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Old Jun 30th 2008, 11:49 PM   #1
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Steel wire

A uniform vertical steel wire is stretched by hanging a weight from its lower end. Which of the following would put a greater strain on the wire?(The Young modulus of steel is greater than that of brass.)

(1) Using a steel wire of greater unstretched length.

(2) Using a steel wire of greater cross sectional area.

(3) Replacing the steel wire with a brass wire of the same unstretched length and cross sectional area.

Could anyone explain each statement for me?Thanks
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Old Jul 1st 2008, 04:21 AM   #2
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Originally Posted by werehk View Post
A uniform vertical steel wire is stretched by hanging a weight from its lower end. Which of the following would put a greater strain on the wire?(The Young modulus of steel is greater than that of brass.)

(1) Using a steel wire of greater unstretched length.

(2) Using a steel wire of greater cross sectional area.

(3) Replacing the steel wire with a brass wire of the same unstretched length and cross sectional area.

Could anyone explain each statement for me?Thanks
For the first two, the larger the stress on the wire, the greater the strain because the wire will tend to stretch more. In both of these cases the mass of the wire is greater than in that of the original case, so the force on the wire is greater, thus causing a larger strain.

For the second one Young's modulus for brass is smaller than that of steel, but the density of brass is less than the density of steel, so there is no way to make a comparison of the forces involved. I don't see any way, then, to compare the strain without knowing the actual dimensions of the wire.

-Dan
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