Shinra Tensei is useless!!
for all you haters: I know that this is manga This explains what would happen in the real world if you had Shinra Tensei
here is an excerpt from a Cracked article called 6 Awesome Superpowers (That Would Suck in Real Life)
and here is the part about force fields (which is Shinra Tensei):
So let's say you lean a little more toward the defensive side when choosing which superpower you fantasize about, and you prefer your superheroes of the "destined to appear only in shitty movies" variety. Fine -- you're Sue Storm, the Invisible Woman. You can conjure up an indestructible, invisible force field to materialize around you at will. Worries like car crashes, house fires, and ninja attacks are things of the past -- literally nothing can harm you now. You're the physical manifestation of the old "I'm rubber, you're glue" bit.
We'd like to glue a rubber to Sue Storm, if you know what we mean. (We certainly don't.)
So What's the Problem?
Let's face it -- if you suddenly find yourself with the ability to blow an indestructible bubble around yourself, the first thing you're going to want to do is show it off, Jackass-style. So say you decide to leave a building in the most Hollywood fashion possible: jumping straight off the roof and activating your force field just before you hit the ground. Well, sorry to tell you this, but your shield -- indestructible as it may be -- isn't going to save you.
"Wait ... what?"
You see, when you land, you'll face the same problem that your grade school egg drop experiment did: limited cushioning. If you're suspended rigidly in the center of your force field, as in most portrayals, the impact will absolutely kill you. There's no escaping the physics of it -- you're still losing all your momentum in an instant, and the force of the impact would turn you into a nice human puree. The problem isn't fixed if you're free to move around inside your force field bubble, either, because that would be like standing in a free-falling elevator -- you'll pancake onto the bottom of your shield on impact just the same.
"So I just won't jump off of buildings!" you say. "It'll still protect me from car crashes and whatnot!"
Whatnot, courtesy of Cracked's art department.
Unfortunately, nope. It doesn't matter where the force comes from -- physics is kind of an ******* that way. You face the same problem whether you're falling from a building, getting hit by a car, or being crushed by a comically oversized falling anvil. That force has to go somewhere, and your fragile human bits are as good a place as any.
Even if you do not use it as a shield but to repel things:
Newton's 3rd law: For every action, there is an equal and opposite reaction.
meaning when you send out such a force to push something, there is counter force being sent back to your little body(that is the exact same ammount but is concentrated only on you) and that would be an OP way to die