How does gravity work?
Discussion
Boring_Chris said:
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.
There's a story that some senior people in the US military didn't believe special relatively so the first GPS satellites had a switch to enable or disable the time dilation calculation.Newc said:
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".
But when you walk on the flat - you aren't working against gravity. Your potential energy walking on the flat remains the same - whereas when you climb a hill, you are increasing your potential energy.
Think about the amount of damage a hammer can do if you drop it from a distance of 1mm above the ground. Now think about the amount of damage if you drop it from a height of 10m.
The difference in the amount of damage caused is down to the amount of potential energy you have given the hammer by raising it's height by 10m.
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.
That's pretty much it, but it amazes me at a much closer level than Pluto.I understand the whole notion of potential energy, conservation of energy and so on. I suppose what I struggle with is the difference in the power of gravity on a body at rest relative to its pull (me riding along on the flat) and a body fighting it (me going up the hill).
Look at any given mountain stage of the Tour de France. If I managed to make it up the climb at all (which is extremely unlikely), I'd be half dead by the time I reached the top, thanks to gravity. If I tried to ride back down without using my brakes, I'd be fully dead by about the second bend, again thanks to gravity, yet when I just stand still, whether at the top or the bottom of said mountain, I'd barely notice the effects of said gravity, and whilst a TdF climb might feel like the biggest thing imaginable to ride, it's actually a minute fraction of the distance to the centre of the earth.
alock said:
Boring_Chris said:
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.
There's a story that some senior people in the US military didn't believe special relatively so the first GPS satellites had a switch to enable or disable the time dilation calculation.Kermit power said:
I'd barely notice the effects of said gravity
It's all about work done.When you are standing still you are burning, say, 1 calorie per minute = 4.184kJ.
When you are climbing, to overcome gravity alone (ignoring resistances) assume you weigh 75kg and ascend 10m per minute = 75 * 10 * 9.8 = 7.35kJ s-1 = 441kJ per minute. Also the total powertrain (from your muscles to the rear derailleurs and tyres) might only be 25% efficient so you will be burning 4 times the energy! The difference is night and day.
Moonhawk said:
But when you walk on the flat - you aren't working against gravity.
Why aren't you working against the gravity of the moon? The moon's gravity pulls the tides towards it, surely it must have a lesser effect on me. The Earth is pulling us down towards the centre, but the moon, the sun, Jupiter, Saturn, the plane flying overhead, are all pulling us (with far less force) in different directions. Edited by TwigtheWonderkid on Tuesday 21st February 11:55
A good analogy (albeit one that is almost entirely wrong...) is to imagine a large rubber sheet.
You get lots of people around the edges to pull it taut, you then put various weights on the sheet.
At each point of weight the sheet will drop down and form funnels. The bigger the weight the longer the funnel and the steeper the sides. The funnel shape is obviously curved from the top inwards.
The sheet represents space time and the weights represent mass.
Gravity is the effect of falling down the side of the walls- unless its really massive (ie a black hole) then getiing out of the gravity well isn't impossible, and you can still do things within the well (ie lift up a cup of coffee). OF course the steeper the sides the harder it is to get out, but the closer you are to the top of the well the easier it is (as its less steep).
Its really not a perfect analogy of how the universe works, but it does help people visualise how gravity works.,
You get lots of people around the edges to pull it taut, you then put various weights on the sheet.
At each point of weight the sheet will drop down and form funnels. The bigger the weight the longer the funnel and the steeper the sides. The funnel shape is obviously curved from the top inwards.
The sheet represents space time and the weights represent mass.
Gravity is the effect of falling down the side of the walls- unless its really massive (ie a black hole) then getiing out of the gravity well isn't impossible, and you can still do things within the well (ie lift up a cup of coffee). OF course the steeper the sides the harder it is to get out, but the closer you are to the top of the well the easier it is (as its less steep).
Its really not a perfect analogy of how the universe works, but it does help people visualise how gravity works.,
RTB said:
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? 
PugwasHDJ80 said:
Gravity is the effect of falling down the side of the walls- unless its really massive (ie a black hole) then getiing out of the gravity well isn't impossible, and you can still do things within the well (ie lift up a cup of coffee). OF course the steeper the sides the harder it is to get out, but the closer you are to the top of the well the easier it is (as its less steep).
But it's hard to get out of the hole BECAUSE of gravity.Kermit power said:
That's pretty much it, but it amazes me at a much closer level than Pluto.
I understand the whole notion of potential energy, conservation of energy and so on. I suppose what I struggle with is the difference in the power of gravity on a body at rest relative to its pull (me riding along on the flat) and a body fighting it (me going up the hill).
Look at any given mountain stage of the Tour de France. If I managed to make it up the climb at all (which is extremely unlikely), I'd be half dead by the time I reached the top, thanks to gravity. If I tried to ride back down without using my brakes, I'd be fully dead by about the second bend, again thanks to gravity, yet when I just stand still, whether at the top or the bottom of said mountain, I'd barely notice the effects of said gravity, and whilst a TdF climb might feel like the biggest thing imaginable to ride, it's actually a minute fraction of the distance to the centre of the earth.
Sounds like you're talking about the Force due to gravity - F = (G/r^2)(m1*m2), you are able to lift yourself up the hill/lift up a mug etc, as m2 (you, the mug) is very very small.I understand the whole notion of potential energy, conservation of energy and so on. I suppose what I struggle with is the difference in the power of gravity on a body at rest relative to its pull (me riding along on the flat) and a body fighting it (me going up the hill).
Look at any given mountain stage of the Tour de France. If I managed to make it up the climb at all (which is extremely unlikely), I'd be half dead by the time I reached the top, thanks to gravity. If I tried to ride back down without using my brakes, I'd be fully dead by about the second bend, again thanks to gravity, yet when I just stand still, whether at the top or the bottom of said mountain, I'd barely notice the effects of said gravity, and whilst a TdF climb might feel like the biggest thing imaginable to ride, it's actually a minute fraction of the distance to the centre of the earth.
Gravity holds pluto, for example, in orbit as despite 'r' (the distance between the two) increasing, m2 is huge.
OP, the change in gravitational field strength between the bottom and top of the hill is a red herring. There's a fair amount of confusion and correct but irrelevant stuff on this thread.
Here's the key difference between cycling on the flat and up a slope:
Cycling up a slope at a constant speed is equivalent to ACCELERATING on the flat.
Here's the key difference between cycling on the flat and up a slope:
Cycling up a slope at a constant speed is equivalent to ACCELERATING on the flat.
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