applyGravity method
Implementation
Offset applyGravity(Offset velocity)
{
// mass is assumed to be 1 until further extension.
// ideally, particles do not need to define a mass unless the physics package is being used.
double newVerticalVelocity = velocity.dy + gravityScale * (10/60);
return Offset(velocity.dx, newVerticalVelocity);
}