  In a recent paper [arXiv:1001.0785], Verlinde has shown that the Newton
gravity appears as an entropy force. In this paper we show how gravity appears
as entropy force in Einstein's equation of gravitational field in a general
spherically symmetric spacetime. We mainly focus on the trapping horizon of the
spacetime. We find that when matter fields are absent, the change of entropy
associated with the trapping horizon indeed can be identified with an entropy
force. When matter fields are present, we see that heat flux of matter fields
also leads to the change of entropy. Applying arguments made by Verlinde and
Smolin, respectively, to the trapping horizon, we find that the entropy force
is given by the surface gravity of the horizon. The cases in the untrapped
region of the spacetime are also discussed.
