How does gravity work?

How does gravity work?

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Kermit power

Original Poster:

28,635 posts

212 months

Tuesday 21st February 2017
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I was pondering on this the other day whilst out cycling...

I understand the notion of gravity pulling us towards the centre of the earth, but what I don't understand is why there is such massive resistance to change, but we're not being squashed flat on the floor?

On a bike, I'd be able to keep going on a flat surface with no headwind relatively indefinitely, but as soon as I start to climb, it gets very hard, very rapidly, to the point where you simply can't turn the pedals any more.

Admittedly, the slope looks steep to us, but even if I rode from sea level to the top of Everest, I'd be less than 0.15% further away from the centre of the earth, which strikes me as essentially nothing. I remember reading that if the earth were reduced to the size of a billiard ball, its surface would be smoother, so how can such a small relative change in altitude cause us so much effort?

Eric Mc

121,788 posts

264 months

Tuesday 21st February 2017
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Gravity gets weaker the further you move away from the centre of the mass creating the gravity. As you approach the top of the hill, the force pulling you back is actually weaker than it was at the bottom of the hill.

The fact that you are knackered nearer the top is nothing to do with the gravity getting stronger or weaker. It's because you are unfit smile

And I know someone well step in and say that gravity isn't a "force" as such but the effect has on "space time" - but sometimes the older Newtonian explanation is good enough when the question is straightforward.

Whoozit

3,584 posts

268 months

Tuesday 21st February 2017
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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.

227bhp

10,203 posts

127 months

Tuesday 21st February 2017
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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.
+ Wind resistance, aerodynamic drag etc.

Dr Jekyll

23,820 posts

260 months

Tuesday 21st February 2017
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Kermit power said:
I
Admittedly, the slope looks steep to us, but even if I rode from sea level to the top of Everest, I'd be less than 0.15% further away from the centre of the earth, which strikes me as essentially nothing.
That's not the issue though. If the earth were bigger so that the same height climb took you only 0.015% further away from the centre it would be no easier because earths gravity would be much stronger (though on the plus side you would start further away from the centre). Might actually be harder, not sure.

so called

9,074 posts

208 months

Tuesday 21st February 2017
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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.
And, gravity is trying to pull you back down the hill and so constantly working to cancel out your momentum.

Hugo a Gogo

23,378 posts

232 months

Tuesday 21st February 2017
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Kermit power said:
I understand the notion of gravity pulling us towards the centre of the earth, but what I don't understand is why there is such massive resistance to change, but we're not being squashed flat on the floor?
there is a resistance to change (inertia) but gravity really wants you to change position

take a step off the edge of your roof to see how fast you will change position

Alex_225

6,234 posts

200 months

Tuesday 21st February 2017
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My understanding is that every mass has a gravitational pull, the difference in pull depends on the size of the object.

So life on Earth has learned to evolve within the pull of the Earths gravity, of which I'm sure there is a measurement. If you were able to stand on Jupiter which is considerably larger than Earth, the gravitational pull would be a lot more significant.

So in terms of climbing Everest, it's such a relatively tiny distance that there's almost no difference from the bottom to the top, well not enough for us to notice I'm sure.

If you consider the Earth's moon is kept in orbit by it's gravitational pull and that's about 240,000 miles away it goes to show its pretty strong!

Einion Yrth

19,575 posts

243 months

Tuesday 21st February 2017
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When climbing a hill you are converting the chemical energy in your food, via your muscles, into potential energy; quite a lot of it. If you climb a tall building you will convert enough energy to make a pretty sizeable "splat" if you step off the top.

Eric Mc

121,788 posts

264 months

Tuesday 21st February 2017
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How much "gravity" you feel is down to two factors - mass and distance.

A more massive planet will exert a stronger gravitational force than a less massive one. However, if the planet has a surface (not all do), then gravitational force that a person feels when standing on the surface is not just down to the mass of the planet but also down to the distance the surface of the planet is from its centre.

A person standing on a planet twice the mass of earth might actually feel a gravitational force on that planet's surface not much different to that of earth - or perhaps even less - depending on the diameter of the planet and its overall density.

An example of this is our own moon. It's mass is 81 times less than that of earth, but the gravity felt at its surface is only 1/6th that of earth's.

Newc

1,846 posts

181 months

Tuesday 21st February 2017
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I think the OPs point is more "there's almost no difference in gravity effect between sea level and mountain top, so the amount of effort required to change height compared to walking on the flat seems disproportionately high".

To which I say that working against earth gravity requires a bucketful of effort (see: rockets to orbit), but you've already done all that work when you are walking on the level.

fezst

234 posts

123 months

Tuesday 21st February 2017
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Basic physics really... If you're travelling along a flat surface, the only resistance (mainly) you have is wind resistance and friction on the road surface.
When you start travelling uphill then your own mass x gravity comes into play.

Example if you weigh 80kg and you go up a hill that is 10 degrees steep.

80 * 9.81 * sin 10 = 136.27N

Immediately you have an extra 136 Newtons working against you as you climb the hill.

julian64

14,317 posts

253 months

Tuesday 21st February 2017
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Wow this thread is getting in a tiz with all sorts of funny answers.

The op is complaining about the amount of effort going up hill because he is attaining a new state of potential energy.

Potential energy = mass x gravity x height ( + the work required to travel the horizontal distance in normal flat pedalling)

He gets the energy via his pedals. So the work isn't all because of gravity. Its to do with the problem of him being a fatty, and the height of the hill which are the other two equation variables. I suggest less pies or pick a smaller hill.

TwigtheWonderkid

43,248 posts

149 months

Tuesday 21st February 2017
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I get where the op is coming from. It seems odd that gravity is strong enough for the sun to hold Pluto in an orbit, yet at the same time weak enough for me to be able to pick up my mug of tea.

Eric Mc

121,788 posts

264 months

Tuesday 21st February 2017
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It's amazing, isn't it.

RTB

8,273 posts

257 months

Tuesday 21st February 2017
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TwigtheWonderkid said:
I get where the op is coming from. It seems odd that gravity is strong enough for the sun to hold Pluto in an orbit, yet at the same time weak enough for me to be able to pick up my mug of tea.
What if your mug of tea was the size of pluto though?

SteveO...

465 posts

224 months

Tuesday 21st February 2017
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Gravity is much weaker than the other fundamental forces.

The nuclear forces are vastly stronger, but act over very small distances (atomic nuclei).

The electromagnetic force is an inverse square law, like gravity, but because it's polarized (+ and - charges exist) most large things are roughly charge neutral and it doesn't have a great bearing on the macro scale.

Gravity is "weak", but isn't polarized (no anti gravity), however it's effect decreases *only* with the square of the distance. It's the dominant force of the large scale structure of matter.

You're right, you can resist the entire gravity of a planet by holding up a cup of tea. However, try holding it up for an hour. Gravity is weak, but it is relentless and you can't avoid it.

All this without getting into Einstein & General Relativity and the fact that gravity isn't really a thing, it's a side effect!

Edited by SteveO... on Tuesday 21st February 10:49

Boring_Chris

2,348 posts

121 months

Tuesday 21st February 2017
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Went on a little science binge when looking up at the Moon and getting all confused last year... going in absolutely clueless, this stuff just knocked me for six!

From what I understood, gravity is a consequence of mass warping both time and space. 'Gravity' is a 'weak force' but is absolutely unstoppable... The greater the mass, the greater the pull. I seem to remember reading that even if you were impervious to heat, should you ever visit, you'd be spaghettified - or at least partially torn to bits - by the gravitational pull of the sun... ?

Anyway, the one fact that blew my mind was that GPS satellites have to adjust their clocks to account for time dilation... i.e. the fact that time runs slower down here on Earth, than in does in orbit, which is a practical demonstration of Einstein's theories. After that, everything clicked into place... the oceans, the atmosphere etc etc. And gravity isn't uniform - it varies depending on the geography of the Earth (or whatever the object in question)

But then of course you've got the rock stars of space to explain... Stars and their interstellar tantrums, as they die to form neutron stars or black holes. All under the influence of 'gravity'.

(apologies if I'm stating the obvious on a Science forum, but this stuff has really grabbed me recently and I don't get a chance to talk about any of it because nobody I know seems to care! Ha!)


Boring_Chris

2,348 posts

121 months

Tuesday 21st February 2017
quotequote all
julian64 said:
I suggest less pies or pick a smaller hill.
I want this in an equation.

Eric Mc

121,788 posts

264 months

Tuesday 21st February 2017
quotequote all
Boring_Chris said:
Went on a little science binge when looking up at the Moon and getting all confused last year... going in absolutely clueless, this stuff just knocked me for six!

From what I understood, gravity is a consequence of mass warping both time and space. 'Gravity' is a 'weak force' but is absolutely unstoppable... The greater the mass, the greater the pull. I seem to remember reading that even if you were impervious to heat, should you ever visit, you'd be spaghettified - or at least partially torn to bits - by the gravitational pull of the sun... ?

Anyway, the one fact that blew my mind was that GPS satellites have to adjust their clocks to account for time dilation... i.e. the fact that time runs slower down here on Earth, than in does in orbit, which is a practical demonstration of Einstein's theories. After that, everything clicked into place... the oceans, the atmosphere etc etc. And gravity isn't uniform - it varies depending on the geography of the Earth (or whatever the object in question)

But then of course you've got the rock stars of space to explain... Stars and their interstellar tantrums, as they die to form neutron stars or black holes. All under the influence of 'gravity'.

(apologies if I'm stating the obvious on a Science forum, but this stuff has really grabbed me recently and I don't get a chance to talk about any of it because nobody I know seems to care! Ha!)

This is the place for uber geekness. It's good here.