∫ C ( x 2 + y 2 ) 3 / 2 x d x + ( x 2 + y 2 ) 3 / 2 y d y
If C is the path defined by x = 2 t cos ( 2 π t 2 ) , y = 2 t sin ( 2 π t 2 ) , for 0 ≤ t ≤ 1 , find the integral above.
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Yes, exactly (pun intended)! (+1)
For the members who are not so familiar with the subject, we should point out that a potential of the given 1-form ω = ( x 2 + y 2 ) 3 / 2 x d x + ( x 2 + y 2 ) 3 / 2 y d y is f ( x , y ) = − x 2 + y 2 1 , meaning that d f = ∂ x ∂ f d x + ∂ y ∂ f d y = ω . As you say, the integral depends only on the initial point A = ( 1 , 0 ) and the terminal point B = ( 2 , 0 ) of the path C ; it is "path-independent". Then ∫ C d f = f ( B ) − f ( A ) = 2 1 .
For those who are familiar with single-variable calculus only: The potential f plays the role of an antiderivative, and ∫ C d f = f ( B ) − f ( A ) is a version of the fundamental theorem, analogous to the single-variable case.
In terms of physics, we are looking at an inverse square field here that could represent a gravitational field, while f ( x , y ) could represent the gravitational potential energy (up to a constant multiple, of course).
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The integral may be written as 2 1 ∫ C ( x 2 + y 2 ) 3 / 2 d ( x 2 + y 2 ) = x ( 0 ) 2 + y ( 0 ) 2 1 − x ( 1 ) 2 + y ( 1 ) 2 1 = 2 1 Remark: Since the integrand is an exact differential , its integral does not depend on the whole path of integration; only the initial and the final points matter.