Why does Gravity appear to have different effects at different scales? Why can't we observe the gravitational fields of objects on Earth?

[UTOPIAHELL]

Hey friends. I hope someone can help me here because I'm starting to feel like I may be going crazy, and I hope someone can set me straight. My question comes in two parts:

1. Why does gravity often seem to have contradictory effects at different scales?

The Earth is spinning at an extremely fast speed, while also moving through space at an extremely fast speed. This force called Gravity keeps me adhered to the Earth's surface, so it must be quite strong. I can easily imagine how strong a force would have to be to keep a tiny object adhered to the surface of a basketball while it was both spinning and moving. So I can empirically deduce that the pull of gravity must be extremely strong.

However, this extremely strong force is also what keeps the moon orbiting the Earth, and the planets of our solar system orbiting the sun. The moon orbits the Earth at a relatively constant distance, and the planets orbit the sun at a relatively constant distance. I understand that the distances vary slightly as the orbits are actually elliptical, but my point is that the planets are not pulled towards the sun, and the moon is not pulled towards the Earth. Man has observed Mercury, for example, orbiting the sun for millenia, and it has apparently failed to be pulled into the sun by the force of the Sun's massive gravitational field, which paradoxically is strong enough to act on a planet such as Neptune, which is orbiting the sun at a distance that is orders of magnitude larger than Mercury's distance from the Sun.

So gravity simultaneously pulls me towards the Earth with such immense force that I adhere to it despite the massive motion of the Earth, yet also keeps the planets and moon orbiting at a relatively constant distance, failing to pull them the same way I am apparently pulled towards the Earth. What gives? How can gravity apparently pull sometimes, yet paradoxically also be the force that allows planets and moons to obtain stable orbits?

2. The Earth is the only object that I can empirically verify as possessing Gravity.

In my physics classes, when learning about Gravity, someone of course asked the obvious question of, "Why don't mountains have gravity?" or why we fail to observe gravitational fields forming around sufficiently large masses on Earth. The answer I received for this is that the gravity of the Earth is so extreme that it dwarfs the force of gravity of these smaller objects and renders them negligible.

This seems to be in opposition to the apparent fact that perpendicular forces are supposed to not effect each other in this universe. For example, you are most likely familiar with the thought experiment of a car moving forward on a frictionless surface at 100 mph. This car is then struck from the side at a perfect 90 degree angle to it's current path by another object. I have always learned that on this frictionless plane, the car would still be moving forward at 100 mph, because the two perpendicular forces do not affect each other - it would just now also have additional momentum pushing it along the horizontal axis.

If this is the case, then sufficiently large masses on Earth should have demonstrable gravitational fields. The gravity of the Earth is pulling down (we'll call this the Y axis), but a sufficiently large mass, such as a massive lead cube, would, if you were holding an object next to it, be exerting it's own gravity horizontally (on the x axis). The vectors of these two forces are perpendicular, and thus should not cancel each other out.

To give a basic example, if you were throwing a football past a 200 x 200 ft. lead cube, the gravity of the Earth would be pulling the football down, but the gravity of the cube should also be able to pull the football towards it, to the left or to the right depending on where you are. This obviously does not happen. On a more practical level, it seems like objects of normal mass should be able to attract sufficiently small masses, as long as the ratio of their masses is similar to the ratio of my mass to the Earth's. This also does not happen.

If perpendicular forces cannot cancel each other out, why can't we observe the gravitational force of masses on Earth?

I sincerely look forward to any input someone can give me on these questions, as they have been vexing me for some time and despite a decent (albeit elementary) education in basic physics, I really cannot explain this.

Thanks!

Edited May 7, 2016 by UTOPIAHELL

[Moontanman]

Hey friends. I hope someone can help me here because I'm starting to feel like I may be going crazy, and I hope someone can set me straight. My question comes in two parts:

 

1. Why does gravity often seem to have contradictory effects at different scales?

 

The Earth is spinning at an extremely fast speed, while also moving through space at an extremely fast speed. This force called Gravity keeps me adhered to the Earth's surface, so it must be quite strong. I can easily imagine how strong a force would have to be to keep a tiny object adhered to the surface of a basketball while it was both spinning and moving. So I can empirically deduce that the pull of gravity must be extremely strong.

 

However, this extremely strong force is also what keeps the moon orbiting the Earth, and the planets of our solar system orbiting the sun. The moon orbits the Earth at a relatively constant distance, and the planets orbit the sun at a relatively constant distance. I understand that the distances vary slightly as the orbits are actually elliptical, but my point is that the planets are not pulled towards the sun, and the moon is not pulled towards the Earth. Man has observed Mercury, for example, orbiting the sun for millenia, and it has apparently failed to be pulled into the sun by the force of the Sun's massive gravitational field, which paradoxically is strong enough to act on a planet such as Neptune, which is orbiting the sun at a distance that is orders of magnitude larger than Mercury's distance from the Sun.

 

So gravity simultaneously pulls me towards the Earth with such immense force that I adhere to it despite the massive motion of the Earth, yet also keeps the planets and moon orbiting at a relatively constant distance, failing to pull them the same way I am apparently pulled towards the Earth. What gives? How can gravity apparently pull sometimes, yet paradoxically also be the force that allows planets and moons to obtain stable orbits?

 

2. The Earth is the only object that I can empirically verify as possessing Gravity.

 

In my physics classes, when learning about Gravity, someone of course asked the obvious question of, "Why don't mountains have gravity?" or why we fail to observe gravitational fields forming around sufficiently large masses on Earth. The answer I received for this is that the gravity of the Earth is so extreme that it dwarfs the force of gravity of these smaller objects and renders them negligible.

 

This seems to be in opposition to the apparent fact that perpendicular forces are supposed to not effect each other in this universe. For example, you are most likely familiar with the thought experiment of a car moving forward on a frictionless surface at 100 mph. This car is then struck from the side at a perfect 90 degree angle to it's current path by another object. I have always learned that on this frictionless plane, the car would still be moving forward at 100 mph, because the two perpendicular forces do not affect each other - it would just now also have additional momentum pushing it along the horizontal axis.

 

If this is the case, then sufficiently large masses on Earth should have demonstrable gravitational fields. The gravity of the Earth is pulling down (we'll call this the Y axis), but a sufficiently large mass, such as a massive lead cube, would, if you were holding an object next to it, be exerting it's own gravity horizontally (on the x axis). The vectors of these two forces are perpendicular, and thus should not cancel each other out.

 

To give a basic example, if you were throwing a football past a 200 x 200 ft. lead cube, the gravity of the Earth would be pulling the football down, but the gravity of the cube should also be able to pull the football towards it, to the left or to the right depending on where you are. This obviously does not happen. On a more practical level, it seems like objects of normal mass should be able to attract sufficiently small masses, as long as the ratio of their masses is similar to the ratio of my mass to the Earth's. This also does not happen.

 

If perpendicular forces cannot cancel each other out, why can't we observe the gravitational force of masses on Earth?

 

I sincerely look forward to any input someone can give me on these questions, as they have been vexing me for some time and despite a decent (albeit elementary) education in basic physics, I really cannot explain this.

 

Thanks!

 

 

The gravitational pull of a human can be measured, I'm not sure where you are coming from on this, the force of gravity falls off by the square of the distance and orbital speed is what keeps things like planets or moons from falling into the earth or sun. Oh and gravity is the weakest of the four forces, not the strongest...

Edited May 7, 2016 by Moontanman

[pzkpfw]

1. Gravity is always "pulling", orbits are about speed: https://en.wikipedia.org/wiki/Orbit

 

In essence the thing orbiting (whether a Planet around the Sun, or a satellite around Earth) is falling, but it's got enough sideways speed to keep missing.

 

If you were placed above Earth, "stationary" (e.g. flung at high speed out the back of an orbiting space station), you'd fall towards Earth and burn up on re-entry to the atmosphere. If you were just "let go" from that orbiting space station, you'd continue to orbit Earth. The strength of the effect of Earths gravity on you is the same in both cases, the difference between falling and orbiting is your "sideways" speed.

 

2. Mountains do have gravity. The pull of a nearby mountain can be measured.

 

Check this out: https://en.wikipedia.org/wiki/Schiehallion_experiment

 

Bear in mind that Earth is smoother than a billiard ball ( http://blogs.discovermagazine.com/badastronomy/2008/09/08/ten-things-you-dont-know-about-the-earth/#.Vy5olWZf2Uk ). Mountains seem big, but you won't see any noticeable effect like walking leaned-over when near one.

 

Gravity has been seen to have effect other than on Earth: by twelve Men who have walked on the moon.

 

(And by unmanned landers on the Moon and Mars etc, and the fact that many of the other Planets have their own Moons ...).

Edited May 7, 2016 by pzkpfw

[swansont]

Hey friends. I hope someone can help me here because I'm starting to feel like I may be going crazy, and I hope someone can set me straight. My question comes in two parts:

 

1. Why does gravity often seem to have contradictory effects at different scales?

 

The Earth is spinning at an extremely fast speed, while also moving through space at an extremely fast speed. This force called Gravity keeps me adhered to the Earth's surface, so it must be quite strong. I can easily imagine how strong a force would have to be to keep a tiny object adhered to the surface of a basketball while it was both spinning and moving. So I can empirically deduce that the pull of gravity must be extremely strong.

 

However, this extremely strong force is also what keeps the moon orbiting the Earth, and the planets of our solar system orbiting the sun. The moon orbits the Earth at a relatively constant distance, and the planets orbit the sun at a relatively constant distance. I understand that the distances vary slightly as the orbits are actually elliptical, but my point is that the planets are not pulled towards the sun, and the moon is not pulled towards the Earth. Man has observed Mercury, for example, orbiting the sun for millenia, and it has apparently failed to be pulled into the sun by the force of the Sun's massive gravitational field, which paradoxically is strong enough to act on a planet such as Neptune, which is orbiting the sun at a distance that is orders of magnitude larger than Mercury's distance from the Sun.

 

So gravity simultaneously pulls me towards the Earth with such immense force that I adhere to it despite the massive motion of the Earth, yet also keeps the planets and moon orbiting at a relatively constant distance, failing to pull them the same way I am apparently pulled towards the Earth. What gives? How can gravity apparently pull sometimes, yet paradoxically also be the force that allows planets and moons to obtain stable orbits?

 

The earth isn't spinning all that fast. Once a day.

 

Gravity also isn't that strong. It might seem that way, but the reason we notice it is that it's the only long-range force that can't be screened. But you generally don't fall through your chair or the floor.

 

As for orbits, you need a force for something to move in a circle. Absent any force, the motion would be in a straight line. But given the right speed, you can continually deflect an object into what we call an orbit.

 

 

2. The Earth is the only object that I can empirically verify as possessing Gravity.

 

In my physics classes, when learning about Gravity, someone of course asked the obvious question of, "Why don't mountains have gravity?" or why we fail to observe gravitational fields forming around sufficiently large masses on Earth. The answer I received for this is that the gravity of the Earth is so extreme that it dwarfs the force of gravity of these smaller objects and renders them negligible.

 

This seems to be in opposition to the apparent fact that perpendicular forces are supposed to not effect each other in this universe. For example, you are most likely familiar with the thought experiment of a car moving forward on a frictionless surface at 100 mph. This car is then struck from the side at a perfect 90 degree angle to it's current path by another object. I have always learned that on this frictionless plane, the car would still be moving forward at 100 mph, because the two perpendicular forces do not affect each other - it would just now also have additional momentum pushing it along the horizontal axis.

 

If this is the case, then sufficiently large masses on Earth should have demonstrable gravitational fields. The gravity of the Earth is pulling down (we'll call this the Y axis), but a sufficiently large mass, such as a massive lead cube, would, if you were holding an object next to it, be exerting it's own gravity horizontally (on the x axis). The vectors of these two forces are perpendicular, and thus should not cancel each other out.

 

To give a basic example, if you were throwing a football past a 200 x 200 ft. lead cube, the gravity of the Earth would be pulling the football down, but the gravity of the cube should also be able to pull the football towards it, to the left or to the right depending on where you are. This obviously does not happen. On a more practical level, it seems like objects of normal mass should be able to attract sufficiently small masses, as long as the ratio of their masses is similar to the ratio of my mass to the Earth's. This also does not happen.

 

If perpendicular forces cannot cancel each other out, why can't we observe the gravitational force of masses on Earth?

 

I sincerely look forward to any input someone can give me on these questions, as they have been vexing me for some time and despite a decent (albeit elementary) education in basic physics, I really cannot explain this.

Cavendish measured the gravitational force between two objects, neither of which is the earth. But small masses exert small forces.

https://en.wikipedia.org/wiki/Cavendish_experiment

 

The mass of the earth is 6 x 10^24 kg. So masses of a few kg aren't going to compare; since gravity is a 1/r^2 force, you could actually make a numerical comparison. We're about 6400 km from the center of the earth. If something is a meter away, that's around 3.6 x 10^13 times more of an effect from the distance, but even then 1 kg will exert more than 10 orders of magnitude smaller force than that of the earth.

 

Large masses do have a measurable effect. Scientists have measured the gravitational field of the earth, and it is not a constant. This is one way of looking for e.g. natural gas deposits — you have less local mass (gas vs rock), so the local gravitational acceleration is reduced.

[MigL]

Both of your questions have the same answer.

 

As Swansont has mentioned, gravity isn't strong at all, in fact, it is extremely weak.

So weak, in fact that you can counteract the gravitational 'pull' of the whole planet on an apple with one hand.

[Janus]

Hey friends. I hope someone can help me here because I'm starting to feel like I may be going crazy, and I hope someone can set me straight. My question comes in two parts:

 

1. Why does gravity often seem to have contradictory effects at different scales?

 

The Earth is spinning at an extremely fast speed, while also moving through space at an extremely fast speed. This force called Gravity keeps me adhered to the Earth's surface, so it must be quite strong. I can easily imagine how strong a force would have to be to keep a tiny object adhered to the surface of a basketball while it was both spinning and moving. So I can empirically deduce that the pull of gravity must be extremely strong.

As pointed out, the spin of the earth is not that fast just. 7.29 rad/sec (0.0007 rpm). Rotating at that speed, the force needed to hold a 1 kg mass from flying off(at the equator) is just 0.034 newtons( 0.122 oz). The force of gravity acting on that same object is 9.8 newtons(2.2 lb) The fact that Earth is also moving through space has no effect on how much force it takes to hold an object to it surface. If we no consider your basketball spinning at the same speed, the force needed to hold a 1 milligram object to it surface works out to be 6.45e-16 newtons. The force of gravity of the basketball at its surface works out to 3e-15 newtons, or almost 5 times as great, so the basketball's own gravity would be more than enough to hold an object to its surface.

 

 

 

However, this extremely strong force is also what keeps the moon orbiting the Earth, and the planets of our solar system orbiting the sun. The moon orbits the Earth at a relatively constant distance, and the planets orbit the sun at a relatively constant distance. I understand that the distances vary slightly as the orbits are actually elliptical, but my point is that the planets are not pulled towards the sun, and the moon is not pulled towards the Earth. Man has observed Mercury, for example, orbiting the sun for millenia, and it has apparently failed to be pulled into the sun by the force of the Sun's massive gravitational field, which paradoxically is strong enough to act on a planet such as Neptune, which is orbiting the sun at a distance that is orders of magnitude larger than Mercury's distance from the Sun.

While the force of gravity acting on Mercury is some 6050 times that of Neptune(77.78 times further), Mercury also orbits the sun with an angular velocity that is 684 times that of Neptune. The force needed to hold an object in a circular path is proportional to the square of its angular velocity multiplied by the radius of the circle, and 684^2/77.78 = 6015. Which is in the range( allowing for rounding, etc.) of the number of times stronger that the Sun's gravity on Mercury is to that on Neptune. Of course this is a bit of an over-simplification, due to actual elliptical nature of orbits, but the upshot is that orbital speeds vary at different distances from the Sun in response to the change of gravity with distance.

 

So gravity simultaneously pulls me towards the Earth with such immense force that I adhere to it despite the massive motion of the Earth, yet also keeps the planets and moon orbiting at a relatively constant distance, failing to pull them the same way I am apparently pulled towards the Earth. What gives? How can gravity apparently pull sometimes, yet paradoxically also be the force that allows planets and moons to obtain stable orbits?

As I pointed out earlier, the force needed to hold you to the surface of the Earth, despite its spin is quite small compared to the force of gravity pulling on you. You may be moving at roughly 1000 mph at the equator, but you would have to be moving at 18,000 mph for your speed to cancel the force of gravity. For comparison, gravitational force acting on the Earth due to the Sun is 3.54e22 newtons (7.96e21 lb) This may seem like a lot but a object of the same mass sitting on the surface of the Earth would weigh 5.85e25 newtons (1.32e25 lbs) or 1652 time as much.( Or put it this way: If you able to "stand" at a point the same distance from the Sun as the Earth is, and you weighed 165 lbs(on the Earth), you would only weigh 1/10 of a lb.) The Earth takes one year to orbit the Sun, so it speed of revolution is 1/365 that of the Earth. But it is also 1.496 million km from the Sun. Again, if you work out the speed the Earth would have to travel around the Sun to cancel the pull of gravity from the Sun, it works out to be 1 orbit per year. The trick is to actually work out what the numbers say rather than relying on your intuition.

 

2. The Earth is the only object that I can empirically verify as possessing Gravity.

 

 

 

 

In my physics classes, when learning about Gravity, someone of course asked the obvious question of, "Why don't mountains have gravity?" or why we fail to observe gravitational fields forming around sufficiently large masses on Earth. The answer I received for this is that the gravity of the Earth is so extreme that it dwarfs the force of gravity of these smaller objects and renders them negligible.

 

This seems to be in opposition to the apparent fact that perpendicular forces are supposed to not effect each other in this universe. For example, you are most likely familiar with the thought experiment of a car moving forward on a frictionless surface at 100 mph. This car is then struck from the side at a perfect 90 degree angle to it's current path by another object. I have always learned that on this frictionless plane, the car would still be moving forward at 100 mph, because the two perpendicular forces do not affect each other - it would just now also have additional momentum pushing it along the horizontal axis.

 

If this is the case, then sufficiently large masses on Earth should have demonstrable gravitational fields. The gravity of the Earth is pulling down (we'll call this the Y axis), but a sufficiently large mass, such as a massive lead cube, would, if you were holding an object next to it, be exerting it's own gravity horizontally (on the x axis). The vectors of these two forces are perpendicular, and thus should not cancel each other out.

 

To give a basic example, if you were throwing a football past a 200 x 200 ft. lead cube, the gravity of the Earth would be pulling the football down, but the gravity of the cube should also be able to pull the football towards it, to the left or to the right depending on where you are. This obviously does not happen. On a more practical level, it seems like objects of normal mass should be able to attract sufficiently small masses, as long as the ratio of their masses is similar to the ratio of my mass to the Earth's. This also does not happen.

 

If perpendicular forces cannot cancel each other out, why can't we observe the gravitational force of masses on Earth?

Let's say that you are ~100 ft from the side of the cube. you throw the ball at 60 ft/sec. So after 1 sec it has moved 30 ft parallel to the side of the cube, You are 1/100,000 the distance from the center of the cube than you are from the center of the Earth, but the Cube is only 1/2.3e15 that of the Earth. Since gravity is directly proportional to the mass and inversely proportional to the distance, the cube would exert a force that is 1/23346 that of the Earth. In one sec your football would fall 16 ft in the 60 ft it traveled forward, and would be deflected to the side by the cube by 1/23346 of that or 1/125 of an inch. Even if you were right alongside the Cube, this only increases the gravity of the cube by a factor of 4 and the deflection would be 4/125, or very roughly 1/30 of an inch. So if you could find a 200 ft cube of lead and could throw the ball accurately enough, and could measure it deflection with high enough precision, you could conceivable measure its deflection due to the cube. But to notice such an effect under everyday conditions,even though it is there, is asking a bit much. It isn't that all objects around us don't exert a gravitational pull on us, it is just that it is too small to measure except with extremely precise instrumentation. Again, the actual math belies intuition.

Edited May 8, 2016 by Janus