Professor: Dr. Tovar Contact Information
Text: Serway & Jewitt
1. A 50-kg ball is suspended from a steel wire of length 5 m and radius
2mm. By how much does the wire stretch?
2. A force F is applied to a long wire of length L and cross-sectional area
A. Show that if the wire is considered to be a spring, the force constant k is
given by k=AY/L and the energy stored in the wire is PE=1/2 F (Delta L).
3. An elevator cable is to be made of a new type of composite developed by
Acme Laboratories. In the lab, a sample of the cable that is 2 m long and has
a cross-sectional area of 0.2 mm2 fails under a load of 1000 N. The cable in the
elevator will be 20 m long and have a cross-sectional area of 1.2 mm2. It will
need to support a load of 20,000 N safely. Will it?
4. If you are given a wire with a circular cross-section of radius r and a length
L, show that (Delta r)/r = -1/2 (Delta L)/L, assuming that (Delta L)<<L. Hint:
the density and volume remain constant.
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