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That is really interesting to know, thank you! are such data sets publicly available?

But what keeps the train moving? I know the answer to this question is inertia, but intuitively it makes sense that there must be some force that is making the object continue to move, even at a constant velocity. I guess a better question is do we know why objects with no net force can remain in motion? Like, it makes sense to me that when net force = 0 = no net movement, but not the constant velocity part.

Yes, but why does only change in motion require external force, and not the motion itself? How can an object like Earth just move in space with constant tangential velocity? Intuitively, it makes more sense to me that something is exerting force on the earth to make it move at all. If I throw a ball in space, I know my hand exerts force on the ball for it to accelerate, but when I let go, it will keep moving forever in a straight path, with no force acting on it. How is that?

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Also, when I push a box on a table at constant velocity, my applied force is equal to the friction force. The net force is 0, but how can the box move? It intuitively makes sense to me that net force = 0 = no movement. I know the answer to this question is inertia, but I don't know this property of matter confuses me. Do I just need to accept this as a fact?

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>object can travel at constant velocity without any added force. It in fact it will take a force to slow it.

I understand that the forces acting on the object opposite of its motions causes it to decelerate. With your space example, I know if you give something an initial push, it will keep moving forever unless another net force acts on it. But how does it keep moving forward? I know that something must act on the object for it to change acceleration, but how can it move in the first place without any net force. It intuitively makes sense to me that when net force = 0, then that means no motion, but how can an object move at all when net force =0? Is that just inertia by definition?

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Thanks for the reply, and sorry if my original question was unclear.

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thanks for the response! I get that friction and opposite forces causes objects like the skateboard to decelerate, but how is it that the object itself can keep going forward at constant velocity? I am just wondering what causes the object to keep moving if there is no net forces acting on it? Is that part of inertia something we just have to accept is true?

Another example, if I push a box on a table with a constant velocity, the friction force is equal to my applied force, but since they are both equal it makes intuitive sense that net force = 0 = no movement at all. How is it that the object can till move with no net force?

So then how does a root canal affect the tooth/jaw relationship?

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Would you mind answering the real question he asked about how prevalent it was about 20 years ago and used as a cosmetic device for developing children? If not it is fine. I just wonder how the practice has changed in time.

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That would be great. I've got permanent brain fog. I wish I could take something that would help my brain repair itself. It's ridiculous how bad my recent memories are compared to my old ones. Also I have to try to work like a human thesaurus because very frequently I'll forget words and have to reach for a replacement.

Such a cool and informative answer. Thank you.

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This is exactly why Newton's laws were revolutionary and why it took humanity until 1687 to discover them! Until then, it was perfectly reasonable to believe everything naturally slows down and stops, unless someone is working hard to keep it moving. That's because everything we could see around us did just that! We know now that everything that we see slowing down does so because of friction and air resistance, but it was not obvious if you've never seen anything different. It took Galileo's physics experiments with carefully constructed artificial conditions to inspire an alternate explanation.

Maybe the closest everyday example of a self-moving object they had at the time was a runaway horse cart. Later on it was an easier idea to swallow when we had trains around. Trains have such low friction that you have to work hard to slow them down. It's more natural to believe that an object in motion remains in motion if you've seen trains your whole life!

Well, the way Newton laws are thought in US schools is weird, and it creates a lot of confusion. The original newtons laws are ais they thought in schools, but nobody teaches the Scholium that precedes the laws.

In the Scholium Newton talks about absolute and relative motion. He mentions absolute coordinate frame, defined by immovable stars. Other frames are relative. Newton says that you cannot really distinguish two relative motions when forces are not actet on two objects. But if you connect these objects be a string and make them spin, by observing the tension of the string you can distinguish that spinning motion.

What Newton actually talks about in the Scholium is about inertial frames of reference, and all other laws can be applied only in such frames (actually Newton thinks that there is one true non-moving frame of reference).

These definitions in Scholium and explanation when we can replace absolute motion with relative motion is quite important. So in some other textbooks they are included as the first Newtons law, which is condensed to "all mechanical processes in inertial frames flow the same", or that "all inertial frames are indistinguishable". This definition of inertial frames may sound confusing at first, but it makes a lot of sense the more you think about motion and general applicability of Newton's laws. You can think about inertial frames as frames that stay in rest or move with constant velocity relative to each other.

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Everything inside the event horizon must travel towards the singularity, but the speed at which they travel inwards can still be different. This allows things like "blood" or "nerve impulses" to subjectively travel "upwards", such that from your perspective you will feel physically normal, even as you cross the horizon.

Here's an example using fake units: you are falling through the horizon, at t=0 your head is 1m above it, your legs 1m below it. Your toe sends a nerve signal. At t=1, your head enters the horizon, your leg is 2m below, and the nerve signal is 1.75m below. The signal is "deeper" into the black hole than when it started, but it still travels up your leg. At t=8, your leg is 9m below, your head is 7m below, and the nerve signal finally reaches your head. You can feel you toe!

You could even have two spaceships fall in separated by 1km (a rope linking them together optional!) and you could see your buddy on the front spaceship fall in past the event horizon and wave at them and then see them wave back in response. Of course, by the time you see the response, you yourself will already be inside the event horizon and way past the point where they were when they waved back.

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An object with nonzero velocity has kinetic energy, potential energy normally is due to the position of one object relative to others.

Lightning is the product of static electricity within a cloud. It’s literally static discharge—the same as you scuffling your feet across a carpeted room and then reaching for a doorknob or something metal. Your body is superb at transmitting that charge and you produce that mini lightning bolt you feel as a shock. So what does this have to do with a rain or snow cloud? Heat in the air from below the cloud helps it pile high and get countless billions of water droplets rubbing against each other up and down inside the cloud. Eventually enough static electricity is built up to be released as lightning, However, in snow clouds it’s just too cold! Not enough heat to get ice crystals to rub each other and produce static electricity. That’s why places like Arizona or Florida have spectacular lightning storms—they’re warm or hot climates. Not cold and snowy like Minnesota or New York.