# There is no gravity on the moon...

Posted by Akano Toa of Electricity , in Math/Physics Oct 06 2012 · 178 views

Gravity Moon Intro lab
Or, so some of my students in my Intro Physics lab think.   Hopefully when you read the title you were ready to get your typing fingers ready to disprove me.  You probably would have made an argument akin to the following mini-lecture.

Gravity is a force between objects/particles proportional to the objects' mass.  Newton's universal gravitation looks like this:

Fg = - G m1m2/r2

where G is a proportionality constant, the m's are the masses of the two objects in question, and r is the distance between the two objects.  This is why we feel the Earth's gravity affect us, but we don't feel the moon's or sun's gravity affect us.  They most definitely influence the Earth (since the sun causes our orbit and the moon causes the tides), but we don't feel the effects of their presence.

So, if we have an object with mass m on Earth in free fall, its equation of motion is determined by

Fg = m a = - G m ME/r2

where ME is the mass of the Earth and a is the acceleration of the object.  Note that, if we divide both sides by m, we find that

a = - G ME/r2

which means that the acceleration of an object in free fall has nothing to do with the mass of the object.  In fact, you can see a video of this on the moon at Wikipedia's Gravitation page that shows Apollo 15 astronaut David Scott dropping a feather and hammer simultaneously.  Since there is no air on the moon, the feather is not afloat longer than the hammer, and they fall at the same rate and hit the ground at the same time.

Also, while I said earlier that gravity affects things with mass, it also affects light, which does not have (rest) mass.  However, light has energy, and as Einstein showed with his Special Theory of Relativity, energy and mass are equivalent:

E = m c2

So, you can construct the relativistic mass of light, thereby finding the equations that govern the changing of the straight path of light in a gravitational field.  Using Einstein's General Theory of Relativity, you can also view the gravitational field as a curvature of spacetime, which influences straight lines to be curved in the space near the massive object, affecting the path of light.

Another interesting thing about mass:  objects actually have two different masses associated with them:  gravitational mass and inertial mass.  Gravitational mass tells you how much an object interacts gravitationally, while inertial mass tells you how much an object resists a change in motion.  In other words, more massive objects take more force/energy to alter their paths than objects with less mass.  Here's the interesting thing, though:  both these masses are equal, even though there really is no physical law stating that they have to be.  The only reason we know these masses are equal is because empirical evidence says they are; there is no indication that these two masses are different to an appreciable/statistical extent.

So, if you think that there are no unanswered questions in the realm of physics, you are sorely mistaken.

Makaru

So, if you think that there are no unanswered questions in the realm of physics, you are sorely mistaken.

So young. So naive. Wait until you get to the point in physics where you understand that gravity makes pretty much NO sense at all.
Akano Toa of Electricity
Oh, no, I'm fully aware, I just didn't feel like confusing my audience any more.

Wait, why did they think there's no gravity on the moon?
Cosmic Titan
I'm confused, why would gravity not make sense?
Xaeraz

I'm confused, why would gravity not make sense?

Because Gravity really only works well on the scale of BIG. On the scale of small, it kinda breaks down, effectiveness wise. IIRC.
Tekulo the WindWriter
Nothing physics can't explain, eh?

Then how do bicycles stay balanced while in motion (aka when a person is riding one)? And if this has been answered, then KK has lied to me...

Also, many students thought there were no ponies on the moon. And they were wrong... until they weren't.
Brickeens

Then how do bicycles stay balanced while in motion (aka when a person is riding one)? And if this has been answered, then KK has lied to me...

I'm sure my dad explained how that works to me once, but his explanations are always really technical and I don't think I understood a word of it.
Cosmic Titan
But that makes perfect sense. D: I think I missed something.
Akano Toa of Electricity

Wait, why did they think there's no gravity on the moon?

I think they confused it with the fact that there is no air on the moon, which is definitely true. It may also be because the moon is in space, and in space you're "weightless," so they may have just extended that idea to the moon. It's a very interesting pedagogical question.

Cosmic Titan, on , said:

I'm confused, why would gravity not make sense?
Because Gravity really only works well on the scale of BIG. On the scale of small, it kinda breaks down, effectiveness wise. IIRC.

Something to this effect. For instance, we understand pretty well how gravity works at the planetary and galactic scales (even though galaxies do not rotate at the proper rate for Newtonian gravity to be the only power at play, leading to the concept of dark matter), but not how it works in quantum mechanics. There are theories, such as String Theory and Quantum Loop Gravity, that attempt to reconcile this incompatibility, but so far we have been unable to empirically determine whether one or the other is right. For instance, we can physically describe gravity outside a black hole, but near the singularity, which is, for all intents and purposes, at a single point in space, we have no model that accurately describes how gravity, space, and time behave.

Nothing physics can't explain, eh?

Then how do bicycles stay balanced while in motion (aka when a person is riding one)? And if this has been answered, then KK has lied to me...

Also, many students thought there were no ponies on the moon. And they were wrong... until they weren't.

That's actually a good question. A lot of people think that conservation of angular momentum keeps a bike upright, but here's the thing; a research group designed a bicycle with four wheels, two that touch the ground and two that spin in the opposite direction (but same rotational speed) of the other two. This was done to completely take away all angular momentum from the system. However, they were able to also design it so that it would be able to right itself without a rider, and it worked. So, conservation of angular momentum is not necessary to keep a bicycle upright, meaning that there are other forces/physics at play for a bicycle to stay upright.

Paleo
With the bike thing, I think it has something to do with the speed. Something about it occupying most of the movement, preventing you from falling over.

Then again, I'm probably completely wrong, considering Biology is my strong point.
Judge Alex Humva
Bikes use a combination of inertia and micromovements by your hands to maintain balance. Unicycles take these micromovements and transfer them to your legs, which because your legs are less nimble than your hands, is why riding a unicycle is so much harder. Far from impossible as my own brother proves, but still rather tricky.

Your body constantly shifting just slightly goes a long way to maintaining balance.

Akano Toa of Electricity

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