# Basic question about gravity

1. Feb 24, 2008

### jj8890

[SOLVED] basic question about gravity

This is just a simple concept question. A person is moving on a ferris wheel around in a circle and the magnitude of the work done on them moving from top to bottom is W=2*g*m*r where m=mass and r-radius of ferris wheel. When plugging in the numbers into the equation, the answer would always be positive correct because gravity is down, positive and mass is positive and the radius is positive. For some reason I am thinking that maybe the gravity would be negative when moving down. I would appreciate any help.

2. Feb 25, 2008

### physixguru

It's all about defining your directions. Once you pick one as positive, the opposite direction must be negative.

Up is usually defined as positive. In this set of coordinates, since gravity is a downwards force, g is -9.8 m/s

There's nothing to stop you from defining down as the positive direction. In this case, since gravity now acts in the same direction as you have defined as positive, it would have to be +9.8 m/s
Sign convention of direction is a matter of choice.You can take the desired axes to be positive and negative.It depends on your accessibility and easiness.There is no hard and fast rule for it.But in most cases the cartesian convention is used.