gravity

Finding the normal force in planar non-uniform circular motion using polar coordinates

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We find an expression for the normal force on a mass which is in planar non-uniform circular motion using polar coordinates.

Just a minute: why do large and heavy ships not sink?

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Until they do due to a mistake, ships do not sink, not even the large and heavy ones. Now and then, textbooks say this is because of dissimilar density. Though not a a wrong statement, it is also not a fundamental one. While ships may sink to the bottom of the ocean thanks to gravity, they also float thanks to gravity.

Why your coffee does not have tides

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A computer-generated 3D image of a cup of coffee. The coffee, however, looks like a moderately turbulent sea. There's a tiny sailboat trying to avoid a lighthouse. All in this cup.

The moon orbits the earth and its gravity is causing the tides. But why don’t swimming pools have tides? Or a cup of coffee? Human bodies consist of water, mostly. Aren’t they tidally influenced by the moon? If you’re asking all these beautiful questions, then what you thought is causing the tides is probably wrong, and here’s why.