When turning a corner, the acceleration is toward the corner while the perceived force is away from the corner. Such forces are called inertial forces . The right panel of figure 6.2 shows the inertial force observed in the reference frame of an object moving in circular motion at constant speed. In the case of circular motion the inertial force is called the centrifugal force . It points away from the center of the circle and just balances the tension in the string.
General relativity says that gravity is nothing more than an inertial force.
This was called the equivalence principle by Einstein. The obvious problem with this interpretation of gravity is that we don't appear to be moving away from the center of the Earth, which would seem to be a natural consequence of such an acceleration. However, relativity has a surprise in store for us here. Spacetime diagram showing the world line of the origin of a reference frame undergoing constant acceleration. It follows from the above considerations that something can be learned about general relativity by examining the properties of accelerated reference frames.