If you have 2 iron spheres and a magnet, the lighter sphere will move first to the magnet. But with earths gravity fall tests we know that every object falls with the same speed when dropped no matter theirs weight. What makes it different?
In: Physics
I don’t think there is a true ELI5 explanation for this, in part because this thought experiment needs refinement. Here are a few things to consider.
Forces are what cause an object to accelerate or decelerate (aka change velocity).
It is understandable why someone would say “every object falls with the same speed when dropped”. But a more accurate statement is that they experience the same acceleration due to earth’s gravity.
If you are at home and drop an object from standing height, the only significant force on that object is gravity acting downward. (Until it hits the floor and the floor pushes back to change its velocity to zero).
In your example of two iron spheres and a magnet you have probably imagined all three resting on a horizontal surface like a table. While both spheres will move toward the magnet, the heavier sphere may experience slower acceleration than the smaller sphere. This is because it experiences greater friction against the table and greater rotational inertia (search the definition of “inertia”) before it begins rolling.
The important point here is that the free fall example only involves the force of gravity. The magnetic objects example actually involves multiple forces – magnetism, gravity, friction – acting in multiple directions and axes. So these two situations actually cannot be compared as a basis for answering the question that you asked.
Lighter objects do indeed move faster when tugged, but with gravity, the heavier object is also tugged more. In fact, gravity’s tug is exactly enough to offset the fact that you have to tug a heavy object more to get it going as fast as a lighter object.
Magnetism’s tug isn’t based on mass. You still need to tug harder to move a heavier object, but magnetisim doesn’t tug heavier objects more. The result is that lighter objects move faster.
Not as physicist so correct me if I am wrong:
Objects falling at the same height are pulled towards the Earth at the same speed no matter how much they weigh.
But objects falling at different heights are pulled towards the Earth at different speeds. Since Earth is enormous and it’s magnetic field is weak this effect is not noticeable.
A small bar magnet will actually have a stronger magnetic field in a smaller sized area so the pull is more dramatic and noticeable.
Gravity is best thought of as a distortion in space around an object, rather than some magical tractor beam.
The huge planet on the other side of the cosmos attracts both a speck of dust and a giant galaxy just the same. Because gravity is really just a “hole” that makes everything slope towards it. The classic analogy is a steel ball on a rubber sheet.
Doesn’t matter how big that ball is, the rubber sheet is going to deform and form a slope, and no matter WHAT size the things resting on the sheet are, they will “roll” down that slope towards the hole just the same. A boulder will slide down a hill just the same as an acorn, and they’ll both do it at the same acceleration.
Magnetism doesn’t operate like that. Magnetism is far closer to being a “tractor beam”. The magnet is trying to pull on other objects, so the larger the other objects are, the stronger the magnet needs to be to even affect it.
Gravity is distortion of space-time. I think we have Einstein et al to thank for noticing that.
You’re comparing apples and oranges. Gravity and electromagnetism are two very different forces.
Suffice to say that gravity pulls more strongly on more massive objects, but a magnet pulls equally regardless of the objects mass.
While gravity causes objects to fall at the same rate, regardless of mass, it does so because the more massive the object, the stronger the pull of gravity, but also the more inertia the object has which needs to be overcome in order to make it move. So the more massive object is pulled harder, but because it is more massive it takes more energy to change it’s velocity. It balances out and all objects fall at the same rate (in a vacuum)
Magnetism on the other hand doesn’t operate the same way. The more massive object still has more momentum and inertia, but the magnetic attraction or repulsion of it does not grow just because it is heavier. If the magnet were larger, then it would be attracted faster, but so would the smaller object and it would still arrive first.
The reason is that the magnetic pull on the 2 iron spheres is essentially the same regardless of their mass, but their difference in mass causes one to accelerate slower than the lighter one.
The thing the helped me understand why gravity works the way it does is understanding that gravity doesn’t work on objects, it works on each individual bit of mass in that object. So it doesn’t matter if you’re talking about bowling balls or feathers, the amount of force working on each proton and neutron is the same and you can picture those being affected separately.
With magnetism this isn’t the case, as the force doesn’t act on each bit of mass but on each bit of charge, and that doesn’t always increase with mass.
Most and of these answers just repeat the question with more complicated words – the opposite of a good eli5 answer
Mass, and how it fits.
1) Gravity is proportional to mass. Double the ball’s mass, get double the pull.
2) Magnetism isn’t proportional to mass. Double the ball’s mass, get the same pull.
3) In both cases, there’s also inertia trying to keep things the same. Inertia is proportional to mass. Double the ball’s mass, get twice the reluctance to accelerate.
So. See what happens when you double the mass.
a) Gravity. Double the pull; double the inertia. The two cancel out. A heavier ball accelerates in just the same way as a lighter one.
b) Magnetism. Same pull; double the inertia. Inertia wins. A heavier ball accelerates more slowly than a lighter one.
Gravity is like rubber bands, no matter how many they all pull with the same tension.
Magnetism is like jumping ropes, the more of your neighbors ropes you tangle with the stronger the attraction.
This harks back to one of the most basic puzzles of physics.
Objects have two different ‘types’ of mass – there is gravitational mass, which changes how gravity affects an object, and there is inertial mass, which governs how the velocity of an object changes with an applied force.
There is, on the surface, no reason why they should be related, at all. As you say, they should be different like an object’s inertia and its magnetism are. But to the limit that we can measure them, they are identical.
It is this puzzle – why are these two types of mass seemingly identical? – that lead to Einsteins general relativity – he thought, if these two totally different things, inertial and gravitational mass, are the same value, might they actually be the same thing? That lead him to see gravity as a curvature in spacetime instead, and what we see as gravitational forces are in fact objects inertia as we force them to follow a curved spacetime.
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