An Yb 31 -doped optical fiber is laser cooled in vacuo from 298 to 282 K. Cooling results from anti-Stokes fluorescence of the ytterbium ions after optical pumping at a wavelength of 1015 nm. The sample temperature is deduced from the emission spectrum, which is sensitive to the populations in the excited-state multiplet of the ions. The temperature change is limited by the coupling between the fiber and ambient blackbody radiation, as confirmed when samples suddenly exposed to the pump laser are found to exponentially relax towards thermal steady state with the expected time constants. [S0031-9007(97)02372-7]
A simple model is developed that predicts the coefficient of rolling friction for an undriven laboratory cart on a track that is approximately independent of the mass loaded onto the cart and of the angle of inclination of the track. The model includes both deformation of the wheels/track and frictional torque at the axles/bearings. The concept of rolling friction is contrasted with the static or kinetic friction that in general is also present, such as for a cylinder or ball rolling along a horizontal or inclined surface.
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