# Calculating gravity - is your down plus or minus?

• Femme_physics
In summary, most physicists say that "down" is more or less "down" depending on the coordinate system used, but some prefer to use +Y to be up instead.
Femme_physics
Gold Member
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?

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.

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

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.

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)

Last edited by a moderator:

## 1. Is gravity always negative?

No, gravity can be either positive or negative, depending on the direction of the force. When an object is pulled towards the center of the Earth, the force of gravity is considered negative. However, when an object is pulled away from the Earth, the force of gravity is considered positive.

## 2. How do you calculate gravity?

Gravity is calculated using the formula F = G * (m1 * m2 / r^2), where F is the force of gravity, G is the gravitational constant, m1 and m2 are the masses of the two objects, and r is the distance between the two objects.

## 3. Does the mass of an object affect gravity?

Yes, the mass of an object does affect gravity. The greater the mass of an object, the stronger its gravitational force. This means that larger objects have a greater force of gravity than smaller objects.

## 4. Is gravity constant on all planets?

No, gravity is not constant on all planets. The force of gravity depends on the mass of the planet and the distance from its center. Therefore, the force of gravity can vary on different planets.

## 5. Does gravity only exist on Earth?

No, gravity exists on all objects with mass. This includes all planets, stars, and even objects on Earth. However, the strength of gravity may differ on different objects depending on their mass and distance from other objects.

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