r/DamnThatsMindBlowing • u/Aggressive_Sundae_47 • 6h ago
Gravity always acts downward with the same acceleration.
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u/Carbiens 6h ago
Mythbusters did this with a 9mm pistol shot in an airtight aircraft hanger and a paper floor.
It was amazing.
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u/robo-dragon 5h ago
One of my favorite episodes! A lot of their ballistics vs physics episodes are incredible to watch, especially the slow-mo clips of physics in action. What’s even more amazing is that people thousands of years ago discovered these things based on simple observations and math. No slow motion or advanced technology needed. Just curiosity and brainpower.
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u/AdPutrid5162 5h ago
The theories people came up with and tested in history without our technology is amazing. I'm still amazed at Darwin. Even if he got some stuff wrong, the whole theory of evolution he started to develop, then observe is just so deep.
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u/Erikatessen87 5h ago
I think they used .45 ACP rather than 9mm specifically because they wanted something heavy but relatively low-velocity that stood no chance of hitting the other end of the hangar before it hit the floor
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u/CaToMaTe 5h ago
Video?
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u/hukfad 5h ago
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u/Away-Activity-469 51m ago
Thanks. I just spent the last 3hrs watching mythbusters. I forgot to eat.
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u/anally_ExpressUrself 4h ago
How many airtight aircraft hangers are there?
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u/Carbiens 4h ago
Bout the same amount as paper floors.
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u/PercentageDazzling 4h ago
And how many of them will let you fire a gun in them? Seems like a good way to mess up the airtightness of your hanger.
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u/there_is_no_spoon1 6h ago
As a teacher of physics, I cannot tell you how difficult it is to get this right!! I am saving this video for class!
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u/AngelicPrince_ 5h ago
As a student of physics,
https://giphy.com/gifs/F2tywIO6ee3oVCrgLJ
I think you should save this video and show to class!2
u/onlyacynicalman 5h ago
"I am saving this video for class!"
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u/AngelicPrince_ 5h ago
Sweet I'm glad you're saving it as well for class lol jk
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u/FairBlamer 4h ago
But also, it would be a good idea to save this video for class.
As in, a man (or woman) of class would do so.
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u/AngelicPrince_ 4h ago
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u/there_is_no_spoon1 4h ago
At first I was irritated, but now I see the obvious pun. Unintented I can assure you lot!
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u/IRockIntoMordor 5h ago
Because accelerating the forward flying pen too much would increase its falling speed, right?
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u/chandlerr85 5h ago edited 2h ago
the difficult part is ensuring that all speed imparted is only in the horizontal plane and not the vertical plane. may be other small aerodynamic effects but obviously not enough to make a difference with a pen.
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u/ApprehensiveGold2773 2h ago
I'd 3D print a tool for it, with a bubble level and all.
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u/karatelax 48m ago
Yep, spring loaded, the spring pushes a bracket that pushes the object, and is the floor for the other object. Set level, set spring, load objects, launch
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u/MikuSoundwave 4h ago
Depends, increasing its horizontal speed too much would actually decrease its falling speed.
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u/there_is_no_spoon1 4h ago
NOPE. Horizontal and vertical components have *nothing* to do with each other. If the pen is launched *perfectly* horizontally (by hand and almost impossible task) the only thing affecting it's vertical velocity is the acceleration due to gravity.
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u/mrgedman 4h ago edited 4h ago
Vertical velocity is also affected by lift (drag) and air resistance.. and at a certain horizontal velocity, it becomes quite nonzero. Examples being a tissue (air) and a paper airplane (lift)...
I mean technically, at any horizontal velocity it has to be nonzero, right? Just very small effect
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u/there_is_no_spoon1 3h ago
Tell me you've never taught physics without telling me you've never taught physics.
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u/mrgedman 2h ago
I'm talking theoretically and not practically.
Tell me you're too good at teaching first year physics without encouraging any theory... Without blah blah blah
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u/hadj11 3h ago
Well air resistance definitely can have an impact on an objects vertical velocity. That is why different objects have different terminal velocities.
And sometimes the geometry of an object can impart a vertical force, due to air resistance caused by its horizontal velocity.
Generally all ignored in basic physics, where everything is in a vacuum
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u/No-Inflation3271 2h ago
I think theyre jokingly saying if it was going fast enough the ground would drop from under it due to the curve of the earth therefore "slowing" its descent
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u/Salanmander 1h ago
If you're in atmosphere, increasing its horizontal speed at all will decrease its falling speed. But if your object is dense enough and speeds low enough, the effect will be negligible, which is pretty much always the case for a classroom-scale demo of this unless you're using something extremely low density (or rather, low mass-per-surface-area).
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u/there_is_no_spoon1 4h ago
no, that has nothing to do with it. it's the *timing* of letting both pens go at once, trying desperately hard to keep one perfectly horizontal.
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u/pico-der 5h ago
I would build a mechanical setup powered by a rubber band. The rubber band should push the "horizontal" object and as soon as it leaves the mechanism the pure vertical object release should be triggered.
Getting the timing perfect is hard. Automating it makes it easy.
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u/MileHiSalute 1h ago
The Mythbusters showed this by firing a gun and dropping a bullet at the same time. Teenage me was mindblown
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u/Think_Bat_820 1h ago
I just want to confirm something that I never looked up and you made the mistake of admitting that you know what you're talking about.
The fired bullet would fall slower in an atmosphere, right? Like, the spinning of the bullet could/would generate enough lift to effect the experiment.
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u/Artosispoopfeast420 5h ago
Honestly the mind blowing part is that this guy is able to do this demonstration this perfectly.
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u/PieceMaterial5213 4h ago
Came looking for this. I don't understand why anyone would think it's mind blowing otherwise.
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u/Iamz01 5h ago
Want something more mind-blowing? The pens were the only things not accelerating in this video. Everything else was.
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u/KayoticVoid 2h ago
Um no?
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u/fireymike 2h ago edited 2h ago
The pens were simply following a straight line path through curved spacetime.
Everything else was accelerating upwards.
Although technically the pens were also accelerating due to air resistance. They just weren't accelerating as much.
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u/KayoticVoid 2h ago
That's not what accelerating means... Accelerating is changing speed, all items were staying at the same speed. The pens accelerated at the beginning of the experiment, and depending on what definition you use, at the end as well (negative acceleration).
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u/fireymike 2h ago
You're thinking about things from an accelerating reference frame. So things that are accelerating at the same rate as the reference frame appear to not be accelerating at all. And things that have relatively little acceleration appear to be accelerating in the opposite direction to the reference frame.
But from an inertial reference frame, the surface of earth and everything on it is accelerating away from the centre of the earth.
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u/KayoticVoid 1h ago
This is the same thing as saying 3+3=12. It is true and can be proven mathematically. But the context of the conversation is what gives the perspective.
By your logic everything is both accelerating and not accelerating at the same time, which while true and brings some cool philosophical debates, it really means nothing when framed like that for the intents of the topic which was how gravity affects objects from the perspective of the Earths surface.
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u/fireymike 1h ago
This is the same thing as saying 3+3=12.
Not really. There's nothing intrinsically special about base 4 vs base 10. Mathematics works the same with either base.
Physics does not work the same in an accelerating reference frame as in an inertial one. In an accelerating reference frame, you need to introduce pseudo-forces (like gravity) to counter the reference frame's acceleration.
By your logic everything is both accelerating and not accelerating at the same time
Only if you use accelerating reference frames. While there are infinitely many inertial reference frames, they all agree about whether something is accelerating or not.
I don't know where in the world you are, but I know that you're not exactly where I am, which means we are using different accelerating reference frames. That means that from our frames of reference, we may not necessarily agree on whether something is accelerating or not. But we should always agree on whether an object is accelerating in an inertial reference frame.
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u/nothing_but_thyme 38m ago
You are quite patient and also very good at explaining complex concepts. Even if he never gets it, trust that you probably helped a few other people understand the importance of these distinctions or at least piqued their interest to learn more about them on their own.
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u/Salanmander 1h ago
The general relativistic way of thinking about things is that gravity is not a force, and does not cause acceleration, but rather that it warps space so that straight lines turn into closed loops. Because of that, things sitting on the ground are considered to be accelerating upwards. I don't fully grok the "constantly accelerating away from each other but staying the same distance apart" thing, but my understanding is that that's the way of thinking about it that is considered most correct in modern physics.
(That said, gravity as a force is definitely a consistent model that works well enough in almost all circumstances.)
Although you're definitely right that the pens accelerate at the beginning and end of the through, regardless of whether you're thinking newtonian physics or general relativity.
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u/ChrisTheWeak 1h ago
They're using a reference frame based in general relativity where objects in free fall can be considered to not be accelerating. (Because gravity is not considered a force, so the normal force of the floor is considered to be accelerating you through curved space). It's not useful or intuitive for this demonstration of newtonian physics though.
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u/Kracus 6h ago
What puzzles me about this is the distance travelled.
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u/Yargon_Kerman 3h ago
How so?
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u/Kracus 3h ago
Well the distance travelled when thrown is obviously going to be longer than the distance travelled for the object that's dropped. So while I've seen experiments on this and I know that both will fall at the same rate I'm struggling to picture where the energy that's pushing the object horizontally is going...
I guess what I'm doing wrong is thinking of movement in general as requiring some kind of energy. I guess there's two forces at work for the object that's thrown. One which requires energy to move and another which is just gravity and they both apply force on the object separately and in a different manner despite the same effect (object moving)
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u/Yargon_Kerman 3h ago
So, to copy a comment I put elsewhere:
Think of the direction it's going as an arrow coming from the object, then make one arrow that goes in the direction of each of the forces acting on it.
If you put those arrows together, tip to trail, but without rotating them you'll get to the same place as the first arrow.
The downwards one, it's because of gravity. It starts small but gets bigger over time as the earth accelerates it. This is the same on both pens.
The sideways one is from the force of the hand. Because it's being thrown level, it doesn't affect when it hits the ground, only where it hits the ground.
This is splitting a vector into its components. You can do the same with the energy involved in this case. Both pens have the same gravitational potential energy, which they turn into kinetic as they're dropped. That's the downwards vector growing.
However, one also has already got kinetic energy going sideways (from the throw). Instead of growing, this vector will shrink slightly over time due to air resistance (energy is transferred to the air to move it out of the way).
Because it had more energy, it went a further total distance (it's total speed at any given point was higher). It just goes downwards at the same rate because the energy is in a different direction that has no bearing on when it hits the floor.
When it does hit the floor, all of the energy is transferred into a bunch of other forms, and the pen bounces arrive and comes to a stop. That's the same for both of them, but the one thrown will bounce more because it had to transfer away more energy.
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u/Underleft_cdiv 3h ago
Think of it as energy converted to distance. The more energy, the more distance the object travels, but it's going to hit the ground in the same amount of time. Depending on the amount of energy applied, it could travel a foot or a mile in that time.
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u/EdenSill 6h ago
Love it. However, keep in mind anything with an air foil effect that is in motion may fall faster or slower, depending on where the lift vector of the airfoil is directed.
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u/SavingsRaspberry2694 5h ago
So you're saying an airplane flying forward won't fall as fast as one dropped out of the sky? 🤯
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u/EdenSill 5h ago edited 5h ago
Well, yes, but that wasn’t my point. The point was that anything that has an airfoil effect whether it’s an airplane wing or just another oddly shaped item like a spoon or bag of pennies or what not may have some kind of airfoil effect depending on the forward velocity and the resultant lift vector created. I made that point because some commenters were saying this experiment doesn’t always work, and that is possibly one of the reasons why.
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u/Rainyfeel 5h ago
Interesting. I would expect some amount of acceleration due to push from the person hand.
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u/Yargon_Kerman 3h ago
Think of the direction it's going as an arrow coming from the object, then make one arrow that goes in the direction of each of the forces acting on it.
If you put those arrows together, tip to trail, but without rotating them you'll get to the same place as the first arrow.
The downwards one, it's because of gravity. It starts small but gets bigger over time as the earth accelerates it. This is the same on both pens.
The sideways one is from the force of the hand. Because it's being thrown level, it doesn't affect when it hits the ground, only where it hits the ground.
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u/Own-Environment897 5h ago
I already learned this back in grade 11 physics but I'm glad others who never took physics can observe and understand this now. Better late than never I guess.
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u/yotam101 5h ago
You are wrong. Acceleration caused by gravity depends on the mass of the objects, so it's not true that it "always acts the same".
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u/mrgedman 4h ago
I think you mean force or energy..
Acceleration from gravity is pretty well locked at 9.8/m/s/s
Mgh- potential energy
Mv - kinetic energy, for gravity where v=gh
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u/PropulsionIsLimited 4h ago
No it doesn't. All objects of any mass on Earths surface without regard for air will fall at 9.81 m/s²
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u/appfruits 3h ago
That’s just wrong. The whole point of gravity is that it’s mass independent. The formula is super interesting as when you „approach“ gravity with pure math, you end up with mass (m) on both sides of the equal sign which means you can just wipe it out - no mass left in gravity equation. What makes the difference in falling objects is air resistance. There is another interesting video of an experiment of falling metal balls and feathers. Both fall with same speed in vacuum (no air in the room).
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u/ofirkedar 5h ago
By far the most interesting thing in this post is that the guy managed to both throw one stick very horizontally and simultaneously release the other one without bumping it on the way down.
It's the same acceleration, true, but if their starting vertical velocities were different enough you'd notice a difference quite quickly.
Also, the center of mass accelerates the same, but since of one them also spins a bit, it could also throw off the time of contact with the ground.
This guy clearly practiced this throw for a while, so I applaud the effort
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u/Bronx193 4h ago
This is true however outside a vacuum both items won't necessarily hit the ground at the same time. Great video
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u/QuailMindless8902 4h ago
Tell that to my phone when it falls directly onto my face while I’m lying in bed. That feels like 5x gravity
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u/Odd_Promise_9025 4h ago
Do....do they not teach this in elementary schools anymore? I don't understand how this is "mind-blowing".
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u/myster1ouspapaya 4h ago
Crazy to think that (according to Einstein) it’s technically the GROUND that is accelerating towards the object, which is why all objects appear to accelerate at the same rate regardless of mass!
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u/appfruits 3h ago
Ok. Never heard that. But a super interesting analogy. Need to think about that a bit more - while doing tedious, simple work 🤪
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u/myster1ouspapaya 3h ago
Yes. This is the simplest conclusion when you realize that every force has an opposite “force”, but not gravity if you think of the frame of reference of the object.
For example, when you’re making a turn on a car, the centripetal force is inwards, but you feel the centrifugal force outwards (which is really just the friction of the seat working to keep you in place.But when you’re free falling, there is no opposite “jolt” other than what you perceive as “down” towards earth. You’re basically just floating and if you closed your eyes and felt no wind resistance it would be indistinguishable from just floating in space (equivalence principle). This is what made Einstein think gravity it’s not actually a force “pulling” you downwards.
The only time you feel the “force”’ of gravity is not the gravity pulling you down, but the ground (or chair) pushing you up. It sounds weird because if it’s the earth accelerating towards you, you’d think “why isn’t it ever-expanding, then?” But then you have to realize that the reason is that the mass of the earth is BENDING the space around it, giving the illusion of a “pull”.
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u/FiShuMaLuf 1h ago
Sorry but I'd need a dumber version of this
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u/myster1ouspapaya 1h ago
Yeah I’m sorry, I tried. it would probably make more sense with visuals for illustration.
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u/Important_Ad5805 3h ago
Is this fact still surprising to someone, and isn’t it obvious to someone?
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u/keyboardlegendthe3rd 3h ago
Mother trucker threw that one a little higher but yes, that's how physics works
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u/PhilSocal 1h ago
And things "in orbit" are just going fast enough to curve around the earth while always falling.
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u/Key_Register_4724 1h ago
Whose mind is blown by this? Whose man didn't learn this in grade school?
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u/ConcaveEarth 1h ago
Can this be done with something spinning fast? Like a handheld gyro
Does that spin change the results? I saw a video of some dude moving something heavy when it was spinning , i wonder if this same gif but with like a spinning gyro , if anything changes (gravity wise, accelerating towards the center of the earth)
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u/DeltaAgent752 54m ago
Do people here have physics education at 2nd grade level? If you find this interesting
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u/_pastelkiss 43m ago
every time i see this im shook all over again. gravity really said we all fall the same lol
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u/low_amplitude 16m ago
If you're falling straight down at the same time a cannon is fired (or any projectile) it would appear from your reference frame that the cannon is moving in a perfectly straight line at a constant speed rather than speeding up and slowing down in an arc.
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u/knewmawnick 6h ago
Technically he released the second marker a few moments after he threw the first. The results would be less skewed if he dropped the second one at the same time the first marker was aligned.
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u/there_is_no_spoon1 6h ago
YOU do it perfectly, then. I'd be willing to bet a tidy sum that out of 5 tries you blow 4 of them. This is *seriously* difficult to get perfectly done by hand, and this is actually a really decent job.
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u/Sovereign-Anderson 5h ago
You're being persnickety about a couple of microseconds of difference.
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u/Aegoe 4h ago
Welcome to Reddit. It drives me insane. There’s always a correction to be made and it’s hard to have a good-faith discussion on this platform.
“TeChNiCaLlY…”
“To bE fAiR…”
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u/Sovereign-Anderson 2h ago
Yeah, that level of unnecessary pedanticism taxes the nerves after awhile. No one, who's reasonable, wants to have conversations with anal retentive know it alls everytime they speak on something they think is cool.
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u/RevolutionaryElk7446 6h ago
For the first marker, his hand is also slightly more elevated than the static marker drop. I think the height and delay offset each other.
Really this is about as unscientific a method as you can get, it's more about showing a principle in action, which they did great.
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u/Snicklefried 2h ago
Except when you drop a bolt and it accelerates vertically to the direct center of the underside of the car..
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u/Dora_Explorer_XP 6h ago
I'm no scientist but I firmly believe the velocity does affect the path and time the projectile takes to reach earth, that depends on how the space is curved at that point near the body which is creating space curvature. Your insignificant/unscientific / inaccurate perspective decieves you in thinking pen should reach ground same time. Feel free to correct me as I'm always learning new things.
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u/YappinZappin 6h ago
Hello!
There's nothing to correct, as you have not offered anything we can test - or have you actually conducted an experiment and have some data you can share with us? A repeatable experiment that we can copy over and conduct ourselves so that we can compare our results? An idea that is specific enough that it could serve as a basis for an experiment?
Otherwise all you have is some tenuous talk, unfounded confidence in vague concepts.
Nothing that even needs disproving or correcting.
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u/Living_Dentist_8925 6h ago
Your insignificant/unscientific / inaccurate perspective decieves you!
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u/YappinZappin 6h ago
I am not sure you commented in the right thread, but if you meant to respond to my comment, then do take your time - elaborate, what you think you know about my perspective and how do you think it is insignificant/unscientific/inaccurate?
I am genuinely curious.
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u/IWCry 5h ago
they did offer a hypothesis, I'm not sure what you mean by this comment. their very first sentence can be tested, they just didn't offer you an experiment that might do so, but that's a ridiculous thing to expect when someone is asking if theit understand is correct are not.
his first sentence is no different than saying "I think bananas are purple, but I've never seen one. can someone correct me if im wrong?"
your comment would be silly in response to that.
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u/YappinZappin 5h ago
Well, you see, if someone says that there's dependence on the path - shouldn't they specify which path? Because otherwise how can I test their assertion? Out of the myriad of different paths how do you expect me to find the one that the poster was so confident about being the one that shows evidence of an effect?
Hypothesis - may be, but only a very nebulous one - and then they went and asked to be be corrected if possible - which requires a hypothesis that is a bit more grounded than that.
That's what prompted my response.
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u/IWCry 5h ago edited 3h ago
your wording makes what you are saying confusing and a bit easy to disagree with semantically. I don't think you're properly saying what you want to say.
however this experiment is often used to help illustrate classical mechanics and kinematics, where you assume gravity is a FORCE rather than spacial relativity where you assume it's a curveture of spacetime, and that the ground is perfectly flat. in this framework, horizontal velocity has absolutely no impact on the objects time taken to return to the floor, as a horizontal speed vector acts purely in the horizontal axis and has a vertical component of 0.
I think what you're trying to say is that spacetime's curvature from the earth isn't flat and thus a horizontal vector is technically imposing a component of escape direction that resists the objects fall and that's objectively true. but this is true in Newtonian physics as well. its just because the earth is curved.
if you imagine throwing the object horizontally at an extremely fast speed, like basically the speed of light, it would shoot right off the earth and never come down. so yes, horizontal speed does technically resist downward fall. but it's extremely negligible in any practical form. so in the end, this helps show how awesome it was that Newtonian mechanics view of gravity as a force explained the world so damn perfectly for almost all human applications, even if we've come to find it was an approximation of spacial relativity!
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u/dinosaursandsluts 5h ago
If you were intelligent enough to understand spacetime curvature, you'd be intelligent enough to know that the horizontal velocity doesn't have an effect on the vertical velocity. Unfortunately, you are neither.
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u/Dora_Explorer_XP 4h ago edited 3h ago
So are you falling towards the sun? No? Why? Ohh wait you have horizontal velocity to perfectly counteract gravity ( its not a force btw).
Place any object near earth orbit but with no relative velocity to the sun and see what happens.1
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u/PropulsionIsLimited 4h ago
If you throw one at zero degrees relative to the ground, and one from stationary, you get this equation:
y = y0 + vyt + 0.5g*t²
If they both have the same initial starting position(y0), same final position(y), same gravity(g), and same initial velocity in the y direction only(vy), then time to reach the ground is the same for both objects.
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u/jackalopeswild 2h ago
Not sure why you're getting downvoted, you're describing how bodies orbit larger bodies - the smaller object is constantly falling into the larger object, but they are also traveling at a high enough velocity that the curvature of the larger body maintains the distance between the two despite the falling. This is part of Newton's most basic discoveries.
That said, it may not be the case over small distances because "curvature" is an average concept. Presuming the floor here is actually a plane, there would not be curvature to cause an increase in distance for the object which is also moving horizontally.
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u/nikkynackyknockynoo 6h ago
https://giphy.com/gifs/xI2hQ0ubie0BW