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    A guitar string has length 0.7m and cross sectional area o.2mm^2. A constant tension of 4.2N is applied to the string causing a strain of 0.015. Calculate the stress of the string.

    I got: F/A = 4.2/0.2*10-3 = 21000

    But the actual answer is 2.1* 10^7

    I really don't understand how they got that answer, can someone please explain it to me?
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    you haven't converted to meters properly

    work out what the radius is in mm, then convert to meters, then find the cross sectional area in meters squared.
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    You have to be careful when converting areas given in units other than square metres into square metres.

    For example, there are 100 cm in 1 m. Does this mean that there are 100 square cm in a square m? This cannot be right - you could arrange 100 squares, each of size 1 cm x 1 cm, along the edge of a metre ruler. But you would need 100 such rows to cover an area of one square metre (i.e. 1 m x 1 m), so that is 10,000 square cm in a square m!

    Can you see how to apply this idea in the case of your problem?
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    Have a think about what Pangol said first. Once you've made some progress, look at the way I've done it, using basic mathematical laws.

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    1mm = 1m*10^-3

    square both sides

    1mm*1mm = 1m*1m*(10^-3)*(10^-3)

    1mm^2 = 10^-6*1m^2

    So you need to multiply by 10^-6 not 10^-3 to convert mm^2 into m^2
 
 
 
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