Trajectoires orthogonales

Orthogonal trajectories are two families of curves that intersect each other at right angles. A classic example occurs in electrostatics, where electric field lines representing the direction of electric force intersect equipotential lines—which mark constant potential energy—perpendicularly. This perpendicular relationship also appears in gravitational fields, where force direction lines intersect surfaces of equal potential energy at right angles.

To find orthogonal trajectories, first calculate the slope of the tangent to the original family of curves. Orthogonal trajectories must have slopes that are negative reciprocals of the original slopes. This perpendicularity condition ensures the two curve families intersect at right angles and can be expressed as a differential relationship between x and y variables.

Begin by differentiating the equation of the original curve family to express slope in terms of x and y. Apply the negative reciprocal condition to obtain a new differential equation. Solve this equation using separable differential equations methods, which involves separating variables and integrating to find the general expression for the orthogonal trajectory family.

A family of parabolas opening along the x-axis, expressed as x = ky², has orthogonal trajectories that form a family of ellipses centered at the origin. Each ellipse intersects every parabola at right angles. This geometric relationship demonstrates how two distinct curve families can maintain perpendicular intersection throughout their domains.

In electrostatics, electric field lines show the direction of force on a charge and intersect equipotential lines perpendicularly. Similarly, gravitational fields exhibit the same pattern: lines representing gravitational force direction intersect surfaces of constant potential energy at right angles. Both examples illustrate how orthogonal trajectories help visualize and analyze vector fields and potential functions in physics.

The negative reciprocal slope ensures perpendicular intersection between curve families. If one curve has slope m, its perpendicular has slope -1/m. This mathematical relationship translates the geometric requirement of right-angle intersection into an algebraic condition that can be solved using differential equations problem solving techniques.

Eliminating the constant parameter k from the original curve equation expresses the slope in terms of only x and y variables. This step generalizes the slope formula to apply to all members of the curve family simultaneously. The resulting expression allows you to derive the differential relationship needed to find orthogonal trajectories without referencing the specific parameter value.