This week’s lesson covers the moon’s affect on Earth as it revolves around it.

Gravity and Mass

All objects exert the force of gravity on one another. Objects with greater mass exert more force than objects with a lesser mass. I reviewed gravity and mass at this point. When the moon revolves around the Earth, it exerts enough force on the water on Earth to pull it towards it. The sun also pulls the water to it, however the force from the moon is about twice as strong. Around Earth there is a tidal bulge towards the moon, but also one on the other side of Earth. To explain those two observations we need to discuss some of Newton’s Laws.

Newton’s Laws of Motion

We discussed these previously. You can see a demonstration for laws 1 and 3 here (minutes 51-54). You can also demonstrate them yourself.

1)A body in motion tends to stay in motion until acted on by a force, which is referred to as inertia. (This one is particularly important for our current discussion)

3) For every action, there is an equal and opposite reaction

2) We discussed Force= mass x acceleration in detail in a previous lesson. I did a small demo with a tube and marbles to review this.

Tidal Bulges on Opposite Sides of the Earth

As the Earth spins, the water in the oceans bulges out around the equator. Just like water droplets spin off of you if you spin when you are wet, the water on Earth spins out. Earth’s gravity keeps it from flying completely away. The moon and the sun pull this water towards them on the side of Earth that they are nearest to. The tide also bulges on the side of the Earth opposite the sun and moon because on that side the inertia of the water is stronger than the gravitational force from the sun and moon. Inertia and the force of gravity are balanced on other parts of the globe.

The Force of Gravity Decreases with Distance

Newton’s Law of Universal Gravitation states that the force of gravity is proportional to the product of the two bodies divided by the square of the distance between them. This means that the force of gravity decreases rapidly with distance. At this point we went over fractions and the details of the equation, because this is a type of formula the kids had never seen before. I explained that even though the product of the masses of the Sun and Earth is much greater than the product of the masses of the Earth and the Moon, the distance is so great between the Earth and the Sun compared to the Earth and the Moon, that the Moon actually exerts more gravitational force. I used a bunch of mini M&Ms (product of the masses of the Earth and Sun) and a Kit Kat bar (product of the masses of the Earth and Moon) to explain that even though the M&Ms have a greater mass (total amount of candy), they are divided into so many pieces (distance squared) that even of you take a few of them, you will still have less candy than 1 segment of the Kit Kat bar.

Tidal Zones

There are a few areas of the tidal zone. The Splash zone is the farthest from the water and only gets occasional water on it so it is often dry. Animals must be able to live through strong sun, fresh water, and strong waves.  In the high zone, animals must also have thick shells so they won’t be crushed by the waves. It is often wet in this zone. The mid zone is submerged in water during the two high tides and out of water during the two low tides. The animals must be able to survive in both conditions. The low zone is always under water. It has the most types of animals living in it. They are exposed to more sunlight than animals in the deeper ocean.