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1. 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?
2. 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.
3. 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?
4. 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.

Spoiler:
Show
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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