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They're traveling away from their origin at constant velocities, so they're traveling relative to each other at constant velocities as well.
The magnitude of the resulting vector (i.e., speed) can be calculated trivially since their movement is perpendicular on a plane, as the root of sum of squares, which many could recognize as the Pythagorean theorem:
√((5 ft/s)² + (1 ft/s)²) = √26 ft/s ≈ 5.1 ft/s
You can verify this by finding that their average speed apart is the same at all times (for all t > 0):
Vavg = √((t * 5 ft/s)² + (t * 1 ft/s)²) / t = √(t² * ((5 ft/s)² + (1 ft/s)²)) / t = √26 ft/s
Don't forget to calculate the location where everything about them began and then include the curvature of Earth considering the latitude of said location into your speed calculation.
No, they're spherical children in a vacuum.
for approximation we can assume that the boy is a point mass and the girl is a lie
Oh, so we have to calculate the gravitational attraction pulling them back. Fucking hell
Augustus! Save some room for later.
https://en.m.wikipedia.org/wiki/Spherical_geometry
I couldn't find 'potatoy geometry' for a better approximation of earth.
You'll note that I already assumed that they were on a plane, not the surface of a sphere.
I'm also noting the stick up your ass. 🙄
If the potato remark and subreddit don't tip you off that I was being flippant, I don't know what will.
No, the stick would be a one-dimensional line.