9702_s22_qp_13 Question 21

The extension of a copper wire is measured for different forces applied to the wire. A graph is plotted to show the variation of the force on the wire against extension. The maximum force is applied at point P. 

Which statement must be correct? 

A Point R is the limit of proportionality. 

B The elastic potential energy of the wire at point S is given by the area under the graph between points T and S. 

C There is no plastic deformation between points Q and P. 

D The wire obeys Hooke’s law up to a point between R and Q.  

ANSWER: B

The Physics Behind answer

• The limit of proportionality is point S, contrary to option A.
• The statement in option C is incorrect.
• The wire obeys Hooke's Law only till the limit of proportionality S, in contrary to option D.
• The correct answer is C as explained below.
• The elastic potential energy, as long as the limit of proportionality is not exceeded is equal to the work done, W = average force x extension [W = 0.5Fx].
• This is just the same as the area under the graph.