# The Naked Scientists Forum

### Author Topic: ?how does gravity affect a top?  (Read 1595 times)

#### CZARCAR

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##### ?how does gravity affect a top?
« on: 15/07/2011 16:39:40 »
gravity pull is from 1 direction & not earth's gravity which is holding the top down. Q1= does it make any diff if the top is spinning or not [assuming top remains upright without spin]? Q2= does it make diff if external gravity pull [not earth's] pulls from above the top or from the side of the top?

#### PhysBang

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##### ?how does gravity affect a top?
« Reply #1 on: 16/07/2011 13:33:39 »
What makes a difference to the top on the Earth are tidal effects. These are the differences in gravity due to the separation of points along an object. If two objects experience the same acceleration in both direction and magnitude (due to the pull of gravity or anything else), then there is no difference in their relative behaviour.

If we think of the tidal effect between two bodies, then we can think that the gravity toward the near end of each body is greater than that towards the middle and much greater than that toward the far end.

In the Earth, this leads to the tides, as the near water and crust is pulled a little closer to the moon, the middle of the Earth is not pulled as close, and the far side of the Earth is not pulled as much.

In the case of the vertical extent of the top, the difference is probably too small to have significant impact from the moon since the top is probably not very large. Objects farther away that are directly above the top are also going to have little influence on the top. They will have pretty much the same effect on the top as they have on the Earth or at least on the table on which the top is sitting or spinning.

In the case of distant bodies then the real difference is caused by the different angles pointing from different ends of the object to the source of gravity. The different angles between then moon and the Earth towards the sun introduce changes in the orbit of the moon around the Earth. The sun has less of an effect on the moons of Jupiter since the distance between the sun and Jupiter means that the angular difference between a line to Jupiter and a line to any of its moons is pretty small.

Because the top is so small and because the planets, the sun, and the moon are so far away, I doubt that there would be significant tidal effects from them. In every case, the angular difference between a line drawn to the Earth (or the table that the top is on) and any point on the top will be very, very small.

It is theoretically possible to use the spinning of the top to measure these tidal effects, though. One would have to watch the spin very carefully and for a long time, but one could use the wobble or the leaning of the top to account for each gravitational source in the solar system and beyond. But the top would most likely stop spinning well before the amount of rotations that would be required. In the history of gravitation research, the difference in the course of the orbits of planets and moons was used to measure the gravitational interactions of the planets, the moons, and the sun. Here we have time, distance, and size on the side of making the physics appreciable. Still, the change in orbits is often calculated in terms of arc seconds (that is 360ths of a degree) per century.

#### CZARCAR

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• Posts: 686
##### ?how does gravity affect a top?
« Reply #2 on: 16/07/2011 15:03:33 »
gravity pull is from 1 direction & not earth's gravity which is holding the top down. Q1= does it make any diff if the top is spinning or not [assuming top remains upright without spin]? Q2= does it make diff if external gravity pull [not earth's] pulls from above the top or from the side of the top?
maybe a better example would be= top is made of steel & pull is from an electromagnet. Does the spin affect how much the emagnet moves the top sideways?

#### The Naked Scientists Forum

##### ?how does gravity affect a top?
« Reply #2 on: 16/07/2011 15:03:33 »