# The Naked Scientists Forum

### Author Topic: gravity, electromagnetism and flying  (Read 1474 times)

#### LetoII

• Full Member
• Posts: 64
##### gravity, electromagnetism and flying
« on: 04/09/2012 20:14:24 »
Hi guys,

i have a pretty simple straightforward question for you all:
gravity is what keeps us tied to this planet however time after time now i've heard "gravity is weak, look this magnet can pick up this weight with no problem even though the entire earth is pulling on it".
So if magnetism is so much stronger than gravity why don't we use it to fly?

now before leaving you with this question i'd love to correct myself because we do in a way use magnetism to fly, we have some sattelites going for the outer edge of our solar system by slingshotting themselves using the planets to do so.

#### CliffordK

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##### Re: gravity, electromagnetism and flying
« Reply #1 on: 04/09/2012 21:15:19 »
You might say that maglev trains do use magnets to "fly".

I think the problem is that magnetic force falls off with the square of distance between the magnets.
Likewise, gravitational force falls off with the square of the distance between the two objects.

However, in the case of Earth, you calculate gravity with the distance between any object and the center of Earth.  So the gravity at the surface of Earth is 9.8 m/s.  But, the gravity in Low Earth Orbit (LEO) is still about 9 m/s.

If, on the other hand one had a magnet strong enough to lift 1000 kg at 1cm, it would only be able to lift 250 kg at 2cm, and 10 kg at 10cm.

The gravity would almost be the same at low earth orbit (2,000 km altitude), but your magnet would be virtually undetectable.

#### JP

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##### Re: gravity, electromagnetism and flying
« Reply #2 on: 04/09/2012 21:30:33 »
now before leaving you with this question i'd love to correct myself because we do in a way use magnetism to fly, we have some sattelites going for the outer edge of our solar system by slingshotting themselves using the planets to do so.

Slingshotting is a gravitational effect.  A planet in orbit around the sun pulls on a satellite, and that satellite gets dragged along in orbit with it, entirely due to gravity.  If the trajectory is chosen appropriately, the satellite gets a boost in velocity because of this extra pull along with the planet's orbit.

#### LetoII

• Full Member
• Posts: 64
##### Re: gravity, electromagnetism and flying
« Reply #3 on: 04/09/2012 22:45:52 »
i would think so too (that it's purely gravitational) however i remember the scientist in the show saying that wasn't the case and i'd hate to argue about that here since it's not really along the direction of my main question.

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Re: gravity, electromagnetism and flying
« Reply #4 on: 05/09/2012 10:51:43 »
Clifford.  Magnetism is a bipolar field always coming with two poles (called north and south) separated by a short distance.  It only follows an inverse square law close to a single pole when one pole is a long way from the other.  When you are further away from the two poles than their separation the field falls off as the inverse cube of the distance.

This is because only the field difference between the two poles creates the magnetic attractive or repulsive force.

This makes magnetic fields only suitable for short range effects in normal circumstances.

There are some exceptions to this for example where whole planets stars or even galaxies have magnetic fields but mostly these are quite weak. again with a few exceptional cases.  Creating even a weak field on a planetary scale requires vast quantities of energy.

#### stu

• Jr. Member
• Posts: 23
##### Re: gravity, electromagnetism and flying
« Reply #5 on: 05/09/2012 19:27:21 »
Yep inverse square law at work. Grav constant and relative distances between relative masses

#### The Naked Scientists Forum

##### Re: gravity, electromagnetism and flying
« Reply #5 on: 05/09/2012 19:27:21 »