# If vibranium absorbs kinetic energy, how can Captain America hurt people with his shield?

So two questions previously asked about vibranium, links below, they both came down to vibranium absorbs kinetic energy, IE absorbing the force. And it seems that way for most vibranium based questions.

Question #1

Question #2

My questions is, if Captain Americas shield is made from vibranium, how does he hurt people with it? I originally thought it was because his shield was not pure vibranium, but a mix between vibranium and proto-adamantium. But that would only reduce the amount of force it absorb, not take away its ability to do so.

From the Wiki

It is able to absorb all kinetic energy and transfers very little energy from each impact, meaning Cap does not feel recoil or transferred impact forces from blocking attacks......

In both cases there is no recoil and very little energy transferred. If these are both true, then there is no energy being transferred to his opponents right?

My question again slightly refined, how does take down his opponents with his shield, if there is no kinetic energy being transferred?

• @einpoklum If the shield does not transfer kinetic energy, then there is no force hitting the person. – Clyde May 8 '15 at 13:38
• @einpoklum It absorbs all the force from the impact, so there shouldn't be anything left to be transferred to his opponent - like hitting them with a feather. Plot hole and writers with no concept of physics. – JRE May 8 '15 at 13:38
• It also emits it, but only at bad guys. – Paul D. Waite May 8 '15 at 13:42
• @JRE - Your own comment doesn't really make sense to me from a physics standpoint--what does it mean for force to be "absorbed"? Newton's third law means forces must always be equal and opposite in a collision. The reason it doesn't hurt to get hit with a feather is a combination of its low mass, air resistance slowing it down, and the fact that the time you and the feather are in contact is spread out more compared to a more rigid object because the feather can deform more, which means the force needed to drop its momentum to zero is smaller (momentum change = avg. force * time in contact). – Hypnosifl May 8 '15 at 14:02
• Because comic book physics. – KSmarts May 8 '15 at 14:07

He can deal damage with his shield because it's designed to work as a story element in a comic book. In terms of real physics, it makes no sense.

Newton's Third Law says that for every action there is an equal and opposite reaction. This is a pretty fundamental property of how masses interact with each other, and makes this concept fail.

The problem is that if we postulate some sort of material that absorbs the kinetic energy, that precludes the shield from imparting kinetic energy upon its target. This is because whatever is going on at the molecular or atomic level to do this absorbtion would also prevent the imparting of energy due to Newton's Third Law.

It's perfectly acceptable to me to postulate magical substances in comic books with physically impossible properties. However, there isn't a way to explain this one due to its asymmetric nature, unless the shield somehow "knows" which energy to absorb and which not to. I'm not aware of it being assigned any kind of consciousness, so that's out.

• Haha. I like this answer. – Clyde May 8 '15 at 19:15
• Physics for the win. – jgallant May 8 '15 at 19:55
• I'll comment that I'm prefectly fine with ridiculous comic book physics. Just because I have a degree in physics and understand these things doesn't mean I can't enjoy the comics. :) – Almo May 8 '15 at 19:59
• while this is obviously the right answer, it's also the right answer to every single question ever asked on this site, thus making is kinda useless. We still like to pretend that our fantasy universes have some kind of internal logic... – KutuluMike May 8 '15 at 23:46
• I don't agree. Sometimes we are able to make sense out of these questions. In this particular egregious example, we're not. This is why, despite having read countless questions like this, I only chose to post this answer on this one. – Almo May 8 '15 at 23:52

You have to keep in mind that Vibranium only absorbs kinetic energy directed into it. If the shield absorbed ALL kinetic energy from any impact, it would be useless as a tool or weapon. Also, Vibranium does not absorb 100% of an impact's kinetic energy, only most of it. Let's speculate!

When Captain America throws his shield at a person or object, keep in mind Newton's First Law:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

So what you have here is a near-invulnerable object travelling through the air at whatever speed it was thrown. Normally, the only thing that would stop such an object is the outside forces applied against it. In the case of the shield, however, most - but not all - of that outside force is nullified by the properties of the material.

Captain America has obviously learned how hard & fast to throw the shield in order to have it impact an object and still bounce back. In fact, his ability to accurately predict the trajectory of the shield is what makes him one of the few people in the Marvel universe who can wield the shield with any sort of effectiveness. In most people's hands, it would be no more effective than a big, near-invincible Frisbee.

As for transferal of energy, it actually works the opposite of what you're thinking - the material prevents kinetic energy from being transferred through it, not out of it. For instance, when someone like Thor or The Hulk hits Captain America's shield, any other material would transfer that kinetic energy directly into the Captain's body. At the very least, his arm would be shattered, and after that his legs would probably turn into pulp. However, because of the Vibranium's properties most of that energy is negated, resulting in a much smaller impact as far as the Captain's bones & muscles are concerned.

In regards to Newton's Third Law, which states

For every action, there is an equal and opposite reaction.

perhaps we can state that the Vibranium is actually re-directing the energy instead of absorbing it. Obviously, a bit of hand-waving is needed here, but we might speculate that - say 80% - of any kinetic force being applied to the shield is reflected away from the shield, or even back upon its source. This would be supported by a scenario like in Marvel's "The Avengers", where an attacker (in this case, Thor) is knocked backwards at almost the full force of their OWN blow. However, we would then be forced to explain other scenarios - like that in Captain America: Winter Solder - where an opponent strikes the shield with no obvious effect.

We might even go so far as saying that Vibranium is polarized, with one side of the shield having a positive charge and the other having a negative charge. This would attempt to explain why impacts on one side of the shield reflect/re-direct while the other side somehow absorbs/nullifies. However, it's far more likely that both sides react to kinetic energy in the same way.

Of course, even with all this in mind... it's also important to keep in mind that applying real-world physics to a comic book will almost always result in frustration.

• The only problem with "directed at it" is I don't think it is worded correctly. If a someone with a bat hits my shield it absorbs kinetic energy, but if I hit my shield against a bat with the same force it doesn't? Force isn't just one way, every time something has force acted on it, it "pushes" back with force. If it pushes back equally nothing happens and if it pushes back with less it moves. – Clyde May 8 '15 at 14:23
• If it prevents kinetic energy from being transferred through it, shouldn't that apply to the kinetic energy of Captain America's own arm when he tries to move the shield? If he hits someone in the face with the front of the shield, didn't the kinetic energy they feel from the shield have the origin in his arm movement? Or if you say that's a case of kinetic energy being "transferred out of it", then why doesn't it work from front to back (Thor hits front, shield transfers kinetic energy to Cap's arm) as it does back to front (Cap pushes out with arm, front of shield transfers KE to villain)? – Hypnosifl May 8 '15 at 15:09
• ...basically, I think we should just leave it at the last line of your answer (or KSmart's comment above)--it's comic book physics, any attempt to analyze it in terms of real physical principles isn't going to work. – Hypnosifl May 8 '15 at 15:12
• you can't have newton's first law without newton's third law... – njzk2 May 8 '15 at 16:53
• What about conservation of momentum? If Thor's fist comes in at the shield in a leftward direction, then if the shield stops his fist, the shield itself must gain the same momentum to the left, and if it doesn't go flying that must be because Captain America planted his feet in the ground and the momentum was transferred through his body to the Earth. But whatever momentum mv it initially gains before Cap has time to decelerate it (equal to the momentum MV of Thor's punch), it must transfer kinetic energy (1/2)mv^2 to Cap, there isn't really any way to get around this in Newtonian physics. – Hypnosifl May 8 '15 at 17:31

While it's possible that something absorbs enormous amounts of kinetic energy, thermal energy, electrical energy, etc., there is one thing that (according to physics in this universe) can never, ever be destroyed, subverted, or "absorbed":

# * Momentum *

It must be transferred.

Captain America takes some heavy hits, but he never just stands upright and lets his shield absorb the impact with its magical Vibranium. He braces for impact.

Thor

Winter Soldier

If he doesn't brace against something, he gets knocked over like everyone else.

Adamantium claws strike the the adamantium-plus (vibranium) sheild with a cascade of sparks. The sudden lunge tips Captain America off balance.

The shield doesn't magically absorb blows. Cap' does that.

It is strong enough to withstand them. The blow is distributed over a wider area, and Rogers can use his arms and body to more gradually absorb the energy of the punch/strike/explosion and transfer its momentum.

That doesn't quite line up with your quote, but that's physics in this universe. And it means that shield makes for a very fine weapon.

You can accept that and take canon less literally, or (my suggestion) you can accept the canon explanation and take this universe's physics less literally. Either way works :)

• Momentum can be absorbed/converted to another form. For example: electromagnetic locks, explosions, and gravity can change the momentum of objects. The important point is that total energy is conserved. – Trisped May 9 '15 at 2:16
• @Trisped, right, it can only be transferred. – Paul Draper May 9 '15 at 5:44
• Yeah I mean he just wouldn't be able to even move it if it absorbed all kinetic energy. – Lightness Races in Orbit May 10 '15 at 2:21
• @Trisped In all of the examples that you list, momentum is actually conserved. It's just being transferred to very large things (such as the Earth and/or its atmosphere,) resulting in a non-noticeable difference in velocity. Also, note that momentum is a vector quantity. An explosion radiating out in all directions produces a net momentum change of zero. – reirab May 10 '15 at 2:57
• @Trisped, right. There is no way to completely resolve physics in this universe and that description. And without mechanical physics (a generalized description of how things move), an answer to the OP's question is "just 'cause". – Paul Draper May 11 '15 at 18:50

# Energetics

If the hypothetical Vibranium absorbs kinetic energy, it probably does so in its own frame of reference. In this frame, the target of the shield is coming with great speed towards the shield. Upon impact, the shield somehow absorbs all of the target's kinetic energy, reducing the target's relative velocity (in the shield's frame) to zero. What happened from an observer's perspective? The shield collided with the target, somehow caused the target to match the velocity of the shield upon impact. In the observer's frame of reference, it could be that the shield actually imparted kinetic energy to the target.

### Where does the kinetic energy go?

Presumably Vibranium heats up or deforms ever-so-slightly when it absorbs kinetic energy. Beyond that, it's comic book physics. :-)

# Momentum

### How can momentum be conserved?

It's a textbook example of a purely inelastic ("sticky") collision. The shield is moving, hits its target, and the two move with the same final velocity. The shield would have to have changed its velocity during the impact in order to conserve momentum.

# How does the shield inflict damage by absorbing kinetic energy?

When the shield impacts its target and somehow makes target match its own speed, the new speed of the target may be very different from its original speed. This impact does damage to the target because the target potentially experiences very large accelerations. It's the same kind of damage that a punch delivers: the painful part of a punch isn't usually in the impact, it's in the sudden (whiplashing) change of velocity that results from the impact.

If a boxer punches an opponent's teeth and knocks a few teeth out, it's not the impact that knocks them out. The teeth break near their roots, yet the boxer's glove doesn't impact the roots. The impact changes the tooth's velocity, which puts incredible shear stress on the tooth's roots as a result. The tooth, being unable to withstand the shear stress caused by the sudden change in velocity, breaks loose at its weakest point.

# Other remarks

It might be easier to think of how it works if you consider the limit of the shield having nearly-infinite mass (but Cap somehow is strong enough to wield it).