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 We are all familiar with the effects of centrifugal force, we experience it for example every time we are in a car and take a bend - we feel a force pushing us to the outside of the curve. If, for example, you have placed your sunglasses on the seat next to you it would come as no surprise if, when taking a sharp bend at speed, they slide across the seat.  Centrifugal force is sometimes referred to as a 'fictitious' force, because it is present only for an accelerated object and does not exist in an inertial frame. An inertial frame is where an object moves in a straight line at a constant speed. But Einstein's general theory of relativity allows observers even in a non-inertial frame to regard themselves at rest, and the forces they feel to be real. Centrifugal force is not fictitious, it is a real force.  Centrifugal force arises due to the property of mass known as inertia - the reluctance of a body to change either its speed or direction. A body that is at rest will stay at rest until some force makes it move, and then will continue to move at the same speed and in the same direction unless and until some force changes the way it is moving. This is all neatly summed up by Isaac Newton's three laws of motion.