How Everything Works
How Everything Works How Everything Works

Question 1414

If two planets were really close together and you were between them, how would the gravitational force affect you? — MB & Class
If you were directly between the two planets, their gravitational forces on you would oppose one another and at least partially cancel. Which planet would exert the stronger force on you would depend on their relative masses and on your distances from each of them. If one planet pulled on you more strongly than the other, you would find yourself falling toward that planet even though the other planet's gravity would oppose your descent and prolong the fall. However, there would also be a special location between the planets at which their gravitational forces would exactly cancel. If you were to begin motionless at that point in space, you wouldn't begin to fall at all. While the planets themselves would move and take the special location with them, there would be a brief moment when you would be able to hover in one place.

But there is something I've neglected: you aren't really at one location in space. Because your body has a finite size, the forces of gravity on different parts of your body would vary subtly according to their exact locations in space. Such variations in the strength of gravity are normally insignificant but would become important if you were extremely big (e.g. the size of the moon) or if the two planets you had in mind were extremely small but extraordinarily massive (e.g. black holes or neutron stars). In those cases, spatial variations in gravity would tend to pull unevenly on your body parts and might cause trouble. Such uneven forces are known as tidal forces and are indeed responsible for the earth's tides. While the tidal forces on a spaceship traveling between the earth and the moon would be difficult to detect, they would be easy to find if the spaceship were traveling between two small and nearby black holes. In that case, the tidal forces could become so severe that they could rip apart not only the spaceship and its occupants, but also their constituent molecules, atoms, and even subatomic particles.


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