If there was a large enough vacuum chamber, and it was a perfect vacuum, could we potentially throw something and have it move infinitely? When you throw something, say a football, it travels through the air. The force behind it is stronger than the gravity pulling it down, so it doesn't fall until the air resistance slows it down enough for it to fall. This is why the space station stays up, gravity is still affecting it but its moving too fast for it to come down, and since it is in space, there is no air resistance to slow it down. So, if there was a large enough vacuum chamber, and it was perfect (no air inside, I have to specify this because vacuum chambers are never 100% a void), could something move for an infinite distance because there is no air resistance from one initial push? would gravity come into play? Atmospheric pressure? Would it have to be a lot of force or could it be just enough to overcome gravity? I'm really curious, have a mediocre understanding of physics, and I don't trust google. Could someone who knows what they are talking about explain this for me? Thanks
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2 Answers
For example, if you imagine a perfect infinitely large vacuum chamber where one can throw a ball upwards against a constant force, then no, the ball will not move upwards forever. Even in the absence of air friction, the opposing force will constantly do work on the ball, slow it down and return it to the initial point of launch.
Satellites are not perpetually travelling upwards against a gravitational force, rather, they are in orbit. Given an inverse square attractive force like gravity, one can provide an object with sufficient kinetic energy so that it's path becomes an orbit, either circular or elliptical depending on the initial conditions. Further, it given even greater kinetic energy the object can escape altogether and head into space free of the Earth's pull. So, here is an example, where a finite amount of initial energy allows for an infinite amount of motion. Because the force of gravity is not a constant, but an inverse square law: $$F=G{m_1m_2\over d^2},$$ i.e. the force lessens as the distance between the gravitating masses increases, then it is possible to overcome the attractive force and escape forever.
When you throw something, say a football, it travels through the air. The force behind it is stronger than the gravity pulling it down, so it doesn't fall until the air resistance slows it down enough for it to fall.
This is not true. While the football is traveling through the air, there is not a stronger force behind it. There's no force pushing it forward after it leaves the hand. Gravity is accelerating it the entire time downwards. A football player usually throws a ball with an upward velocity. Acceleration of gravity eventually slows this upward velocity, then starts to create a downward velocity. At some point enough time has passed with that downward velocity that it hits the ground.
Air friction does play a part in a football, but not in the up/down axis. It matters in the forward/back axis. Air friction slows the ball's forward progress. If it flies long enough, this slow deceleration means it would not fly as far as it would without air friction. But this is separate from hitting the ground. It doesn't need to bleed off any velocity in order for it to hit the ground.
Consider this experiment. You fire a gun perfectly horizontally in a vacuum and drop a rock at the same time from the same height. Which hits the ground first? The answer is they hit the ground at the same time. Both are affected by gravity the same, so they both accelerate downward the same. Marksmen h ave to consider this effect when hitting far off targets. The bullet has to fly long enough that the pull of gravity would ruin their aim if they did not account for it.
This is why the space station stays up, gravity is still affecting it but its moving too fast for it to come down, and since it is in space, there is no air resistance to slow it down.
This is correct, but we need to change models a bit for it to make sense. In the football example, we assume gravity pulls "down." This is 99.99999% true. In actuality, gravity pulls towards the center of mass of the planet. For something as slow as a football, this matters little, so we get away with our assumption of gravity pulling "down." For the ISS, it's going fast enough that that little assumption starts to matter. As it moves (quickly), the direction of gravity's pull changes. It's always pointed towards the center of mass, but the position of the center of mass relative to the ISS is changing, so the direction of that vector is changing. At orbital speeds, that vector is changing fast enough that you never actually get closer to the ground. This is the effect you are talking about "... moving too fast for it to come down..."
As for drag, this is where it gets interesting. There actually is drag in space. It's astonishingly small compared to what we're used to in the atmosphere, but it exists. The ISS is constantly losing energy, dropping into lower orbits. Were we not firing thrusters regularly on the ISS, it would fall into the atmosphere and burn up.
There's drag everywhere, even in deep space. It's estimated that there's something like 4 atoms of hydrogen per cubic meter out in interstellar space. That's a ridiculously low number, but non-zero. But if you could somehow make a perfect vacuum (which is actually impossible, as it turns out), an object would indeed fly forever in it according to Newton's laws.
Of course, if you bring in gravity, there's a catch. The object may never hit ground, but that doesn't mean it gets to fly out into the furthest distances of space. For objects in orbit, while they never hit ground, they never escape either. It takes a minimum amount of energy to escape. This is often phrased in velocity, and we call it escape velocity (this is the velocity where you have enough kinetic energy to escape the gravity well). So if you gave an object a "light tap" for an initial push, it may orbit the planet forever. But if you gave it a rather strong push (11,200m/s for Earth's gravitational pull), you would send it on its way forever... or at least out of Earth's gravitational pull. The pull of the sun would still pull on it enough to drag it into an orbit around the sun. It takes 42,100m/s of velocity to escape the sun's pull, as the Voyager probes did.
