Re[11]: [ATM] Use of bearings in (static) mirror cells

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Vlad, Don, Richard,

To cut things short:

The force required to move an object over a surface is proportional to the
normal force excerted by the object on this surface. The proportionality is
given by the (dimensionless!) coefficient of dynamic friction, k(v).

F = k(v).N

Note: The value of k(v) is only VERY SLIGHTLY dependent on velocity, in the
sense that it increases with increasing velocity.

The force required to set an object into motion is also proportional to the
normal force, but by the coefficient of static friction, k(0).

F = k(0).N

k(0) is usually larger than k(v), but for some materials (like teflon, PTFE)
this difference is only marginal. Obviously this is the characteristic you
need in dob bearings.

Summarizing, the dependency of k on velocity is that at v=0 it has a
infinitesimally narrow peak (like a delta function), and for higher
velocities it can be assumed constant.

> I ask in public the question you didn't answer when I asked in
> private. Does it require more force to pull a heavy object on a
> sled when moving faster?

Getting back to the argument of Richard: Pulling the object with higher
velocity requires more power, i.e. energy per unit of time. This is not the
same as force.

Hope this settles the debate?

Arjan

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