Vector function, position and tangent vectors

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SUMMARY

The discussion centers on the mathematical proof that if a curve's position vector r(t) is always perpendicular to its tangent vector r'(t), then the curve lies on a sphere centered at the origin. By differentiating the dot product of r(t) with itself, it is established that the derivative 2r'(t)·r(t) equals zero, indicating that the magnitude |r(t)| is constant. This constancy confirms that the curve is constrained to a spherical surface.

PREREQUISITES
  • Understanding of vector calculus, specifically position and tangent vectors.
  • Familiarity with the concept of dot products in vector analysis.
  • Knowledge of differentiation techniques in calculus.
  • Basic principles of geometry related to spheres and their properties.
NEXT STEPS
  • Study the properties of vector functions and their derivatives.
  • Learn about the geometric interpretation of dot products in vector spaces.
  • Explore the implications of constant magnitude vectors in physics and engineering.
  • Investigate the relationship between curves and surfaces in three-dimensional space.
USEFUL FOR

This discussion is beneficial for students studying calculus, particularly those focusing on vector functions, as well as educators and professionals in mathematics and physics who require a deeper understanding of geometric properties of curves.

Maianbarian
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Homework Statement


If a curve has the property that the position vector r(t) is always perpendicular to the tangent vector r'(t) show that the curve lies on a sphere with center at the origin


Homework Equations



The Attempt at a Solution



I have no idea how to even approach this problem, if somebody could give me a nudge in the right direction it would be much appreciated.
 
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Differentiate r(t).r(t) (the dot product).
 
Ah, I get it now.

We know that r(t).r(t)=|r(t)|^2. Differentiating the dot product we get:

d/dt [r(t).r(t)]= 2r'(t).r(t) = 0 (as r'(t) is orthogonal to r(t))

thus |r(t)|^2 is constant, and so |r(t)| is constant, which corresponds to a sphere with the centre at the origin.

Thanks Dick :)
 

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