Physics...May/June 08

[larina]

May/June 08 Question 23...
A spring of original length 100 mm is compressed by a force. T he graph shows the variation of the length L of the spring with the comperessing force F.

What is the energy stored in the spring when the length is 70 mm?
A)0.090J B)0.21J C)0.27J D)0.63J.

The answer in the marking scheme is (A).
I'll be really thankful if someone tells me that

 

[larina]

FORCE IS IN ''NEWTONS'' IN THE GRAPH...SORRY FOR THAT

 

[Xenon]

Since the spring is compressed, initial length is 100 mm and final length is 70 mm and thus the energy stored will be the area under the graph from 70 mm to 100 mm.
energy = 0.5 x (6-0) x (0.1 - 0.07 ) [converted to meter]
= 0.09 J

hope that helps

 

[larina]

Thanks Xenon!
bt shouldn't it be the area of the trapezoid...the area under 40 and 70mm load??
i did that before and i was getting (C) 0.27J.
can u please tell me that why did u use the area between 70 and 100mm??

 

[Xenon]

the area under 40 to 70 is not taken as the spring has been compressed from 100 to 70 but not 70 to 40.
the initial length was 100 when there was no force. as the force is applied, the spring compresses and energy is stored. when the force is 6 N, the spring becomes 70 mm. a decrease in length of 30 mm for which 6 N of force was required.

hope u get it

 

[larina]

yup i got it...thanks Xenon