards from spheres and we know when dealing with gravity at the surface of Earth, it's pointing towards the center and thus, against our radial vector, so we would have to pick the negative guy for the correct way of doing things.

 

[Dale]

Either way works, and I have used both depending on the problem. But I do use negative down more often than positive down.

 

[D H]

Sometimes. Sometimes positive, sometimes negative, other times, just a vector somewhere in three space. Depends on the coordinate system of interest.

• Some (pseudo) inertial frame such as J2000: It's just a vector somewhere in three space. Over the north pole it is roughly -z, +z over the south pole, some combination of x and y over the equator. In short, "down" is all over the map.
• Local vertical, local horizontal: +z is toward the center of the Earth, so roughly speaking, +z is down.
• North east down: +z is toward the center of the Earth (geocentric NED) or is normal to the reference ellipsoid (geodetic NED). So again +z is more or less "down" (more rather than less with geodetic NED).
• East north up: +z is "up", more or less.
• North east up: A pox on people who work in left handed coordinate systems.

 

[JDługosz]

Working in ray-tracing, I prefer +Y to be up, so facades match my doodles. On the other hand, sometimes I'm http://www.dlugosz.com/POV/another_world.html" .

(I'll reboot the server tonight. Sorry the link isn't working at the moment but it's worth it to try again)