Ask a physicist anything. (4)

[Steffenfield]

Nope. I'm thinking more of an atmospheric bullet.

 

[Assyrian]

Don't people get killed shooting themselves with guns loaded with blanks, that is rapidly expanding gas, rapidly moving gas. But dry ice would be much more fun for the CSIs.

 

[Gracchus]

Don't people get killed shooting themselves with guns loaded with blanks, that is rapidly expanding gas, rapidly moving gas. But dry ice would be much more fun for the CSIs.

Blanks are usually sealed with wads of paper, cloth, or wax. That can make them deadly at close range.

 

[Assyrian]

I have come across that too, one poor unfortunate was killed by wadding from a man of war's 21 gun salute for him. I have come across the claim, news report of accidental death, that the hot gasses could be responsible. I don't know if wadding was excluded though. It raise an issue about the original question though, which proposes compressed gas rather than gas from propellant. Could compressed gases reach the same sort of pressure as gasses from propellant, or would they liquefy first?

 

[corvus_corax]

ESCAPE VELOCITY-
I know I'm going to get in trouble with this, simply due to the math involved, so if it can be explained to be in plain English (so to speak), that would be great.

If I were in a rocket that had (hypothetically) an unlimited fuel supply (let's say the kick-butt fuel supply of matter/antimatter annihilation on board the Enterprise), and it was launched at a mere (steady, non-stop) 50 miles per hour, assuming a vertical flight (i.e. non orbital, as the rocket would continue on due to it's fuel supply), I could escape Earth orbit, correct?

 

[Dr.Strangelove]

Question for a physicist:

Is there any actual, real, non-theoretical scientific proof that the Earth is either spinning on its axis or orbiting the sun?

 

[corvus_corax]

Question for a physicist:

Is there any actual, real, non-theoretical scientific proof that the Earth is either spinning on its axis or orbiting the sun?

"Is there any actual, real, non-theoretical scientific evidence that the Earth is either spinning on its axis or orbiting the sun?"


Sorry, I felt it needed to be corrected.
Great question though! Love it

 

[mzungu]

If one were to travel towards the centre of the earth; At what point would gravity start to diminish (considering that at the exact centre; Gravity is nill or equal in all directions).

 

[Dr.Strangelove]

"Is there any actual, real, non-theoretical scientific evidence that the Earth is either spinning on its axis or orbiting the sun?"


Sorry, I felt it needed to be corrected.
Great question though! Love it

Evidence or proof. I'll take either.

 

[Wiccan_Child]

Is it possible to build a deadly air gun?

Imagine this...

Have a container build pressure to something like 300 psi.

Expell that down the long nozzle of a gun, the end of it would be covered so the air wouldn't escape just yet.

Have some type of mechanical seal draw behind that pocket of air, compressing the chamber greatly down to the size of a bullet.

Open up the end of the nozzle to discharge the round.

I've seen some of the stuff they have in Mythbusters which just sprays air. It only becomes deadly when it's loaded witth an object.

Just wondering if air alone could be pressurized and condensed enough to make it deadly say at least a distance of fifty yards.

It would travel at the speed of sound, and wouldn't do much more than burst your ear drums and knock you over. It wouldn't actually penetrate you like a real bullet...

That said, sonic weaponry is reaching fruition

 

[Wiccan_Child]

You're a geocentrist!

Hehe. You don't actually remain perfectly still. The normal force from the floor doesn't exactly cancel out the gravitational force of the Earth. The residual force is exactly the centrifugal force required to make you follow the Earth's 24-hour-rotation about its center.

Hush

 

[Nostromo]

ESCAPE VELOCITY-
I know I'm going to get in trouble with this, simply due to the math involved, so if it can be explained to be in plain English (so to speak), that would be great.

If I were in a rocket that had (hypothetically) an unlimited fuel supply (let's say the kick-butt fuel supply of matter/antimatter annihilation on board the Enterprise), and it was launched at a mere (steady, non-stop) 50 miles per hour, assuming a vertical flight (i.e. non orbital, as the rocket would continue on due to it's fuel supply), I could escape Earth orbit, correct?

The problem is where you state "a mere (steady, non-stop) 50 miles per hour".

In order to counteract the "pull" of gravity, you'd have to accelerate in the opposite (upward) direction.

If you set something going at 50mph and it's not accelerating, it will just fall back to Earth. If you set it going at 50mph and continue to accelerate it at the same rate that gravity is pulling it down (~10m/s^2), it'll carry on at 50mph and escape earth orbit.

 

[Wiccan_Child]

Question for a physicist:

Is there any actual, real, non-theoretical scientific proof that the Earth is either spinning on its axis or orbiting the sun?

Without getting into the quibbles of evidence and proof yes, there's a very good proof:

The stars move.

Considering they're at least four lightyears away, for them to orbit the Earth every day means they have to be travelling a minimum of about 60 trillion km in 24 hours - a speed which is eight orders of magnitude larger than the speed of light

So, if the Earth was not spinning, the stars would be travelling far faster than the speed of light.

 

[Wiccan_Child]

ESCAPE VELOCITY-
I know I'm going to get in trouble with this, simply due to the math involved, so if it can be explained to be in plain English (so to speak), that would be great.

If I were in a rocket that had (hypothetically) an unlimited fuel supply (let's say the kick-butt fuel supply of matter/antimatter annihilation on board the Enterprise), and it was launched at a mere (steady, non-stop) 50 miles per hour, assuming a vertical flight (i.e. non orbital, as the rocket would continue on due to it's fuel supply), I could escape Earth orbit, correct?

In principle, yes. If your speed was sustained, and you didn't topple, you could escape the Earth. The trick is making a system that can withstand Earth's gravity for the time it takes to leave its gravitational well, at a mere 50 mph, without the massive fuel requirements making your ship too heavy.

 

[Dr.Strangelove]

Without getting into the quibbles of evidence and proof yes, there's a very good proof:

The stars move.

Considering they're at least four lightyears away, for them to orbit the Earth every day means they have to be travelling a minimum of about 60 trillion km in 24 hours - a speed which is eight orders of magnitude larger than the speed of light

So, if the Earth was not spinning, the stars would be travelling far faster than the speed of light.

How do we measure star distances? Isn't it theoretical?

 

[shernren]

Nope. I'm thinking more of an atmospheric bullet.

Won't work. A gaseous "bullet" would immediately mix with its surroundings and disperse all its energy; you'd have to hold the nozzle of the gun right up to a person's skin for any kind of serious damage to occur.

ESCAPE VELOCITY-
I know I'm going to get in trouble with this, simply due to the math involved, so if it can be explained to be in plain English (so to speak), that would be great.

If I were in a rocket that had (hypothetically) an unlimited fuel supply (let's say the kick-butt fuel supply of matter/antimatter annihilation on board the Enterprise), and it was launched at a mere (steady, non-stop) 50 miles per hour, assuming a vertical flight (i.e. non orbital, as the rocket would continue on due to it's fuel supply), I could escape Earth orbit, correct?

If you have an antimatter ship I claim SHOTGUN right now.

Well yes you can escape the Earth's gravitational field if you go at a steady 50mph. But to do so, you need to keep exerting a force on the ship that cancels out the gravitational force which is slowing the ship down. That doesn't fit the definition of escape "velocity" (really speed), though: the escape velocity is the speed at which a projectile can be initially fired and then, without any further external force, escape from the gravitational field of its parent body.

If one were to travel towards the centre of the earth; At what point would gravity start to diminish (considering that at the exact centre; Gravity is nill or equal in all directions).

When you get past the crust of the Earth. A radial shell of matter outside you exerts no net gravitational force on you. Thus, were you to make your way into the mantle (and survive), you would only experience gravity due to the mantle and the core but not due to the crust; were you to reach the core, you would only experience gravity due to the core; and when you hit the center of the Earth, you experience no gravity at all due to the mass of the Earth (or rather, it all cancels out symmetrically).

How do we measure star distances? Isn't it theoretical?

Perhaps, but star parallax isn't. We can measure how the actual positions of the stars in the sky change with the seasons, and they match up with a heliocentric rather than a geocentric model.

Mind you, the heliocentrism vs geocentrism discussion is moot if we only refer to the Earth and the Sun. If these two were the only objects in the universe (or the only objects we are considering) then either description is interchangeable. But in the days when the question was being asked, the "fixed stars" were considered a unique inertial frame, and so the question:

"Which is moving relative to the fixed stars: the Earth or the Sun?"

is a valid and important question. It turns out that the "fixed stars" are in fact themselves not fixed, but move around in galaxies; nevertheless, all the visible stars in the night sky are the Sun's neighbors in the Milky Way galaxy (IIRC), and the Sun is more or less fixed relative to them, while the Earth certainly does move relative to them, which gives a very real sense in which geocentrism is wrong and heliocentrism is accurate.

 

[Dr.Strangelove]

Perhaps, but star parallax isn't. We can measure how the actual positions of the stars in the sky change with the seasons, and they match up with a heliocentric rather than a geocentric model.

Doesn't star parallax assume that the Earth has moved to the other side of the sun every 6 months?

Isn't it assumption based trigonometry?

all the visible stars in the night sky are the Sun's neighbors in the Milky Way galaxy (IIRC), and the Sun is more or less fixed relative to them, while the Earth certainly does move relative to them, which gives a very real sense in which geocentrism is wrong and heliocentrism is accurate.

Uhm....say again?

 

[Wiccan_Child]

How do we measure star distances? Isn't it theoretical?

shenren is right: we use parallax to work out the relative distance of the stars (star A is twice as far away as star B, for instance). Then it's a matter of finding at least one star with an objective distance, which we do by using what're called 'cephid variable stars': stars which physically pulsate regularly. We can work out their distance, and, from there, the distance of everything else.

 

[Wiccan_Child]

Doesn't star parallax assume that the Earth has moved to the other side of the sun every 6 months?

Well, we do. The fact that parallax works demonstrates this: if the Earth weren't moving round the Sun, the stars would have the same relative position day after day.

Isn't it assumption based trigonometry?

Mathematics is anything but assumption, my dear doctor

 

[Dr.Strangelove]

Well, we do. The fact that parallax works demonstrates this: if the Earth weren't moving round the Sun, the stars would have the same relative position day after day.

But this is surely circular reasoning.

Heres a schematic of trigonometric star parallax:

At the bottom we see that the measurements assume that the Earth is orbiting the sun.

So they take a measurement to the target, and then six months later take another measurement ASSUMING the Earth is 186 million miles away on the other side of the sun.

So you're saying because we can measure the stars using a system that assumes the Earth orbits the sun, we can prove the Earth orbits the sun.

It's circular.

It's theoretical trig because one of the points is completely made up.


Doc.