I think the answer to your question would be that the power Juggernaut posses is divine in nature and thus not follow the laws of physics as we know it. Gods stand above such matters.
I took a bit of a page (http://marvel.wikia.com/Cain_Marko_%28Earth-616%29
) to help out, but.. there is a lot of text there and this particular powerhouse bores me. Sorry *grins*
The Unstoppable Juggernaut
Artful DodgerAdded by Artful Dodger
The Unstoppable Juggernaut possesses the conventional superhuman attributes of an Elder God, however many of his abilities are greater then the average member of his race. Cytorrak grants his godly body to his avatar, the Crimson Ruby of Cyttorak then bonds to his avatars soul and makes him a god by giving him this new body, however, this body is Cyttorak's original Elder God body instead of his degenerated demonic body hence it is superior to Cyttorak's current body.
Superhuman Strength: The Juggernaut possesses vast superhuman strength the limits of which are unknown. Using his physical strength he was able to knock out the Thing, Colossus, and even the Hulk, He proved strong enough to match the highly strong Prime in a fight, knocked out several mutants with a thunderclap, and even toppled Stranger with one punch after he grew to about 50 feet tall. He was also able to go toe to toe against an enraged World War Hulk, The Juggernaut has stated his strength eaisily rivals Thor's physical strength.
Superhuman Stamina: Juggernaut's body generates no fatigue toxins during physical activity, granting him limitless superhuman stamina in all physical activities.
Superhuman Speed: Despite his immense size, Cain's superhumanly strong leg muscles allow him to run and move at speeds beyond the natural physical limits of the finest human athlete, He can run at speeds of 111 miles per hour or more.
Superhumanly Dense Tissue: The Juggernauts skin, muscle, and bone, is exactly 4 times denser than the same tissue in the body of a human being, greatly contributing to his superhuman weight.
Invulnerable: Cain is completely invulnerable to all forms of physical injury. The Juggernaut's skin cannot be broken, and he is incapable of feeling pain, or sustaining any form of injury from any purely physical attack regardless of magnitude or intensity. Although Cain is able to be harmed by exceedingly high levels of mental attack, mystical attacks or weapons with mystical properties can hurt Cain, only because they counteract, in some way alter, or reduce the mystical energys that make Cain invulnerable, however, these attacks have rarely been shown to cause any permanent damage. When The Juggernaut's force field, and hence some of his power, was temporarily absorbed by Mjolnir, he was still durable enough to exchange blows with Thor.
Regenerative Healing Factor: Despite his complete resistance to physical attack, Cain can be damaged by sufficiently powerful mystical forces. If damaged, Cain possesses a regenerative healing factor that enables him to completely regenerate with superhuman speed. For instance, he has been injured by Shatterstar's sword, which is forged from a mixture of extra dimensional technology and magic and healed instantly. The demon being D'Spayre once flayed the Juggernaut down to a skeleton only after absorbing almost all of the Juggernaut's power and even then, the Juggernaut could not be stopped and was able to instantly regenerate all the damage done once he regained his full power. Essentially as long as there is a molecule of Cain's body, and a molecule of the power of the crimson gem of cyttorak still upon him, and of course he is still the Exempler of the ruby, the juggernaut will regain his full power and be completely healed, suggesting nothing is drawing or blocking the gems mystical energys from resonating, and all of the energy from becoming one again.
Self Sustenance: The Juggernaut is completely self sustained, he has no need to breath, eat, and drink, he cannot tire, hence he cannot sleep, however his self generating mystical engergies provides him with the nourisment of sleep, nutrition, hydration, and oxygen. He is sustained by his mystical energys alone.
Immortality: Cain is immortal. He is not a millisecond past his absolute physical prime, and is immune to aging. Nothing, except perhaps, total molecular disinigration can kill Cain. The Juggernaut is also immune to all toxins, poisons, and disease.
Force Field: The Juggernaut is capable of generating a personal force field around himself, extending to about 1 foot in diameter from his body. He uses this force field for many purposes, such as preventing physical contact, keeping debree and unwanted materials off himself, erecting his force filed to escape piles of debree easier, and a few other purposes as well, however it's most important purpose is the fact, that if he is casted in conrete or other materials, example a plane dropping a large amount of solidifying material that can put his body in a hard cast, or falling into wet concrete that would harden around him in a full body cast, which would keep his motion prohibited, he can erect this force field to break out of a cast, hence without this force field he would not be unstoppable, also this force field is particually resistant to such materials in the first place, as such materials would slide off his force field much faster and more smoothley then his body alone, do to the shape and the fact his force field is pure energy, also his force field can resist mystical attacks better then the juggernaut alone, as his force field is energy created from his actual power, also mystical weapons that can hurt Cain have to penetrate his force field first, making it even more useful, not to mention the force field being able to produce more energy to continually push out an intruder that has managed to break in to his defences.
Embodiment of irresistible force: Once he begins to advance in a certain direction, it is virtually impossible to halt his movement. Some obstacles such as many tons of rocks and plasma-discharge cannons have slowed his pace but can't completely halt him. The only known examples of him being stopped involve physical force with mystical involvement, which is Juggernaut's sole weakness. Thor unleashing his power was able to stop him in his tracks but his powers are mystical in origin, It should also be noted that Thor was severely hampered at the time as he was extremely weakened by an extra dimensional malady. During a battle with "War Hulk", in which the mutant Apocalypse, increased the Hulk's strength and power enormously by grafting Celestial tech to him, which again involved mysticism.
Power Grid 
Juggernaut is able to summon and remove his mystic armor at will. As a result of his days in the military and his experience as a superhuman criminal and hero, the Juggernaut is a formidable hand to hand combatant preferring to use street fighting and brawling techniques that allow him to make full use of his great strength.
Class 100+ ; The Juggernaut possesses vast physical strength the exact limits of which is unknown but he is able to lift far in excess of 100 tons effortlessly. He is one of the strongest beings to ever walk the Earth.
Cain is susceptible to exceedingly high levels of mental and mystical attack. Both have been shown to be able to at least temporarily defeat him but neither have been shown to cause any type of permanent injury. The weakness to mental attacks are nullified by his helmet. If The Juggernaut does not use his powers for his own personal desires, the power of the gem does not work correctly and does not fuse with his sole essentially makeing him a human with only a portion of some of his powers, no longer a perfect god, just a greatly enhanced human.