Captain America is able to throw his circular shield and have it return it to him like a boomerang.
Is this even possible according to physics (especially considering the ranges at which he uses the shield)?
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Captain America does not throw his shield and have it return like a boomerang. It acts instead more like a frisbee disk able to strike and transfer energy between multiple opponents. As to whether the laws of physics would allow this to take place in reality is highly unlikely. There are no known substances which would allow a metal to do what the Shield is able to do.
Using his enhanced strength and his awareness of three dimensional space, he can cause it to return after striking a variety of targets or opponents. Thrown with sufficient force it can act as a cutting edge destroying metallic objects such as guns, helicopter rotors and even tank turrets.
Considering the number of surfaces he is able to use to redirect his shield, his awareness of vector dynamics would make him an extraordinary marksman or billiards player. He is able to calculate intuitively what it will take for his shield to return to him and has through practice been able to predict his shield's behavior with nearly flawless accuracy.
The edge of the shield is somehow able to absorb and transfer energy allowing the shield to behave like a superball, bouncing until it runs out of energy, hits a soft target which absorbs its energy transfer or is caught by Captain America or another sufficient strong and agile person.
The shield is made of, depending on which medium you are discovering it, from either and adamantium/vibranium or vibranium/iron alloy or in the most recent episodes of the Avengers comics, vibranium/adamantium/uru metallic alloy mixture.
The extra-terrestrial material vibranium bestows by nature of the curvature and density of the element within the shield its ability to either deaden or absorb vibration-based energy used against the shield when struck along the front of the shield. The shield is capable of absorbing incredible energies and preventing them from reaching the wielder.
Cap has even used it to absorb the impact of falling from a skyscraper and absorbing the energy of his landing with the shield escaping serious injury.
The shield has also taken blows from Thor and Mjolnir with Captain America's fighting skill allowing him to roll with the impact escaping serious injury.
The adamantium/iron/uru element of the shield supposedly renders the shield relatively invulnerable to almost any energy or effect not considered cosmic or reality-altering in nature.
The nigh-indestructible shield has been broken five or six times, in continuity, by the Molecule Man, Doom, imbued with the Beyonder's powers, Thanos, wielding the Infinity Gauntlet, Thor using the Odinforce and by the Serpent, brother to Odin.
Its breaking is usually a monumental event and when its replaced or repaired it is often by the same force that destroys it. There are numerous retcons in the Marvel universe where the shield was thought to be broken and found to be a replica instead.
When the shield is thrown, edge on, the vibranium alloy allows the shield to resonate and continue bouncing from object to object when struck on the side.
The shield will continue bouncing, if thrown properly until it is either caught amid flight or strikes an object not capable of resisting the shield well enough to continue on its way. With sufficient training, Captain America has learned to be able to throw the shield so it returns to his hand, apparently effortlessly.
Yet we know that this is actually very difficult to do, since only a few other people have shown any aptitude with performing this feat (Hawkeye and later, the Winter Soldier/Bucky) are two of the people to have ever been shown to have learned it well enough to use the shield offensively. Technically, it is to be assumed the Taskmaster could do it since he can photographically copy the reflex actions of any trainable skill.
Though it may appear that Captain America's shield returns to his hand via magical means, it does because of his highly trained and finely-honed skill at throwing it.
Coupled that skill with his enhanced superior agility and mental acuity as well as the unique metallurgical properties of the shield itself and you have the unique defensive and offensive properties of Captain America's shield.
Nearly indestructible, it can be used defensively as a protective device and offensively only in the hands of a skilled user. Few on Marvel's Earth-616 have the capabilities and experience throwing the shield of Captain America.
Because his shield is made of a fictitious alloy, it doesn't need to obey the laws of physics.
From the Wikipedia article:
The vibranium in the shield grants it unusual properties, allowing it to absorb virtually all of the kinetic impact from any blows that the shield receives without injuring Rogers in the process. The vibranium is also a factor in the way Rogers throws his shield: he often uses it to ricochet around a room and strike various opponents with little loss of velocity in its forward movement after each impact.
Given his enhanced strength and intelligence, he can calculate where he needs to hit something and the ricochet paths in order for it to return to him.