Floating magnets and gravity

If a magnet is using it’s opposing force to hold another magnet in the air, what happens to the weight? Is the ‘supporting’ magnet receiving pressure equal to the weight of the ‘floating’ magnet?


Sure it is.

Nothing “happens to the weight,” in any meaningful sense. Weight is merely the downward force on an object due to the acceleration of gravity, f=m•a. ‘F’ here is the weight, ‘a’ the gravity (9 m/sec[sup]2[/sup], or 32 ft/sec[sup]2[/sup] if you like).

The two magnets act on each other just like two identical non-magnetic bricks would - except with an air space between. The lower one provides enough upward force to the upper one to arrest its further downward motion; the upper pushes down on the lower. Because of this, the lower one is pressed into the tabletop (or whatever), with the weight of two magnets/bricks: its own and the upper one’s.

Which is a periphrastic way of saying ‘Yes.’ :wink:

  • Dave

Yes Tim, the lower one bears the weight of the upper one.

yeah, gravity is really really really really weak compared to the other forces

Yes, by Newton’s 3rd Law: “For every action there is an equal and opposite reaction,” a statement known by all but misunderstood by most.

Longer but more understandable version: “If 2 bodies interact (via gravity, electromagnetism, contact, etc.), the force exerted by body 1 on body 2 is equal in magnitude and opposite in direction to the force exerted by body 2 on body 1.”

So if magnet 1 is exerting a repulsive upward force on magnet 2 equal to magnet 2’s weight, magnet 2 must be exerting a repulsive downward force (not pressure) on magnet 1 that is also equal in magnitude to magnet 2’s weight.

True, but how is this relevant here?

um, just saying that the magnet will have pretty much no problem overcoming almost anything that gravity can throw at it. it was basically my way of saying me too, maybe expand a little on the theme.

But the gravitational force on the magnet is equal to the magnetic force on it. The downward pull of gravity is exactly counteracted by the magnetic force pushing it up. Otherwise it’d fly up or fall down. (Or to be exact, it would move up or down to a position where they are equal.)

The entire Earth (5.98 x 10[sup]24[/sup] kg) is pulling down on the floating magnet, but that is overcome by a 5 g magnet pushing up.

Indeed–I have used that very example in class. I didn’t see what that had to do with the OP, though. This whole discussion could have taken place with a normal force replacing the magnetic force.

In any case, I made that comment well before my coffee this morning. :slight_smile: