Fridge Magnets
People use magnets to hang notes on refrigerators. Despite the downward pull of gravity, magnets do not fall, even though the magnetic force doesn't pull up.
How is this possible?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
3 solutions
Correct. Friction is not an "upward" force, in this case it is a resitant force. It is fighting the downward pull of gravity.
Friction has nothing to do with this
So you are saying that if you had a frictionless magnetic surface, and you put a magnet on it, the magnet would slide down due to gravity?
Yes, I think so
Lets look at solutions A, B, and C separately from a logical perspective A: How would a magnetic field affect gravity? We can put two magnets near each other and they'll attract. If we make them fall at the same time no difference is made. This rules out the option of choice A. C: Magnetism attracts. It does not relate to being pulled up or down. It can be up or down depending on the situation. However, in this situation it is sideways, not up. This rules out the option of choice C. B: This is the only option left so it is correct. However, the question is why. We know gravity acts downwards. As the magnet is pushed towards the fridge another force comes to mind. Friction! Friction is when two objects rub against each other. When the magnet rubs against the fridge it is acting "upwards" by negating the \n gravity because of the magnetism. (This is different than A because these forces are acting against each other in this situation since friction is going UP). Therefore, B is the answer.
They aren't moving though.
Log in to reply
The gravitational force is weaker than magnetic force because the mass of the clip is so small in comparison.
How about Static Friction?
If there was no friction, the magnet would have moved down. However, there is friction and it opposed the magnet's tendency to move down by acting upwards.
Then if you put a non-magnet on the fridge, why doesn't the friction hold it up? Obviously the answer has to include "magnetism does something".
Log in to reply
The magnetism attracts the objects to the surface of the fridge that has enough friction to completely cancel the small downwards force of gravity.
To elaborate Maxim's point, the magnetic attraction between the magnet and the fridge causes the magnet to press to the fridge. As a result, there is normal force between the magnet and the fridge. This normal force allows friction between them.
The force of friction is proportional to the normal force. The magnet increases the normal force considerably.
Just to be clear from a logical perspective, two answers being clearly wrong doesn't suffice in making the third right. There is still the scenario that all three answers are flawed.
Log in to reply
I agree. Such a solution is not helpful in figuring out the nature of reality.
Youn guys are wrong. Friction is passive and acts in all directions.
Log in to reply
Could you please explain what that means?
Log in to reply
Friction doesn't "pull" anything in any direction. It's a passive, resisting force that makes it harder to push an object in any direction. Gravity drags the magnet down, but friction weakens the effect. In the same way, if a force would try to push the magnet upwards on the fridge door, friction would again weaken the effect.
It's not that friction is dragging or pushing an object in any direction, it just passively makes it harder for another force to move an object, no matter the direction.
2 is not the correct answer if it was magnetism would be an antigravity force. Magnetic force is in this case a pressure force that holds the paper tightly enough so that is does not fall. All the friction does is add to the magnetic force. No magnetic force and friction would not be sufficient to hold the paper or object.
Log in to reply
All 3 answers are wrong. The magnet acts similarly to a nail or glue which does not allow gravity to move the note because it is connected to the fridge. The fridge can’t move because it is sitting on the floor which stops it from falling.
Log in to reply
This "glue" is called friction. Of course friction itself is not a fundamental force, it actually consists of the many forces acting between the atoms of the two surfaces. Read the english Wikipedia article for a nice picture that shows how, as you described, friction actually just locking up two rough surfaces. Of course in the fridge case the friction is created by the pressure of the magnetic force. So answer 2 is not incorrect, just incomplete.
The 2nd answer is incomplete, I believe that it should highlight the effect of the magnetic forces on the answer can make it more complete, for instance: old: Friction acts upwards and gravity acts downwards new: the friction plus the magnetic interaction acts upwards and gravity acts downwards any suggestion?
sorry... but you have to now that the difference between the gravititional force and magnatic force is 1:1*10^39 - and you can see that when you trying to jump so you can "briefly" break the gravity, but you can never ever get your hand throught the wall............ so i think that the correct anwser based on quantum mechanics is 1.
N one of the posed answers are correct. The magnetic force of the magnet on the steel door exceeds the pull of gravity on the mass of the magnet. There is nothing else to it. A. Gravity does not become weak B. Since the refrigerator door is vertical, friction plays no part. C. The magnetic force acts horizontally and dominates the mostly vertical force of gravity
Log in to reply
- Why can't friction play a part if the door is vertical?
- You say that the magnetic force acts horizontally. If we draw the free body diagram of the fridge magnet, there is gravity downwards. Which force exists in the vertical direction to cancel this force?
You are mistaken. The correct answer isn't available to choose. The magnetic force is stronger than the gravitational force. There isn't any friction unless you are sliding.
Log in to reply
If you draw the directions of forces acting on the fridge magnet, you will see that the magnetic force acts horizontally, whereas the gravitational force acts vertically. There must be some other force (in the vertical direction) since the magnet remains at rest. This force is friction. Friction can act even when there is a tendency for the object to slide. Sliding isn't a necessary condition.
The surface of everyday objects are not smooth to the microscopic level. So there's always some friction involved!
Relevant wiki: Friction
Magnetism provides the normal force which allows the friction to resist gravity.