object is covered by considering the sum of all forces. A possible cause of this problem is that the third law is often stated in an abbreviated form: For every action there is an equal and opposite reaction, without the details, namely that these forces act on two different objects. Moreover, there is a causal connection between the weight of something and the normal force: if an object had no weight, it would not experience support force from the table, and the weight dictates how strong the support force will be. This causal relationship is not due to the third law but to other physical relations in the system. Centripetal and centrifugal force Another common mistake is to state that "the centrifugal force that an object experiences is the reaction to the centripetal force on that object." If an object were simultaneously subject to both a centripetal force and an equal and opposite centrifugal force, the resultant force would vanish and the object could not experience a circular motion. The centrifugal force is sometimes called a fictitious force or pseudo force, to underscore the fact that such a force only appears when calculations or measurements are conducted in non-inertial reference frames. See also Ground reaction force Reactive centrifugal force Isaac Newton Ibn Bajjah Reaction engine/jet engine Shear force References Bibliography Feynman, R. P., Leighton and Sands (1970) The Feynman Lectures on Physics, Volume 1, Addison Wesley Longman, . Resnick, R. and D. Halliday (1966) Physics, Part 1, John Wiley & Sons, New York, 646 pp + Appendices. Warren, J. W. (1965) The Teaching of Physics, Butterworths, London,130 pp. Classical mechanics Force