In the case of gravitational forces, the intensity of the force depends on, or is proportional to, the mass density ρ ρ ( x , t ) of the material, and it is specified in terms of force per unit mass ( b i ) or per unit volume ( p i ). These two specifications are related through the material density by the equation ρ ρ b i = p i . Similarly, the intensity of electromagnetic forces depends upon the strength (electric charge) of the electromagnetic field.