  The force can be defined from the linear momentum in the gravitational field
and electromagnetic field. But this definition can not cover the gradient of
energy. In the paper, the force will be defined from the energy and torque in a
new way, which involves the gravitational force, electromagnetic force,
inertial force, gradient of energy, and some other new force terms etc. One of
these new force terms can be used to explain why the solar wind varies velocity
along the magnetic force line in the interplanetary space between the sun and
the earth.
