Year 7 & 8 TOC

Gravity

the Solar System & Moon

LO: Exploring how gravity affects objects on the surface of Earth

Gravity is the force that attracts objects to each other. The Sun’s gravitational pull keeps all the planets aligned. Our earth’s keeps up on the ground, the moon orbiting and holds an atmosphere of gases.

This is why planets with weak gravitational pull, (mercury and mars) have extremely thin atmospheres.

Spinning around with gravity

So, if gravity pulls things in, why don’t all the planets simply gravity into the sun?

Image if you standing with a tennis ball. Now if you just throw the ball a little bit, earths gravity will pull it down and make it crash to the ground. But if we throw the ball hard and fast enough, the ball will follow the curvature of the earth.

Since the Earth is round, the ball you threw would ‘fall’ all the way around the planet. If it goes fast enough it will keep going around and around, the ball would stay in orbit.

Solar system

The Sun has the largest mass of anything in the solar system, it’s mass is 1000 time greater than Jupiter (the largest planet in our solar system). This mass produces a strong gravitational force, making all the other planets, asteroids and dwarf planets in our solar system orbit the sun. As a planet tries to travel forward it’s pulled in by the Sun’s gravity.

Earth & the moon

This is the same for the Earth and the Moon. While the Moon looks like it’s always the same distance from the Earth, it’s actually drifting away from the Earth by 3.7 cm every year! It’ll take many thousand years for the Moon’s increasing distance to make a noticeable difference on the Earth.

Tides

The moon affects the tides. Yep! that’s right. The Moon’s gravitational pull can affect the tides on Earth! While it’s not as big as the Earth it is close, which allows it’s gravitational pull to affect our oceans.

Gravity created by the moon attracts the water toward it, causing the oceans to bulge out in the Moon’s direction. This causes the water to cover a little more land. We call this, high tide. What you’ll notice is that when the moon creates a bulge on one side, it also creates a bulge on the opposite side of the Earth. This is because the Earth is also being pulled towards the Moon.

Remember, gravity is affected by distance and the Earth’s diameter is 12,756 kilometers. This is enough for the moon’s gravitational pull to weaken from one side of the Earth to the other. As the Moon pulls the water from oneside, it also pulls the centre of the Earth too. It’s not strong enough to pull the rest of the ocean from the other side, so that stay in place. Causing a bulge.

When the moon and the sun are on the same side or opposite sides, we have super high tides and super low tides. This is called Spring tides. Neap tides occur when the Moon and sun are at right angles to the Earth. These are smaller than the spring tides.