<P> In an inertial frame of reference, were it not for this net force acting on the stone, the stone would travel in a straight line, according to Newton's first law of motion . In order to keep the stone moving in a circular path, a centripetal force, in this case provided by the string, must be continuously applied to the stone . As soon as it is removed (for example if the string breaks) the stone moves in a straight line . In this inertial frame, the concept of centrifugal force is not required as all motion can be properly described using only real forces and Newton's laws of motion . </P> <P> In a frame of reference rotating with the stone around the same axis as the stone, the stone is stationary . However, the force applied by the string is still acting on the stone . If one were to apply Newton's laws in their usual (inertial frame) form, one would conclude that the stone should accelerate in the direction of the net applied force--towards the axis of rotation--which it does not do . The centrifugal force and other fictitious forces must be included along with the real forces in order to apply Newton's laws of motion in the rotating frame . </P> <P> The Earth, because it rotates once a day on its axis, constitutes a rotating reference frame . Because the rotation is slow, the fictitious forces it produces are small, and in everyday situations can generally be neglected . Even in calculations requiring high precision, the centrifugal force is generally not explicitly included, but rather lumped in with the gravitational force: the strength and direction of the local "gravity" at any point on the Earth's surface is actually a combination of gravitational and centrifugal forces . </P> <P> If an object is weighed with a simple spring balance at one of the Earth's poles, there are two forces acting on the object: the Earth's gravity, which acts in a downward direction, and the equal and opposite tension in the spring, acting upward . There is no net force acting on the object and the spring balance so the object does not accelerate and remains stationary . The balance shows the value of the force of gravity on the object . </P>

What is the centrifugal force of the earth