Hello. I'm new here to the Physics Forum, but I was hoping I could contribute. While I was in high school, my physics teacher told me that gravity at one earth's radius was about -9.8 m/s^2. As it turns out, 'negative means down', and 'down' is in the direction of the earth's core. Isn't 'down' more or less arbitrary? If we were solving a problem on the surface of Mars, we wouldn't use a frame of reference based on the earth's surface; more importantly, if we were to compare the sun to the center of the earth, we might incorrectly arrive at the conclusion that the sun were orbiting the earth. If we have graduated beyond the notion of geocentrism, then it's important to update our methods. Gravitational force, as we all know, is not a uniform force pulling all matter in one direction. With that in mind, I can confidently say that gravity is not negative. If gravity were negative where you sat in New York, then using the same frame of reference - your body - then Australians would fall into space; or, more accurately, Australian gravity would have to be defined as positive in respect to your body. Now, look at the equation for gravitational force. Where, in that equation, is direction defined? The gravitational constant is positive, masses are positive, and distance is fundamentally absolute. The equation addresses only magnitude. If you were to calculate the gravitational force on your body where you are sitting, using this equation, you would determine that you should be collapsed against your ceiling! What I have done is redesigned a few of the classical equations to account for the frame of reference. I'll post two of my equations, first with mathematical notation and then explain it in case you don't understand or can't see the symbols. 1. Angle of incidence. a) Traditional equation: ϴr = ϴi b) Modified equation: ϴr = 2ϴp - ϴi Okay, in what universe does a reflected ray come out at exactly the same angle it enters at? The implications of this equation are disastrous; worse yet, it's authorized by the State of New York, and most likely every other state (although I'm not sure of that). When you actually use the traditional equation properly, it requires you to first measure an angle in degrees (except your measurement is 180 degrees wrong, since an incident ray pointing down and left at 225 degrees will be measured as 45 degrees) and then to mirror your frame of reference in the middle of the equation. In my equation, Angle P is the angle of the plane. Flat, level ground, which is not inclined, would be considered 0 degrees. Angle I is the incident angle: a ray pointing straight down is 270 degrees. Angle R is obviously the reflected angle - the previous ray should reflect straight back up, at 90 degrees. All angles are measured in the same frame of reference. Not only does this equation solve the frame of reference problem and unify the angles of measurement, but it introduces the possibility of an angled plane. This is essential for almost any realistic calculation - like tipped mirrors, sunlight reflecting off of a car, etc. - so you aren't forced to "rotate your graph paper", so to speak. I applied this equation when I was creating a few Flash demos - http://www.soulfox.com/flash/gravitation2.php" [Broken] was my best and most recent - and you can see how well it works. 2. Gravitational Force. a) Traditional equation: Fg = G M1 M2 / r^2 b) My equation: Fg[x,y,z] = G M1 M2 [x2-x1, y2-y1, z2-z1] / r^3 My equation uses two bodies: body 1 has the properties M1, x1, y1, z1, and body 2 has the properties M2, x2, y2, z2. The equation introduces direction and splits the force into three dimensions using a trigonometric equation based on the Pythagorean theorem. If the system is two dimensional, or even one dimensional, the unused variables, Z or both Y and Z, are 0. So... My equations are designed to correct flaws with the traditional equations that made them unintuitive and illogical, and to make them more applicable in programmatic applications. I have successfully used both of my equations without error in Flash programming. I have several examples of the reflection equation, and one example of the gravitation equation, uploaded on my website (the same website in the link earlier in this post). What do you think? Is this worth looking into? I want to redesign all of the equations to correct flaws like this, and to include frame of reference.