Am I getting anything wrong here?
I think a knife uses force over a small area rather than energy, as pressure to cause cuts, splits and cracks.
A club can have the same pressure as the knife, but the club carries greater momentum and will knock around things more.
A bullet causes damage through the energy it carries and causes heat and deformations as it hits the target.
When people talk damage with bullets they use energy, but with melee it’s usually force. But when should I use them? Both are used at the same time but one is doing more of the effect. Don’t both result in work?
Like a force over a distance is work and thus the same as kinetic energy. Joules.
But if it’s pressure doing work I get unsure. I mix up how things break all the time.
In: Physics
Force and pressure are two completely different things. Force is mass*acceleration and pressure is force divided by area. Basically the more mass and the faster the mass is going the more force it has and the more force applied on a smaller area the more pressure you have. When playing baseball the heavier the bat and the faster you swing it the harder you hit the ball in terms of force. For pressure think of a knives edge(suuuuper small area) with a lot of force. Cuts right through. That’s also why the sharper the knife the easier it is to cut. Sharper blade=thinner blade. Hope this helps
So pressure is a bit wonky because pressure is force *per unit area*. It behaves like temperature. If you take two hot things and force them to touch, do they get doubly hot? No, they don’t. Pressures behave similarly. Doubling the material does not necessarily change the pressure.
Force is always a good thing to use. If you wish to use force, but you have been given pressure, then you must calculate what the force of that pressure is. It is equal to the pressure (force per unit area) times area, which cancels out to pure force.
Energy, Momentum, and Force are all sort of interchangeable. Energy is the conservation of force over time (hence why it can and does change with distance). Momentum is the conservation of force over distance (Hence why it can change with time).
You can convert from energy to momentum through kinetic energy and vis versa.
Force is a result of energy. It’s the mass x acceleration. All of the things you mention use this formula, but the variables change a lot. A club has higher mass, but lower acceleration, and also distributes that force over a larger area than a bullet, so its tends to not penetrate, but also transfers most of its inertia into the target. A bullet has lower mass, but much higher acceleration that is also focused on a small area, so it results in a lot more pentration, but that low area of contact might not all get absorbed by the target, so the bullet might pass right on through.
You are on the right track. Saying a club has the same pressure as a knife isn’t quite right, as people have mentioned pressure is a factor of force over an amount of space. However that’s not necessarily inaccurate either, as that is why you can find blunt weapons with spikes and stuff attached to them.
When talking about bullets and mentioning energy, they’re generally talking about kinetic energy, which is another method for measuring how much energy a bullet has. It’s relevant because kinetic energy is equal to half an object’s mass times the square of its velocity. This means that increasing speed has way more of an effect on kinetic energy than increasing its mass, but keeping velocity steady. Increasing velocity along with using harder projectiles is what influences the ability to penetrate armor.
So bullets cause damage for the same reason knife points do. They put a relatively large amount of energy into a rather small spot. High pressure. You can actually see that from an general perspective, most bullets done carry a lot of energy compared to other things. Some bug guns do have a lot of recoil, but it’s not going to throw you off your feet. Simply speaking, any action has an equal and opposite reaction. It’s oversimplifying a bit, but any recoil you feel is basically similar to what propels the bullet out the other end, after you account for energy moving the gun and whatnot.
I am not really sure what your question is, and you also threw around a lot of terms that are completely different from each other. It is also important to note that the way we use physics terms in our day to day lives are not at all representative of the true definition of these terms. Anyway here is the briefest physics course you will ever receive.
**Momentum** is how hard it is to stop something. It is equal to speed times mass.
**Force** is equal to the change in momentum over time. Basically how quickly something would be gaining momentum if it were allowed to move indefinitely.
**Work** is a form of **Energy**. There are many forms of energy, but the energy you are referring to is the work done on an object, i.e. a **force** applied over some distance. ex. you do more work when pushing a box 3 feet than you do when you push it 5 feet, even if it takes the same amount of force to move.
**Pressure** is the concentration of force per area. Ever heard of PSI like for air pressure or when blowing up a ball? That is **P**ounds of force per **S**quare **I**nch of area.
Additionally, the energy of a bullet isn’t *directly* related to the damage it can cause. You can tell very little about the damage something can do from it’s kinetic (movement) energy alone. There are SO many things to consider here. You have to consider the size and shape of the object. You have to consider the angle and distance which the object is traveling, and most importantly you have to consider **the way the energy is transferred from the object, and how it dissipated in the target.** If I shot a big ball of jello, it wouldn’t do much damage as Jello is highly viscous and nonrigid, causing it to dissipate the energy quite well. If I shot a ceramic plate, it wouldn’t fare so well as ceramic is fragile and rigid, and prone to weak areas in the construction.
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