This demo implements a simple physical simulation using only linear forces.
The cyan sphere is being pulled towards the origin by a spring.  It is also 
being affected by gravity and a slight drag force, as well as forces created
by the user.  You can toggle between three different integration methods:
Euler's method, midpoint method, and velocity Verlet.  Note how the Euler
becomes more unstable as the spring force increases, and midpoint as well but
less so.  The Verlet method is most tolerant of strong forces of all three.

Controls
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I,J,K,L: generate forces in the yz directions to pull the sphere around.
W,S: increase and decrease the spring force
M: switch integration method
SPACE:   resets demo
ESC, Q:  quit
