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In Sample Problem 13-3, we saw an example (a skater) in which kinetic energy appeared but no external work was done. Consider the opposite case. A screwdriver is held tightly against a rotating grinding wheel. Here external work is done but the kinetic energy of the screwdriver does not change. Explain this apparent contradiction.

In Sample Problem 13-3, we saw an example (a skater) in which kinetic energy appeared but no external work was done. Consider the opposite case. A screwdriver is held tightly against a rotating grinding wheel. Here external work is done but the kinetic energy of the screwdriver does not change. Explain this apparent contradiction.

Grade:10

1 Answers

Jitender Pal
askIITians Faculty 365 Points
8 years ago
In accordance to work energy theorem, the net work done by the forces acting on a body is equal to the change in the kinetic energy of the body. It applies only to the particles. In the case of screwdriver, the external force is involving the making and breaking of many microscopic welds, which deform the systems and result in changes in internal energy of the rotating system. Because of the difficulty of accounting for these other forms of energy, and because the objects do not behave as particles, it is generally not correct to apply the particle form of the work-energy theorem to objects subject to rotational motion. Therefore in this system, external work is done but the kinetic energy of the screwdriver does not change.

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