Applies the specified force to the physics object (in Newtons).
An entity that Simulates it's own gravity by applying a force downward on the entity based on the force equation :
(Force = mass * acceleration)
Since, by default, entities already have gravity. The default gravity must be turned off by adding
phys:EnableGravity(false) in the entities Initialize function so that the default gravity doesn't interfere with our custom gravity.
NOTE: We can get the mass of the entity by using the PhysObj:GetMass function.