means to add ( or subtract as appropriate) the magnitude of all the vertical forces. Consider up to be the positive direction and down to be the negative direction. Enter F followed by three letters indicating the type of force, on the object, by the other object, in that order. For example, enter -Fgpe for the gravitational force on the person by the earth. Please note the negative sign in front of Fgpe indicating the direction of the force is down.