On Wikipedia they list bullet velocity and energy, some bullets have different sized bullets so have a list.
The measurement for energy is ft lbs. (Or at least I understand that one better.) And while I do understand a bullet can do a significant amount of damage to an object, I don’t understand what the ft lbs means. Case in point, for the 500 S&W magnum round they have 5 different bullets velocity and energy listed. The bullets listed have low 2000 ft lbs to almost 3000 ft lbs listed.
My understanding of “1 ft lbs” is the amount of energy a 1 pound weight hits the ground with. (In a vacuum) Or the amount of energy needed to raise 1 pound 1 foot in a vacuum.
So while the 500SW magnum is a very powerful round, I know it can’t move 2000-3000 pounds a foot. So I want to know what the ft lbs means in this case? Like if a person is shot they don’t really fly back. There is a video on YouTube where a guy shot a 5 gallon water jug. It caused the jug to split and water to fly everywhere, but it didn’t move.
Am I not understanding ft lbs to start with? Or what? How does a bullet have that much energy, but in reality can’t move an object even close to the size listed?
In: Physics
The problem with imperial units is, it’s hard to make sense of the described property from the units.
In school, you may have heard of kinetic energy and potential energy. All objects have mass and when they’re moving, they have kinetic energy. A feather or leaf falling on you is barely noticeable. An apple falling on you is a little bit painful. A rock thrown at you can actually hurt. A car hitting you can kill you. A train hitting you will destroy your body as well.
As you can see, kinetic energy is dependent on the mass and speed (velocity, to be precise) of the moving object.
A bullet may not have much mass, but it has a lot of velocity, and thus,a lot of kinetic energy. Getting shot with a bullet is like getting hit with a hammer – very hard. However, all that energy is focused into the point of the bullet. So instead of crush injury at the impact site, it pierces your body and goes through it.
Since the bullet is typically made of lead (a soft metal), the bullet also disintegrates in the body, destroying organs as the pieces of the bullet go through the body.
If the target is a metal block placed on a surface, it will indeed move when hit by a bullet, since the bullet can’t penetrate the hard metal surface, so it yields all it’s energy into the metal block.
Your understanding of what a foot pound is is correct. The reason that it doesn’t push the jug is because it doesn’t transfer much of its energy to the jug – if you shoot a plastic jug with a gun, the bullet will pass straight through and not lose all that much velocity, which means most of the energy is still in the bullet.
In the case that the bullet embeds itself in the person, a lot of its energy will be spent deforming things (breaking your bones, tearing through muscle etc all takes energy to do).
[deleted]
If you perfectly applied the energy of the bullet to the target, then it would move precisely as far as the foot-pounds calculation suggests, absent any resistive force.
However, a bullet impact is usually pretty messy, as a lot of the energy goes into smashing up both the bullet and the target, and the bullet often carries on moving afterwards.
The energy unit is J which is N.m (Newton meter)
The other way to measure force is, given a mass, how much force it exerts on ground.
So, a pound can be a unit of force, like kg can be. Generally, they write a f suffix when mentioning force from weight, like kgf, but not a hard and fast rule.
ft * pound becomes the unit of energy then.
1ft pound = 1.355 Nm = 1.355J = 0.138 (kgf).m
Yeah you got foot lbs right. There is enough energy to move a pound straight up by one foot, or half a pound by two feet, or two pounds by half a foot, etc.
In reality though, when shit gets hit by a bullet it moves a bit but mostly breaks. One foot pound of energy *would* be enough to move a perfectly unbreakable object a foot in the air straight up, its just no real materials are unbreakable.
You know how old cars are rigid, while new cars crumple like a soda can? Same principle in action. “Crumpling” absorbs energy from an impact.
Latest Answers