Add a continuous force to the rigidbody, using its mass.

Apply the force in each FixedUpdate over a duration of time. This mode depends on the mass of rigidbody so more force must be applied to push or twist higher-mass objects the same amount as lower-mass objects. This mode is useful for setting up realistic physics where it takes more force to move heavier objects. In this mode, the unit of the force parameter is applied to the rigidbody as mass*distance/time^2.