How Does The Moon Move Huge Masses Of Water, But Can’t Move a Small Pebble?


The moon causes the ebbs and flows, but at the same time, other ordinary objects like pebbles and people do not move at all from its force of attraction.

We all have studied Newton’s law of universal gravitation.

Newton’s law of universal gravitation
Newton’s law of universal gravitation

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As we know from this law, the attraction between two bodies depends on their masses and on the distance between them. The more mass is, the greater the attracti on is, and the larger the square of the distance is, the less attraction is.

Knowing this law, we can calculate the force of gravity of the moon acting on ocean water. But since the Earth has a rather large diameter, about 3.5% of the distance between the Earth and the Moon, the force of attraction of the Moon acting on the ocean water will differ on the side of the Earth-facing the Moon and on the opposite side of the Earth.




attraction between two bodies

If we vectorially add these forces to the forces of gravity of the Earth, we get the following picture:

attraction between two bodies

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The forces indicated by the arrows are called tidal. These forces tend to stretch the body towards the gravitating mass. It is these forces that cause the appearance of two tidal humps on the surface of the ocean.

Moon force of attraction
Moon force of attraction

However, we ourselves do not feel any tidal effect of the moon. The reason for this is that the size of people and objects around us is too small for the difference in the attraction of the moon on different sides of the objects to be any significant. For the same reason, tides do not occur in shallow reservoirs, such as lakes.

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