9813

Normal Forces Can Do Work

This Demonstration illustrates the fact that a normal force can do work in moving reference frames. A system consisting of a block sliding on a wedge at angle is seen in an inertial reference frame that moves to the left with speed with respect to the reference frame in which the block and the wedge are at rest. In the normal force is not perpendicular to the trajectory and thus does work. Only in the case of , when the normal force is perpendicular to the trajectory, does the work vanish.

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This problem shows that the work done by a force depends on the reference frame, and consequently its associated potential energy (if any). When the reference frame changes, some work associated with forces that do no work in the initial frame can appear; in the Galilean case this work is easily evaluated from the net impulse.
It is possible to show the expected Galilean covariance of the work and energy theorem. In the more general case of noninertial translational reference frames, the form of the theorem is preserved as long as the fictitious work is included. Additionally, in the center of mass system the total fictitious work vanishes, even if such a system is noninertial.
For more information, see:
R. A. Diaz, W. J. Herrera, and D. A. Manjarres, "Work and Energy in Inertial and Noninertial Reference Frames," American Journal of Physics, 77(3), 2009 pp. 270–273.
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