Force in a gravitational Field If the field strength at a point in a field is the force per unit mass, it doesn't take a huge leap to realise that the total force acting on an object of mass, M, in a gravitational field will be F = M. g Apply that to the equation for field strength in a radial field to get: Note: that all the points made about the field strength equation are just as valid about the force equation. It is a vector, it's an inverse square relationship, it depends on the masses involved, etc. However, force is not a property of the field. It depends on what is inside the field.