It doesn't take that much to experience Earth's curvature. I used to paddle a kayak across fairly large lakes. Because kayakers sit so close to the surface of the water, you only need go get about 3 miles from a beach to have it disappear beyond the Earth's curvature.

 

Well it just does not make a difference from the viewpoint of ballistic physics.

I don't know what the vein of thought that this comment was intended to be read from, but assuming that it had something to do with flat earth versus round earth, it most certainly does make a difference from the viewpoint of ballistic physics. It might be negligible if shooting a .308 Win Magn rifle from 1000 yards, but it is the difference between hitting and missing your target if shooting a 16" rifle from a battleship at 20 miles or so and it is absolutely critical for ballistic missiles at nearly any strategic range.

 

Okay, look at road, it is straight right? I really have no doubt that Earth is truly oblete spheroid or whatever.

But I'm just wondering why can't we feel. it. But yes, when I look videos taken above from helicopter, I can see the bulge of Earth.

Also, I always read "gravity is the attraction between two objects". In Earth, what's that two objects Maybe my pen and earth?

Thanks so much!

Let's consider your "road is straight" assertion. I assume you are talking about a road going directly away from you? Well, that's the road I'm talking about. Imagine the earth was completely flat and you were in, say, Spain on the east coast and were looking due west across the Pacific at New York, roughly 3000 miles away. You should be able to see the larger buildings in New York City using a telescope (or even a large set of binoculars) as long as the lens aperature is larger than about 10cm. You should be able to see Pikes Peak with a cell phone camera and possibly with the naked eye.

But, of course, you know you can't. If you were to make a bunch of experiments you would discover that there is a very hard limit on how far away an object can be and you still be able to see it, no matter how good the telecope you use. But, if you move up in a tower, then you can see further. If you made a bunch of these measurements, you would also see that there is still a limit because the higher you go the less and less the increase in range becomes. You could play around with plotting these and would likely come to the conclusion, eventually, that the simplest answer that fits the data is that the Earth is a sphere (or close approximation to one).

Another thing you could do is take measurements of the angle made by the shadow of a vertical stick at noon at various points along a north-south running road. You could assume that the Earth was flat and that the sun was sufficiently close to account for the change in the angles, but that math just wouldn't work out because the sun would have to be at different distances for different locations. The other explanation is that the sun is far away (and various separate measurements could establish a lower limit for the distance and it doesn't have to be all that far for the "far away" assumption to be valid) and the different angles are due to different slopes, relative to the sun, of the ground. Again, a bit of plotting and playing and you would eventually conclude that the Earth is basically a sphere.

 

Okay. I have one more question.

Why for example when we look at road (short distance), we can't notice its curvature?

 

Several reasons (and certainly more than what I'll put here)

1) We do! If you are looking down the road, then it is curving downward away from you. The sides of the road are also converging toward the same point. Fine a really long, straight, wide road (hard to do) and you will notice (use binoculars if necessary), that the road still has a discernable width when it reaches the horizon. This is because the curvature of the earth has taken the road beyond your line of site after only about three miles, which is well before you could see far enough down it to lose the ability to resolve the distance between the sides.

2) The deviation from a "straight" line is so small that our minds interpret what we are seeing as being "straight". We would need something to compare it to in order to see the difference. For instance, take a long straight board and lay it across two chairs of difference height spaced several feet apart. Now shim one side until you judge it to be "level". Now place a level on it. Do this several times and you will almost certainly discover that there is usually a very noticeable difference between what you thought was level and what really was, but it wasn't discernable to you until you had a reference to judge it by. Or here's another example. Find a water tank that is very large and that you can walk up to (the latter may be tough). Say the tank is 100 ft in diameter. From a distance, you can easily discern the curvature. But if you get up right next to it and block out everything but a one foot length along the circumference and look at where it meets the footing, you may have a very hard time telling that is curved at all. In case you can't get up close to it, take a letter size sheet of paper and draw a curved line from one corner to the diagonal corner such that, in the middle, it deviates from a straight line by about 0.060", which is the thickness of a penny. How easy is it for you to tell that the line is curved without drawing a straight line next to it for comparison?

 

There is a town in Fla that is famous for what they call spook hill. There is a line

line painted in a road, you pull up stop your car put it in neuaral and your car will

roll up hill . This is visable to the eye.

 

It doesn't take that much to experience Earth's curvature. I used to paddle a kayak across fairly large lakes. Because kayakers sit so close to the surface of the water, you only need go get about 3 miles from a beach to have it disappear beyond the Earth's curvature.

good point.

 

There is a town in Fla that is famous for what they call spook hill. There is a line

line painted in a road, you pull up stop your car put it in neuaral and your car will

roll up hill . This is visable to the eye.

And it is one of many, many places, both natural and artificial, where such optical illusions can be found -- and they are all optical illusions. There is something that your mind assumes must be level that is, in fact, tilted. But you interpret the road in relation to the object you are using as a level reference. Frequently that reference is the horizon which you assume is level but might actually be a gentle and consistent slope.

 

I don't know what the vein of thought that this comment was intended to be read from, but assuming that it had something to do with flat earth versus round earth, it most certainly does make a difference from the viewpoint of ballistic physics. It might be negligible if shooting a .308 Win Magn rifle from 1000 yards, but it is the difference between hitting and missing your target if shooting a 16" rifle from a battleship at 20 miles or so and it is absolutely critical for ballistic missiles at nearly any strategic range.

You maybe did not read it correctly.

For example the force of attraction between two object, e.g. a boy and a girl, is 1040.361*10−11 N. Then where's the direction of the force? (I always think of this as like magnet.)

It is irrelevant, totally.

Like the color or the actual name of the missile. This does not take part in the calculation. So it can be omitted from the discussion.

If we wish to discuss the coloring of the missile or it's name and all the like, we raise that seperately.

 

You maybe did not read it correctly.

When you said, "Well it just does not make a difference from the viewpoint of ballistic physics," I had to determine what you meant by "it" since you didn't give any context (something that we (myself most definitely included) are all guilty of more often than we probably realize). The only thing I could think of that made sense was that you were referring to the topic of the thread, namly the curvature of the Earth.

 

When you said, "Well it just does not make a difference from the viewpoint of ballistic physics," I had to determine what you meant by "it" since you didn't give any context (something that we (myself most definitely included) are all guilty of more often than we probably realize). The only thing I could think of that made sense was that you were referring to the topic of the thread, namly the curvature of the Earth.

No I did not actually. Also this would not be an answer to the thread question as such. My lack of clarity maybe.

 

You maybe did not read it correctly.

For example the force of attraction between two object, e.g. a boy and a girl, is 1040.361*10−11 N. Then where's the direction of the force? (I always think of this as like magnet.)

It is irrelevant, totally.

Like the color or the actual name of the missile. This does not take part in the calculation. So it can be omitted from the discussion.

If we wish to discuss the coloring of the missile or it's name and all the like, we raise that seperately.

It's relevant to the Earth being round and rotating. When firing North or South (any direction, actually, but easier to visualize N or S), there is a several second flight time. In that time, the projectile is no longer linked to the earth by the barrel or target, and the Earth rotates the impact point into the path of the projectile.

If this weren't accounted for, Battleship rounds wouldn't have been nearly as accurate as they were, nor would air corridors for flight. If the earth was flat, but somehow maintained gravity, these calculations, and aircraft flight mapping, would be simpler. See "Coriolis Effect" and "Great Circle Route".

The Earth was known to be spherical before Columbus set sail, it was just calculated that he would have run out of supplies 1/2 way through his cross-ocean trip to India. Luckily, he landed in the Caribbean only 1/3rd of the way through his trip.