Gravity: the phenomenon that keeps our feet on the ground:

By Moira Whitehouse PhD

Gravity is a force.

Gravity is a force that pulls.Every object has gravity.

So every object pulls on every other object.

The more mass an object has, the harder it pulls.

We will use two hypothetical planets for our example. Both the blue and green planets are pulling on each other.

Which one pulls harder?

This should help us see that the more mass an object has the stronger its gravity.

The Earth obviously has more mass than the Moon and it pulls harder. So much harder that the Moon is held in an orbit around the Earth as though by some magically strong string.

But the Moon’s gravity is also pulling on the Earth. So hard that the oceans swell toward the Moon where ever it passes. We call this high tide.

EarthMoon

Let’s think about gravity on our favorite little planet, Earth.

Gravity on Earth pulls everything toward its center.

That’s why there is no top or bottom and no one falls off.

If you dug a hole right through the Earth and fell in, how far would you fall?

As soon as you passed it, you would be pulledback towards the center.

You’d fall into the hole and shoot right past the center because you would be going so fast.

So you would bounce back and forth like a bungie jumper till you finally stopped at the center.

On Earth it appears that not everything falls to the ground at the same rate.It seems to us that things with less mass or weight e.g. feathers fall slower than things with more mass or weight e.g. rocks.

Lighter objects on Earth fall slower due to our atmosphere which slows their descent.

This is a misconception.As demonstrated by Galileo in the 1500’s, all objects in a vacuum, fall at the same rate regardless of mass.

On the Moon, an astronaut dropped a feather and a hammer.

Since there is no atmosphere on the Moon the feather and the hammer hit the ground at the same time.

Demo dropping book and paper.

The weight of an object is a measure of how hard gravity pulls on it.

However, the amount of gravity on each planet differs. The Moon has only one-sixth as much gravity as the earth. Consequently, on the moon you would weigh only one-sixth of what you weigh on Earth.

This boy weighs 60 pounds on Earth.

On the Moon he would only weigh 10 pounds.

Since each planet has a different amount of gravity, this boy’s weight would change each time he went to another planet.

On Jupiter, this would be like carrying an extra 100 pounds around on your back all day.

The farther an object is from the center of a planet, the weaker the force of gravity.

So, would this apple weigh more in some place like Death Valley or on top of a very high mountain?

Weight of a one pound apple heading out to space.

Appleweighs1 pound here

Appleweighs1/4 pound here

Appleweighs1/9 pound here

Appleweighs1/16 pound here

It is because the spaceship, being pulled by gravity, is always falling from beneath you.

In a spaceship like the shuttle, you would be weightless. However this is not because you are so far from Earth that there is no gravity.

Both of these men are weightless, still they are both being pulled by gravity. They have weight

only when gravity pushes them against something like the floor or a scale.

What causes gravity?

Even the great Sir Isaac Newton couldn’t answer that one.

But it is a force that effects everything in the universe.

At his time and still today, what causes gravity is a mystery.