Set up the gravity well and add a golf ball and about 20 marbles to it.
Tell the students that the golf ball represents the Earth, and the marbles represent the ocean water.
The gravity of Earth (modelled by the well in the fabric) attracts the water and holds it in place, just as gravity pulls on us and stops us from floating away.
We can model the Moon orbiting the Earth by pushing our hand down into the fabric to make a second well in the fabric, and circling it around the "Earth". As our hand moves, the marbles roll slightly towards the "Moon" and follow it as it orbits.
(Note that it is tricky for students to make the right amount of gravity (pushing into the fabric the right amount) and orbiting at the right speed (moving the hand in a circle around the golf ball), to make the water follow the "Moon" without leaving "Earth". Demonstrate how to make it work.)
The real Moon's gravity also pulls on the oceans, not so much that the water leaves Earth, but enough to make it follow the Moon in its orbit. The water nearest to the Moon will bulge out.
Tides on Earth are a result of this gravitational attraction of the Moon on the Earth's oceans, with additional factors:
The Moon causes the water to bulge out on the side nearest it. Because of the difference in the gravitational pull of the Moon on the near and far side of the Earth, water is also pulled out on the opposite side from the Moon.
The Earth rotates under the tidal bulges, so each point on earth moves through two high tides in one day.
The sun also pulls on ocean water. When the sun and the moon are lined up (new or full moon) the tides are higher (called spring tides). This happens twice a month. At half moon, the sun and moon are pulling water in different directions, so the tides are lower (neap tides).
The land masses and the varying ocean depths mean that the tides are on a more complex cycle than this (https://oceanservice.noaa.gov/facts/moon-tide.html), but they all originate with the pull of the moon on water.
Animation of the tides:
https://oceanservice.noaa.gov/education/tutorial_tides/media/supp_tide0…
https://oceanservice.noaa.gov/facts/springtide.html
Image at https://www.ck12.org/earth-science/Tides/lesson/Tides-HS-ES/
Another interesting phenomenon occurs with the gravity well where if the hand is moved fast the marbles will fly out in a line following the hand. This is a model of the Roche limit, how a satellite breaks up when it gets to close to a planet. See more on page 23 of https://www.spsnational.org/sites/default/files/files/programs/2012/soc…
Also note that students will want to explore more with the marbles, picking them up to make them orbit - follow this activity by using the gravity well more to explore orbit shapes and speeds.
