Homework #8
Phys 222

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.

5.    16.5*
6.    16.6*
7.    16.10*
8.    16.12
9.    16.53
10.  16.60 

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