Teaching Physics with the Physics Suite
Some authors seeking public attention have suggested that when many planets are “aligned” (i.e., are close together in the sky) their gravitational pull on the earth all acting together might produce earthquakes and other disasters. To get an idea of whether this is plausible, set up the following calculation.
Note: In fact it is not the gravitational force itself that produces the possibly dangerous effects, but the tidal forces — the derivative of the gravitational force. This reduces the effect by another factor of the distance, i.e., the tidal force goes like 1/r3 instead of like 1/r2. This weakens the planet's gravitational effect compared to the moon's by an additional factor of rearth-moon / rearth-planet, a number much less than 1.
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Page last modified October 11, 2002 GR02