[Sidefx-houdini-list] Gravity in Pop

Mark Alexander malexander at sidefx.com
Mon Aug 24 12:37:49 EDT 2009

Gravity is a force between two masses, as defined by:

Fg = (G * M1 * M2) / (d^2)

where G is a constant, M1 and M2 are the masses of the objects, and d is 
the distance between them.

For objects near the surface of the earth, M1 is the mass of the earth 
(a constant, Me) and d can be assumed to be constant, as the distance 
between an object and the center of the earth remains roughly constant 
(since our atmosphere is a tiny fraction of the overall radius of the 
earth). So, you can put all the constants together and get:

Fg = (G*Me/d^2) * Mo

The acceleration (a) on that mass (Mo) is then defined by F=Ma, so:

a = F/Mo

If you then substitute the gravitational force into that equation, you get:

a = Fg/Mo
   = (G*Me/d^2) * Mo / Mo
   = (G*Me/d^2)

...and the Mo terms cancel each other out. Turns out that G*Me/d^2 just 
happens to work out to 9.8m/s^2.

So, in almost all earthly cases, you can think of gravity as an 
acceleration. It's only when you blast off into space that it should be 
considered a force.


François Duchesneau wrote:
> Hi, a silly one :)
> Do I understand it properly? A force in real world react differently 
> depending of the mass but since gravity is an acceleration, no matter 
> the mass all objects should fall at the same speed. For that reason we 
> must check "Ignore Mass" on a Force Pop to simulate gravity but when you 
> create it through the shelf it is not checked by default. Who's wrong?
> François
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