[Physics] Why is the damping force on a spring oscillator linearly dependent on velocity

frictionharmonic-oscillatorspring

If you consider the damping force is friction like in:
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then the force should be $$F=\mu N$$ where $\mu$ is the coefficient of kinetic friction. Why then is the damping force assumed to be linearly dependent on velocity?

Best Answer

This question is actually one of the lab exercises I teach. For a spring-mass system, if the damping force is friction, then it is independent of velocity (verified experimentally). However, as mentioned in the comments, the damping force may not always be friction. For example, if the mass is a material like aluminium and it is oscillating over some magnets, the damping force will be linearly dependent on velocity.

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