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AS Youngs Modulus Question

I have gotten a youngs modulus question from my revision book and I can't do it!!! Please can someone explain it for me.

The Youngs Modulus for copper is 1.3 * 1011
A tensile force of 150N is applied to a sample length of 1.50m and cross-sectional area 0.40mm². (1mm² = 1.0*10-6m²
Assuming the limit of proportionality is not exceeded, calculate the extension of the copper.

[EXPAND="Answer:"]4.3mm

[EXPAND="My obviously stupid and wrong way of doing it"]
E = (F/A)/(e/l)
1.3 * 1011 = 150/1.5*(4*10-6) / e/1.5
1.3 * 1011 = 150/6*10-7 / e/1.5
1.3 * 1011 = 250000000 / e/1.5
1.3 * 1011/250000000 = e/1.5
1.3 * 1011/250000000*1.5 = 780

780 WTF? Thats not 4.3mm
Reply 1
Avatar for Robob
Robob
17
Young modulus = stress/strain

stress = force/cross sectional area
strain = EXTENSION/original length

--------------

the second to last line of working is wrong, 1.3 * 10^11 / 250000000 = 1.5/e (not e/1.5)
The Youngs Modulus for copper is 1.3 * 1011
A tensile force of 150N is applied to a sample length of 1.50m and cross-sectional area 0.40mm². (1mm² = 1.0*10-6m²
Assuming the limit of proportionality is not exceeded, calculate the extension of the copper.

[EXPAND="Answer:"]4.3mm

Young's modulus
= stress/strain
= (F/a)/(x/l)
= Fl/ax
= (150 x 1.5)/(0.40 x 10-6x)

1.3 x 1011 = 225/(4x10-7x)
x = 225/(1.3 x 1011 x 4x10-7)
x = 4.3 x 10-3m
x = 4.3mm
Reply 3
Avatar for Robob
Robob
17
wait the your attempt is even more wrong.

This is how it should be:

E = (F/A)/(e/l)
1.3 * 10^11 = (150/(0.4*10^-6))/(e/1.5)
1.3 * 10^11 = (150*1.5)/(0.4*10^-6*e)
e = (150*1.5)/(0.4*10^-6*1.3*10^11)
e = 0.0043269231m = 4.3 mm to 2sf

--------------

Damn widowmaker gt there 1st
Reply 4
Avatar for emdiesse
emdiesse
OP
Cheers Guys. I would rep you but apparently I have given out my maximum repage today. I have only done one! Whats that all about, heh.
Reply 5
Avatar for Robob
Robob
17
emdiesse
Cheers Guys. I would rep you but apparently I have given out my maximum repage today. I have only done one! Whats that all about, heh.


lol, ur only allowed 2 rep once every 24 hours, dw tho, ur rep wont be worth nething, apparently it only works if you've gt over 100 posts or something, i wudnt no, my rep isnt worth nething neways coz its so low, lol
Reply 6
Avatar for emdiesse
emdiesse
OP
I think I will just quickly ask this.

I struggle to rearrage some equations because no one has ever taken the time to explain it to me and give me all of the tricks or tips.

What do you do?
Reply 7
Avatar for Robob
Robob
17
emdiesse
I think I will just quickly ask this.

I struggle to rearrage some equations because no one has ever taken the time to explain it to me and give me all of the tricks or tips.

What do you do?


First of all get all of the x's (ie the variable you want to separate) on one side of the equation and then simplify until you are just left with x = ?

Difficult to explain, comes with practice tbh. Do you do maths?
Reply 8
Avatar for emdiesse
emdiesse
OP
rpotter
First of all get all of the x's (ie the variable you want to separate) on one side of the equation and then simplify until you are just left with x = ?

Difficult to explain, comes with practice tbh. Do you do maths?


I used to.

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