Applies the specified impulse in the mass center of the physics object.
An entity that simulates its 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 entity's 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.