223 Homework 7

Spring 1997

Due: Friday April 18, 1997

In this problem we derive the principle of conservation of Energy. The kinetic energy of a particle moving with speed v is tex2html_wrap_inline56 . For a conservative vector field tex2html_wrap_inline58 with potential function f(x,y,z) (in the physicist's sense) so that tex2html_wrap_inline62 . The potential energy of a particle at position tex2html_wrap_inline64 is tex2html_wrap_inline66 . The Conservation of Energy Principle says that the expression

eqnarray13

is constant for a particle moving in the field with position vector tex2html_wrap_inline64 and velocity vector tex2html_wrap_inline70 . Let P and Q be two points in space and let C be a path from P to Q parameterized by tex2html_wrap_inline82 , tex2html_wrap_inline84 , where tex2html_wrap_inline86 and tex2html_wrap_inline88 .

  1. Let tex2html_wrap_inline70 be the velocity of the particle and tex2html_wrap_inline92 be the acceleration. Show that

    equation32

  2. Use the previous part and Newton's second law of motion, tex2html_wrap_inline94 to show that the work done by tex2html_wrap_inline58 as a particle moves along C is equal to the kinetic energy at Q minus the kinetic energy at P.
  3. Assume that the vector field tex2html_wrap_inline58 is conservative as defined above. Use the fundamental theorem of calculus for line integrals to show that the work done by the force tex2html_wrap_inline58 along the curve C is equal to the potential energy at P minus the potential energy at Q.
  4. Use the previous two parts to show that the total energy at P is the same as at Q.