Similarly, why does The Moon’s gravity affect the tides but won’t, for example, cause a paperclip to slide across a desk?
In: Physics
In both cases, you can plot an orbit around the Earth – the moon, being much further away and moving much faster, manages to miss the Earth and make a complete loop. However, when you jump, you’re so much closer that your path intersects the Earth and you collide
Well, you don’t fly off into space. You can barely jump like maybe the height of your body and then you “fall back to the Earth” pretty damn fast. So in a way the Earth’s gravity is strong enough to keep you on Earth, and weak enough to merely keep the Moon in orbit.
The force of gravity itself increases with mass, but decreases with the SQUARE of the distance, and the Moon, while “immense”, is also quite some distance away, compared to how close you are to the Earth.
But ultimately, tie a rock to a rope and see what force is required to “keep the rock from breaking the rope and flying away”. If you put the rock and the rope on the ground, the force required is 0. If you pick it up and start spinning it around above your head, you’ll have to hold the rope with some force. You’d have to spin very fast to actually have the rock break that rope, beyond what you can do with just your muscles.
So bottom line, the Moon doesn’t spin around the Earth all that fast. Gravity is enough to keep it there, like a rope would.
The strength of gravity depends on the mass of both gravitating objects. You are presumably much, much lighter than the Moon, so it’s not impossible for you to overcome the much weaker force of gravity with your legs and jump.
> Similarly, why does The Moon’s gravity affect the tides but won’t, for example, cause a paperclip to slide across a desk?
It does cause a paperclip to move towards it. However, it also makes the desk, and the building you’re in, to move towards it by approximately the same amount, so they stay fixed relative to each other. The displacement distance technically depends on how close the object is to the Moon, but on the small scale that you’re talking about the difference between the desk and the paperclip would be minuscule and unnoticeable.
There isn’t anything strong pushing the moon away, so gravity would pull it down immediately if it wasn’t moving.
It is moving sideways fast enough that gravity just makes it move in a circle instead. If you throw a ball, it goes in a curve until it hits the ground, if you throw it faster, it goes further before the curve that the surface of earth has and the curve the ball follows meet. If it goes fast enough, the curve of the ball never runs into the earth, and comes all the way back around.
You are thinking of gravity like it’s a giant muscular arm pulling at something. If that was the case, pulling a massive object would have less of an impact than pulling something light. If that’s how gravity worked, the force to keep the moon in orbit would flatten everyone in Earth.
In reality, Gravity is just a constant speed change. It accelerates all objects who are the same distance from the center at the exact same rate. If you shoot a bullet straight up in the air, it might leave the barrel at a speed of 700 meters per second. It’s speed relative to you will decelerate to 690 meters per second by the second second. 10 seconds in, that’s 600 meters per second now. In reality, the bullet slows down a lot faster because of air resistance, but, in a vacuum, a 5 ton boulder and a a bullet weighing a fraction of an ounce would decelerate at the same speed.
The reason for this is that Gravity isn’t necessarily a force. We calculate it as such because it behaves like one, but that’s just a short cut. Gravity alters space time so that the path of the object is altered. This is why the mass of the object is irrelevant (I mean it matters because the object also has gravity which also alters space time, but those are overcomplications).
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I really want to try to explain this in a bit more complex way that really made it all click on my head. Hopefully everyone can follow along.
So to start with, we should note that there are two forces that act on you: gravity pulling you down, and Earth under you, refusing to let you in and therefore pushing you up. A net result is, you stay roughly still.
So, to understand gravity better, lets erase Earth with its upward pulling, space-taking, ball-resembling properties and just leave the gravitational field. Basically, we make it so that you can freely fall through Earth unaffected by anything other than its gravity, to better understand gravity.
Now, for you this would mean you start falling. But once you are at or near the center of Earth, it’s not like you stop. Your momentum now keeps carrying you away from Earths core, and you emerge from the other side at about the same height as you started with, before gravity finally wins and manages to turn you around and make you fall back towards Earths core.
So when you think about path you trace, you notice it would resemble a line, or perhaps an ellipse if you started with sideways motion. And now to high school physics part of this, you notice that all you’re doing is exchange gravitational potential energy to kinetic energy and back. Your total energy stays the same always.
And this means, without using energy to change your trajectory, you could not reduce this total energy you have and stay, on average, closer to Earths core. You make flybys but cannot stop going away again.
And you notice, for Moon these scenarios, with fall-through Earth and real Earth, are identical. Moon just keeps falling but has enough sideways motion that instead of huge yoyo motion, it does something more resembling circular orbit.
And to go back to the example of you jumping, that can be seen as you adding a tiny extra bit of energy to your orbit. But on real Earth, this orbit again ends right as you smash back onto Earths surface.
So gravity is not a force working on you, it is a force created by all objects. It binds us together (like…well…”the force”). You are pulling on the earth just like the Earth is pulling on you, but your mass is so much smaller that your gravity doesn’t do much to the earth. The sad fact is that, while it is a big part of our life, gravity just isn’t that strong. You can overcome the gravity between you and the earth to move yourself away, but not so far that the Earth’s gravity won’t pull you back, but that doesn’t mean it’s super strong; tiny little birds are able to constantly push against Earth’s gravity by pushing on the air.
The Moon is a whole different beast because now it has enough mass to have measurable gravity of its own so it is actually pulling on the Earth, JUST like the Earth is pulling on the Moon, less because the moon is smaller. If the Moon had no gravity, you are absolutely right that it would just fly off, but the Moon is also holding onto the Earth, so much so that it actually causes the Earth to wobble! https://images.app.goo.gl/C7z1vCNXbeReiMZr6
For the water and paperclip comparison water is a liquid and so it can flow in response to the pull while a paperclip has to have its entire mass moved against the friction of the desk.
A comparison would be putting water and a paperclip on a slightly rough surface and tilting it slightly. The water flows off but the clip stays in place.
Basically, because the gravity you need to keep hold of the moon or to create the tide is *tiny*.
Being in orbit is just falling all the time, but moving fast enough to miss the ground by the time you get there. The force of gravity between earth and the moon is so small that it takes about half a month for it to reverse direction. That would be like jumping so high that you don’t come down for 2 weeks.
Obviously we can’t do that here, but gravity gets weaker very fast, as you move away from earth. Out where the moon is you wouldn’t even feel it, I don’t think.
The tide, meanwhile, doesn’t take much because it doesn’t actually move very far. It might seem big to a human on a beach, but the few meters between high and low tide is only a tiny fraction of the depth of the ocean.
It doesn’t take much strength to shift something such a small amount.
Also, the reason a seemingly small amount of gravity can have big effects on big things is that it’s working on every piece of whatever it’s moving.
You might not feel the pull of the moon on a human scale, just like you can’t see the tide in a human-sized pool of water, but it’s there.
When the moon pulls on the ocean, it’s pulling on each human-sized bit of it separately. You still couldn’t see the effect looking at any one of them, but add them all up and you get the kind of forces you need to shift whole oceans a *really tiny* amount. It’s like watching the edge of a puddle with a really good microscope. There’s a tide there, or in a cup of water, it’s just too small to see.
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