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Joat42 wrote:

Loren Pechtel wrote:However, I see a big problem with those backstops--they're in atmosphere. What happens to the air hitting it? If it's powerful enough to stop a bullet it's going to be imparting quite a velocity to the air that touches it. Where does that energy go? Why doesn't it melt whatever eventually stops it?

Well, it was something I wondered about too. Grav-fields in an atmosphere would create some really interesting effects.

You could of course place a low-G field pointing towards the shooter just before the real backstop. That would essentially create a zone with very low air-pressure or even a vacuum.

I considered that but it doesn't work. It's not gravity pushing the air against the backstop, but rather the ambient pressure caused by gravity pulling down on the air. If you're going to create a gravity field sufficient to repel that you need push as hard as the planet is pulling. If this is on Earth that means it's pushing something like 11 km/sec. (Note: Escape velocity is dependent on both surface gravity and density, they don't always move together.) Oops, your nice bullet-destroying backstop is now a perfect spring sending the bullets back at the shooters.

Loren Pechtel wrote:However, I see a big problem with those backstops--they're in atmosphere. What happens to the air hitting it? If it's powerful enough to stop a bullet it's going to be imparting quite a velocity to the air that touches it. Where does that energy go? Why doesn't it melt whatever eventually stops it?

Joat42 wrote:Well, it was something I wondered about too. Grav-fields in an atmosphere would create some really interesting effects.

You could of course place a low-G field pointing towards the shooter just before the real backstop. That would essentially create a zone with very low air-pressure or even a vacuum.

Loren Pechtel wrote:I considered that but it doesn't work. It's not gravity pushing the air against the backstop, but rather the ambient pressure caused by gravity pulling down on the air. If you're going to create a gravity field sufficient to repel that you need push as hard as the planet is pulling. If this is on Earth that means it's pushing something like 11 km/sec. (Note: Escape velocity is dependent on both surface gravity and density, they don't always move together.) Oops, your nice bullet-destroying backstop is now a perfect spring sending the bullets back at the shooters.

What you need is a short range force field pushing out at a bit more than 14.7 pounds per square inch (on Earth), more than enough to cancel atmospheric pressure. That will slow a bullet some, but not enough to bounce it back at the shooter. A speed is not a push and we certainly do not need to accelerate things to escape velocity.

tlb wrote:What you need is a short range force field pushing out at a bit more than 14.7 pounds per square inch (on Earth), more than enough to cancel atmospheric pressure. That will slow a bullet some, but not enough to bounce it back at the shooter. A speed is not a push and we certainly do not need to accelerate things to escape velocity.

Since when is gravity measured in pounds per square inch?

tlb wrote:What you need is a short range force field pushing out at a bit more than 14.7 pounds per square inch (on Earth), more than enough to cancel atmospheric pressure. That will slow a bullet some, but not enough to bounce it back at the shooter. A speed is not a push and we certainly do not need to accelerate things to escape velocity.

Loren Pechtel wrote:Since when is gravity measured in pounds per square inch?

Since when is force measured at "something like 11 km/sec"? The point is that atmospheric pressure (on Earth) is about 14.7 pounds per square inch, which does indeed give a field strength needed for each square inch of target that must be protected.

A pound is a unit of force, and a kilogram is a unit of mass.

Loren Pechtel wrote:

tlb wrote:What you need is a short range force field pushing out at a bit more than 14.7 pounds per square inch (on Earth), more than enough to cancel atmospheric pressure. That will slow a bullet some, but not enough to bounce it back at the shooter. A speed is not a push and we certainly do not need to accelerate things to escape velocity.

Since when is gravity measured in pounds per square inch?

The point is to mitigate the atmospheric pressure, not nullify the speed of something moving at escape velocity.

A horizontal gravity-field in a vertical plane pointing away from a hermetic wall (ie the backstop) will scavenge the atmosphere from the space between very very quickly.

Joat42 wrote:

Loren Pechtel wrote:Since when is gravity measured in pounds per square inch?

The point is to mitigate the atmospheric pressure, not nullify the speed of something moving at escape velocity.

A horizontal gravity-field in a vertical plane pointing away from a hermetic wall (ie the backstop) will scavenge the atmosphere from the space between very very quickly.

But atmospheric pressure doesn't exist at the molecular level. It's just a bunch of molecules moving quite fast. That's what you're going to have to deflect. That's basically the same as the gravity field required to hold onto the atmosphere.

Loren Pechtel wrote:Since when is gravity measured in pounds per square inch?

Joat42 wrote:The point is to mitigate the atmospheric pressure, not nullify the speed of something moving at escape velocity.

A horizontal gravity-field in a vertical plane pointing away from a hermetic wall (ie the backstop) will scavenge the atmosphere from the space between very very quickly.

Loren Pechtel wrote:But atmospheric pressure doesn't exist at the molecular level. It's just a bunch of molecules moving quite fast. That's what you're going to have to deflect. That's basically the same as the gravity field required to hold onto the atmosphere.

Pressure at the macro level can be related to kinetic energy density at the micro level, since it is the average force per unit area exerted on a plane by molecular collisions. As the pressure at sea level is set by gravity, you get the following effect: when air temperature increases or decreases (related to the average molecular kinetic energy) the air density decreases or increases in turn to keep the pressure constant.

PS: this is at the level of a room, trying to explain weather patterns of high and low pressure is more complicated; however it is still related to temperature.

Loren Pechtel wrote:But atmospheric pressure doesn't exist at the molecular level. It's just a bunch of molecules moving quite fast. That's what you're going to have to deflect. That's basically the same as the gravity field required to hold onto the atmosphere.

There is nothing to deflect since an air-molecule only have a miniscule amount of kinetic energy due to air-currents, if an air-molecule enters a gravity field it will be accelerated in the direction the field points to.

You are treating this as if the molecules are mechanically shoved away or stopped which isn't happening, they are just affected by a gravity field. What'll happen is that you'll get some kind of draft, probably in the combined direction of the fields in question.

Loren Pechtel wrote:But atmospheric pressure doesn't exist at the molecular level. It's just a bunch of molecules moving quite fast. That's what you're going to have to deflect. That's basically the same as the gravity field required to hold onto the atmosphere.

Joat42 wrote:There is nothing to deflect since an air-molecule only have a miniscule amount of kinetic energy due to air-currents, if an air-molecule enters a gravity field it will be accelerated in the direction the field points to.

You are treating this as if the molecules are mechanically shoved away or stopped which isn't happening, they are just affected by a gravity field. What'll happen is that you'll get some kind of draft, probably in the combined direction of the fields in question.

I expect the protective field to push the gas molecules outward, away from the focused gravity backstop. If it actually causes a draft, then the field is too strong.

Yes, the kinetic energy of a single gas molecule is small, but its average speed is large; however the direction is mostly random, because of collisions with other molecules.

Hunter.CUNY.edu wrote:So the average speed of a gas molecule is about 500 m/sec.

tlb wrote:I expect the protective field to push the gas molecules outward, away from the focused gravity backstop. If it actually causes a draft, then the field is too strong.

If it pushes air away there will be a "draft" since we will get slightly higher density of air next to the field which in turn will continuously sink towards the floor. If it is noticeable or not is a different matter.

Although, perhaps the field and the atmosphere will reach an equilibrium after the field is powered up and no draft will occur after that.

Ugh, gasses, flows and physics tend to get hairy once you leave the simple realm of Boyle's law.