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Comparing the strength of natural materials like granite and iron with surface tension and surface density, the researchers calculated that such a hollow object could be no more than one-tenth of Earth's radius, making it more likely to be a minor planet than a proper planet.

As the CAR T cells slowly expand, the surface density of PEG becomes diluted, progressively restoring their ability to interact with other cells.

This would be very strange, since very large black holes can form when the surface density of collapsing matter reaches that of, say, ordinary water, and we believe we understand physics at such densities.

We looked at physics papers and read up on fluid dynamics and use surface density calculations so that when the wheels spin or slide, they dig into the surface and find the grip below.

Or time itself might work on the unstable chemistry of a picture, fading it to ghostliness or burnishing it to the surface density of a Dutch master.