To Do and Notice

You can model the gravitational contribution to the tides with the following dance.

Arrange three people in a straight line one pace apart. (A pace is the length of one relaxed step, about a half-meter.) Place another lone person 8 paces away on the same line. The lone person is the moon.

Have the person nearest the moon take 3 paces toward the moon, the person in the center take 2 paces and the person furthest away take 1 pace. These people are modeling the distance that a mass would fall under different accelerations.

Notice that the people have moved apart.

What's Going On?

This is what would happen to masses located on the earth if they fell toward the moon and weren't held together by their own gravity. In reality, the gravity forces between these masses and the others that make up the earth act like springs holding them together. As the earth falls around the moon in its orbit, these springs are stretched by leading to the tidal bulges.

The point nearest the moon bulges out toward the moon and the point furthest from the moon is left behind.

This leads to two tidal bulges.

The earth rotates under these two bulges once each day and so there are two tidal bulges a day.

Many museums have a gravity Well which can be used to model tides.

Material

A Gravity Well

Three marbles or other solid spheres.

To Do and Notice

Arrange the marbles in a straight radial line near the outside of the gravity well.

Release all the marbles at the same time.

Notice that as they fall toward the center of the gravity well the marbles move apart.

If the marbles had been part of a solid object the forces that cause them to move apart would have stretched the object creating tidal bulges toward and away from the center of the well.

Arrange the three marbles along an arc of a circle. Space them out so that there is one marble diameter of empty space between each of the marbles. Release the marbles all at once.

Notice how the marbles move together as they fall into the gravity well.

Tidal forces squeeze bodies together in directions perpendicular to the direction to the planet.

What's Going On?

The force of gravity on a mass that is close to a planet or moon is stronger than the force of gravity on the same mass farther from that planet or moon.

Therefore the closer mass will accelerate toward the planet or moon more rapidly than the further mass resulting in the creation of tidal bulges toward and away from the planet.

Three objects that start out along a circular arc will each fall toward the center of the circle, as they fall they will get closer together.

A flexible track with a groove in the middle can be used to create tidal forces.

Material

A flexible track with a groove in the middle.

For example, use a bandsaw to cut a 2 meter long piece of 3/4 inch inside diameter thick walled (Schedule 40) PC into four parts. You should end up with 4 pieces of 2 meter long flexible grooved track.

Three marbles

To Do and Notice

Bend the flexible groove into a shape so that y = -1/x.

It may help to draw this shape on a large piece of cardboard. and then hold the flexible groove against the cardboard as it is bent into shape.

Arrange the marbles in a straight line near the outside of the flexible groove.

Release all the marbles at the same time.

Notice that as they fall toward the center of the gravity well the marbles move apart.

If the marbles had been part of a solid object the forces that cause them to move apart would have stretched the object creating tidal bulges toward and away from the center of the well.

What's Going On?

The force of gravity on a mass that is close to a planet or moon is stronger than the force of gravity on the same mass farther from that planet or moon.

Therefore the closer mass will accelerate toward the planet or moon more rapidly than the further mass resulting in the creation of tidal bulges toward and away from the planet.

Scientific Explorations with Paul Doherty

© 2005

18 August 2005