How does gravity work?

[TwigtheWonderkid]

Steve Campbell said:

but there is a reason our leg muscles are as big as they are.....as they are constantly fighting that 1g to even stay upright, you just don't notice.

Plus we can't fly. Animals with legs that can't fly are generally adapted to run, whereas things that can't fly don't need to run, and have lightweight puny legs.

[XM5ER]

Gary C said:

Did not the op really ask why does gravity feel so weak, but makes going up hill so hard.

I think we should ask about aeroplanes and conveyor belts and then retreat to a safe distance.

[Thorodin]

If gravity is such a weak force, why doesn't centrifugal force fling us off? Any point on the Earth's surface is whizzing round at an astronomical rate. Or are both forces mysteriously as strong as each other?

[Dr Jekyll]

Thorodin said:

If gravity is such a weak force, why doesn't centrifugal force fling us off? Any point on the Earth's surface is whizzing round at an astronomical rate. Or are both forces mysteriously as strong as each other?

Because the Earth is massive, so generates enough gravity to more than compensate for centrifugal 'force'.

[Goldenballs13]

Thorodin said:

If gravity is such a weak force, why doesn't centrifugal force fling us off? Any point on the Earth's surface is whizzing round at an astronomical rate. Or are both forces mysteriously as strong as each other?

Dr Jekyll said:

Because the Earth is massive, so generates enough gravity to more than compensate for centrifugal 'force'.

The centripetal acceleration is pretty much negligible compared to gravity. Looking at basic info of the speed and radius of the equator, gravity only needs to overcome acceleration of ~0.03 m/s2

[pingu393]

Whoozit said:

When you're on the flat, the only resistance you have to overcome is the rolling resistance of your bikes (pedal and wheel bearings, tyres on the road). The moment you go uphill, you are also lifting a portion of your bodyweight and that of the bike, that portion depending on how steep the hill is.

Nailed it. This is an excellent explanation of why cycling uphill is harder.


Gravity is the attraction of two masses.

I remember an experiment at school where two large spheres of mercury were used to attract other masses, resisted only by the torsion of the very thin wire.


Google has found this - The Cavendish Experiment (it's an excellent video and a good reminder to this ex-A-level student )

https://www.youtube.com/watch?v=jkjqrlYOW_0

[Thorodin]

Goldenballs13 said:

Thorodin said:

If gravity is such a weak force, why doesn't centrifugal force fling us off? Any point on the Earth's surface is whizzing round at an astronomical rate. Or are both forces mysteriously as strong as each other?

Dr Jekyll said:

Because the Earth is massive, so generates enough gravity to more than compensate for centrifugal 'force'.

The centripetal acceleration is pretty much negligible compared to gravity. Looking at basic info of the speed and radius of the equator, gravity only needs to overcome acceleration of ~0.03 m/s2

Thanks both. Fascinating.

[julian64]

pingu393 said:

Nailed it. This is an excellent explanation of why cycling uphill is harder.


Gravity is the attraction of two masses.

I remember an experiment at school where two large spheres of mercury were used to attract other masses, resisted only by the torsion of the very thin wire.


Google has found this - The Cavendish Experiment (it's an excellent video and a good reminder to this ex-A-level student )

https://www.youtube.com/watch?v=jkjqrlYOW_0

Can someone explain to me why this isn't dependant on when the periodicity measurement is taken. Surely given enough time the periodicity will melt away to nothing.

[pingu393]

julian64 said:

pingu393 said:

Nailed it. This is an excellent explanation of why cycling uphill is harder.


Gravity is the attraction of two masses.

I remember an experiment at school where two large spheres of mercury were used to attract other masses, resisted only by the torsion of the very thin wire.


Google has found this - The Cavendish Experiment (it's an excellent video and a good reminder to this ex-A-level student )

https://www.youtube.com/watch?v=jkjqrlYOW_0

Can someone explain to me why this isn't dependant on when the periodicity measurement is taken. Surely given enough time the periodicity will melt away to nothing.

I was surprised to see periodicity in the test. When I did it in school, the bar was stationary, but permanently twisted on the wire due to the mass-attraction.

When you removed the large masses, the hanging bar swung back to its original position.

Mercury was used (different times ) when we did it as there was absolutely no magnetic attraction between the masses. If you use steel (of any kind), there could be a magnetic influence in the experiment.

[Hoofy]

https://www.futurelearn.com/courses/gravity/3

[pingu393]

Gravity is classed as a weak force because macro-sized items do not attract.

Magnetism is a strong force because macro-sized items do attract.



Can you imagine two buses parked one centimetre apart on a frictionless surface? Would they be attracted to each other through gravity? Yes, but so slowly and weakly that you wouldn't see the effects for many millenia.

Now imagine the same two buses made of super-magnetic material (An electro-magnet in a scrapyard can easily lift a car). Would they be attracted through magnetism? Yes, but the force would be so strong that you would need a high-speed camera to measure the speed of attraction.

[TwigtheWonderkid]

pingu393 said:

Can you imagine two buses parked one centimetre apart on a frictionless surface? Would they be attracted to each other through gravity? Yes, but so slowly and weakly that you wouldn't see the effects for many millenia.

Now imagine the same two buses made of super-magnetic material (An electro-magnet in a scrapyard can easily lift a car). Would they be attracted through magnetism? Yes, but the force would be so strong that you would need a high-speed camera to measure the speed of attraction.

If gravity didn't exist and the sun and Pluto were both magnets, would they move towards each other, given the initial distance between them (3.67 billion miles)? If not, then isn't gravity stronger than magnetism, given the hold the sun exerts on Pluto?

[pingu393]

TwigtheWonderkid said:

If gravity didn't exist and the sun and Pluto were both magnets, would they move towards each other, given the initial distance between them (3.67 billion miles)? If not, then isn't gravity stronger than magnetism, given the hold the sun exerts on Pluto?

Good question. I suspect not, as there are many nickel and ferrous objects in the universe.

It looks like there could be a range limit to magnetism's strength, whereas gravity is weak, but over a longer range.

I'm thinking along the lines of a normal curve. Magnetism would be high on the y-axis and narrow on the x-axis (mu=0, sigma=6 for example). Gravity would be more like mu=0, sigma=100. http://faculty.frostburg.edu/math/monline/stat/51_...

Does that make sense?

[deckster]

TwigtheWonderkid said:

If gravity didn't exist and the sun and Pluto were both magnets, would they move towards each other, given the initial distance between them (3.67 billion miles)? If not, then isn't gravity stronger than magnetism, given the hold the sun exerts on Pluto?

Yes they would attract, but not very strongly compared to gravity at that range. We believe that electromagnetic fields, like gravitational fields, have potentially infinite range however the force exerted by electromagnetism follows an inverse-cube curve, whereas gravity falls off as an inverse square - so although electromagnetism is much the stronger of the two forces at small distances, gravity quickly comes to dominate as the separation between objects increases.

[Halmyre]

A quick and dirty (and probably horribly inaccurate) calculation based around the formula in here https://en.wikipedia.org/wiki/Force_between_magnet... suggests that the earth and the sun experience an attractive force of 68 micronewtons. The gravitational force is about 10^22 newtons.

[TwigtheWonderkid]

OK, thanks for those answers. But he's something else I don't get. How come everything stays in the same orbit for millions of years when the forces of gravity must alter so much as the planets orbit at differing rates. Eg. Jupiter and Saturn are neighbours and are both huge. Not huge on the sun's scale but huge compared to Pluto. There must have been a point in the billions of years of the planets orbiting that Jupiter and Saturn aligned with Pluto. So the combined gravity of those 2 giants would be pulling on Pluto. Now I know the sun is much bigger, but it's also much further away. So why wasn't Pluto pulled out of it's orbit?

[Eric Mc]

In the early years of our solar system's formation there were far more objects orbiting in irregular orbits and passing closer to each other more frequently. This resulted in a very unstable situation with orbits being changed by gravitational attraction and collisions occurring fairly regularly.

However, after a few tens or even hundreds of millions of years of this, the system settled down to a more stable state mainly because of this sweeping up process. That is where our solar system is now.

What you are seeing is the end result of a sort of gravitational Darwinism where most of the objects that were going to be pulled out of their orbits and crashed into the sun, or each other, or expelled from the solar system - have now largely been sorted.

There are still lots of irregular orbiting objects in the solar system - chiefly comets and asteroids, but all the large objects as far as we know are in fairly regular or stable orbits.

By coincidence, I've literally heard in the last minute that "In Our Time" on Radio 4 will be discussing the Kuiper Belt just after 9.00 am this morning.

[deckster]

TwigtheWonderkid said:

OK, thanks for those answers. But he's something else I don't get. How come everything stays in the same orbit for millions of years when the forces of gravity must alter so much as the planets orbit at differing rates. Eg. Jupiter and Saturn are neighbours and are both huge. Not huge on the sun's scale but huge compared to Pluto. There must have been a point in the billions of years of the planets orbiting that Jupiter and Saturn aligned with Pluto. So the combined gravity of those 2 giants would be pulling on Pluto. Now I know the sun is much bigger, but it's also much further away. So why wasn't Pluto pulled out of it's orbit?

The simple answer is that a lot of early proto-planets were indeed pulled out of orbit, and became moons - or were consumed by, or coalesced with larger planets. The early solar system was a pretty violent place by all accounts.

And to your specific question - Pluto is about 5x as far from the sun as Jupiter is; or, to put it another way, if you set off from Pluto to the sun, then you won't pass Jupiter until you're 80% of the way there. So fundamentally the sun is not 'much further away' from Pluto than Jupiter is; the distance is, to a degree of approximation, pretty much the same. As the sun is about 1000x the mass of Jupiter, the small difference in separation is vastly outweighed (geddit) by the difference in mass.

[Hugo a Gogo]

all the planets do affect each other's orbits slightly though, and scientists can use this as one way of working out relative masses of the planets

orbits are not static perfect ellipses, but the time scales of changes are huge

[Gary C]

Eric Mc said:

By coincidence, I've literally heard in the last minute that "In Our Time" on Radio 4 will be discussing the Kuiper Belt just after 9.00 am this morning.

That's a shame.

I can't listen to that condescending, smarmy knob for more than 5 minutes.