The strength of MRI machines is measured in Teslas. One Tesla is basically equivalent to the strength of the magnetic field of the earth. Most modern MRIs are 1.5 or 2 Teslas (2T).
The aluminum plate trick is:
If you take an aluminum plate, such as a sign or a pie pan and put it upright in the bore of the MRI, the plate will not fall over!
A magnetic field of that strength induces an eddy current in the plate, and the eddy current literally holds the plate upright. Certainly gravity has some effect, but any tendency the plate may have toward falling over will take place at an amazingly slow rate. It falls much much slower than it would on the moon.
If you ever get an MRI, pay attention to what the tech asks you. You'll be checked for metals, and he/she will ask you if you've been working with metals lately (e.g. drilling, grinding, or welding). There's a simple reason for this. The field is strong enough to pull a sliver of steel completely thru your body.
I think every MRI tech has seen accidents with the MRI. I've heard some horror stories that don't bear repeating, but the one that really gets most guys is the story of a patient who had a penile implant....... and the MRI machine reportedly made it heat up!
Somehow I had visions of it waving around like a light-saber.
No, I'm not an MRI tech. I just work with a lot of them.