How Much Does the Moon Weigh?

How Much Does the Moon Weigh?

How Much Does the Moon Weigh?

It weighs roughly zero pounds.

Okay, that’s probably not the answer you were expecting. Allow us to explain.

When you stand on a scale and weigh yourself (in pounds), you’re actually taking a measure of force. Specifically, the force your body is exerting downwards on the scale, due to Earth’s acceleration of gravity. If you remember Newton, force is equal to an object’s mass multiplied by its acceleration (F=ma). The standard unit for mass is kilograms, which is also how people weigh things basically anywhere that isn’t America.

So weight is the force something exerts on another thing due to gravity (like you exert on Earth). If you’re currently taking a physics class, sometimes you’ll see it arranged as the force something exerts back on another thing due to gravity. But that’s far less intuitive in our opinion, so we’ll be discussing weight in terms of the former.

For all intents and purposes, the Moon is not accelerating towards the Earth (actually, it kind of is, but we’ll get to that). So, in the F=ma equation, F = 0 because a = 0. Since weight is a measure of force, weight = 0.

What Is the Moon’s Mass?

Because the Moon’s acceleration is 0, the value of its mass becomes irrelevant when calculating its weight relative to Earth. But, you might be wondering how much mass the Moon has? That’s a known value. It’s about 7.35 * 10^22 kilograms.

That’s 73,500,000,000,000,000,000,000 kilograms. To put that into perspective, a blue whale is about 200 tons. That comes out to 181,437 kilograms. This means that roughly 405,000,000,000,000,000 blue whales equal the mass of the Moon. Suffice to say the Moon is heavy. But the Moon is only 1.2% of the Earth’s mass (roughly 6*10^24 kg), so the Moon is actually super light in the cosmic scheme of everything.

Related topic: What Color Is the Universe?

Why Does the Moon have Zero Acceleration?

We’ve already established that the Moon isn’t accelerating towards Earth, which is true to a degree. The reason why the Moon isn’t currently on a crash course towards Earth is because of its orbit.

It’s known that the Moon orbits around the Earth. We’ve put together a simplified diagram of how orbits work below. In summary, the Moon is accelerating towards the Earth, but it also has forward momentum perpendicular to its acceleration towards the Earth.

Because these two vectors are roughly equal, the Moon ends up moving in the circular path outlined on the same diagram.

Since one vector is momentum and the other is acceleration, the Moon will eventually (over the course of a very long time) stop orbiting the Earth. Eventually its momentum may decay until Earth’s gravity is far greater, in which case the Moon will collide with the Earth. As it stands though, the Moon is actually gradually drifting away from Earth. Some theorize that instead, the Moon is actually destined to exit Earth’s gravity and drift off into space.

Of course, before any of this happens, the sun will enter its red giant phase. As it begins to expand about 5 billion years from now, it will swallow up the terrestrial planets. And the Moon will have the same unfortunate fate as Earth. So we’ve got that going for us. Which is nice.

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