Why do we rotate along with the earth's rotation?

[Mute]

See, if we draw a free body diagram. We'd draw an arrow from the man to the centre, calling it centripetal force. And another opposite to it calling it centrifugal force, right?
I thought they'd cancel, but I don't think I've clearly finished that thought process, I made a quick reply...

And of course they wouldn't have any component tangential to the surface.

But a body already moving with a velocity tangential do not need a force to keep it moving along the tangent.
But that's along the tangent...

Oh... I think I'm confused. Let me think for a while...

To elaborate a bit on DH's answer to your confusions: Your free body diagram would have neither centripetal nor centrifugal forces drawn on it.

Centripetal force is a net force pointing in the radial direction towards the centre of the rotation, and net forces never appear on a free body diagram! In the intro physics example of a satellite traveling around the Earth in a circular orbit, the only force acting on the satellite is gravity, so gravity does provide the net centripetal force in this case. (but that's a simplified example).

Now, the above all takes place from the point of view of someone in an inertia reference frame watching the rotating object. In an inertial frame there is no such thing as a centrifugal force. If you go to the point of view of the rotating object, however, then you "feel" a centrifugal force on you, but you are now in a non-inertial reference frame, and so you have to introduce "fictitious forces", of which the centrifugal force is one, in order for Newton's second law to continue to work in the non-inertial reference frame. It turns out that in the non-inertial reference frame the centrifugal force is the opposite of the centripetal force, but neither exists in the same frame at the same time, so they don't "cancel out".

 

[Stcloud]

'QUOTE=RedX;3359983]You have it the opposite. If the moon does not spin on its axis, after half an orbit around earth, you would see the opposite side of the moon. '

Suppose someone is walking in a circle around you, always facing you. For example if they are walking clockwise around you, each footstep they take they need to rotate a little bit in order to remain facing you, using their right foot as their pivot foot (hence they are rotating clockwise about their axis, while walking clockwise about you). Suppose the person begins to the north of you. If they don't use their right foot as a pivot to turn while they walk, then when they get south of you, their back will be to you.[/QUOTE]''>>>>

Suppose someone IS walking in a CIRCULAR DIRECTION around you, that person's FACE will ALWAYS be facing the direction in which he/she is travelling. Just like the face of a greyhound racing around a circular track is ALWAYS facing the direction it is travelling. If that greyhound started to spin around on its OWN axis then at some time during its spin it would be moving in the same direction around the track, tail first. ''

 

[Doc Al]

Suppose someone IS walking in a CIRCULAR DIRECTION around you, that person's FACE will ALWAYS be facing the direction in which he/she is travelling. Just like the face of a greyhound racing around a circular track is ALWAYS facing the direction it is travelling. If that greyhound started to spin around on its OWN axis then at some time during its spin it would be moving in the same direction around the track, tail first. ''

Yes, both you and the greyhound always face the direction that you travel. But that direction changes. At first you face north, then east, then south... you're rotating!

Of course, you're not just rotating. You're also moving in a circle.

 

[Drakkith]

'QUOTE=RedX;3359983]
Suppose someone IS walking in a CIRCULAR DIRECTION around you, that person's FACE will ALWAYS be facing the direction in which he/she is travelling. Just like the face of a greyhound racing around a circular track is ALWAYS facing the direction it is travelling. If that greyhound started to spin around on its OWN axis then at some time during its spin it would be moving in the same direction around the track, tail first. ''

This is the last time I will explain this, as this is extremely fundamental.
The moon completes one revolution (orbit) around the Earth in about 27.3 days.
If it was NOT rotating then the same face of the moon would always face the same direction. But, it does not. Because it is revolving around the Earth and the same face of the moon always shows to earth, that requires that it rotate around its axis. If I were looking down upon the Earth and the Moon from up above the solar system somewhere, I could easily see this. From that frame of reference it is obvious that the moon rotates, otherwise how could its direction of facing change? However, because we are here on Earth the situation can be slightly confusing.

The reason that someones face is always facing the way they are walking is because it is natural to face they way you are walking. However objects in space aren't walking, they simply coast through space. The greyhound in your example IS rotating on it's axis as it moves around the track. Just like I am every time I run a lap around the track at the gym. I rotate once per lap. If I did not, I would end up running sideways after the first turn, backwards after the 2nd, sideways again on the 3rd, and back to forwards after the 4th turn.

Again, I refer you to this article on tidal locking. I suggest you read it through thoroughly. Specifically look at this sentence: A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner.http://en.wikipedia.org/wiki/Tidal_locking

 

[cepheid]

Supose someone IS walking in a CIRCULAR DIRECTION around you, that person's FACE will ALWAYS be facing the direction in which he/she is travelling. Just like the face of a greyhound racing around a circular track is ALWAYS facing the direction it is travelling. If that greyhound started to spin around on its OWN axis then at some time during its spin it would be moving in the same direction around the track, tail first. ''

Perhaps the attached diagrams will help visualize the situation. In the first one, entitled, "What would happen if the moon didn't spin", I've depicted the moon at various points in its orbit around Earth. On each moon, I've drawn an arrow that is fixed to a specific point on the moon's surface, and always points radially outward from that point. We know that the moon is not spinning in this diagram, because the arrow never changes direction. As a result, as the moon moves around Earth in a circle, the portion of its surface that it presents towards Earth changes continuously. To aid in seeing that, I've drawn lines through the centre of each moon, dividing it into two hemispheres, the one that is visible to Earth, and the one that is invisible to Earth. If we start with the moon at the top of the diagram, it's clear that half an orbit later, when the moon is at the bottom of the diagram, people on Earth are looking at exactly the opposite side of the moon as they were at the start. If the moon didn't spin, then over the course of one orbit, we'd see its entire surface.

In the second diagram, entitled, "What actually happens", we can see that the moon is spinning. We know it's spinning, because the arrow continously changes direction. It does so in such a way that the same hemisphere of the moon is always presented towards Earth. In order for this to occur, the arrow has to do a full 360 over the course of one orbit. In other words, the time required for the moon to spin once on its axis is equal to the time it takes to complete one orbit around Earth. The moon's rotation period is the same as its orbital period. As has been mentioned, this is not a coincidence, but is due to something called tidal locking.

 

[Stcloud]

You didn't look at the article called Tidal Locking that I linked did you? Here:

Not if it wasn't rotating around its own axis. If the Earth stopped rotating, the stars would never change. We would see the same ones all the time. (Other than the sun, which would appear to slowly move through the sky to make one day last a full year.)

Huh? If the body (Earth in this case) stopped spinning around as it orbited the sun the people on Earth would see stars that 'never changed'? You mean always see the same stars? All the time? Of course they would see all the stars surrounding them, as the moved along their circular path around the sun!... And if people lived on the far side of the moon from the sun, they wouldn't ever see the sun, if the Earth stopped spinning... Just as the case with the moon.. if the moon stopped spinning on its own axis, any people on the far side of the moon from earth, would never see Earth... their 'other side of the moon' would always face earth.

I think it is time to use logic, pure logic.. The moon IF it spun around on its own axis MUST at some time during its spin face earth... Which is to say.. if the moon faced all of itself toward a central object or point.. it is spinning. Regardless as to what is or may at that central point.

THIS is unequivocally understood : The far side of the moon is aways the same side. It never faces earth. It therefore can not be spinning around on its own axis.. because it does not do other than 'keep facing the same side of itself toward Earth'.. with which claim you all who want to say the moon IS spinning on its own axis agree!

 

[WannabeNewton]

THIS is unequivocally understood : The far side of the moon is aways the same side. It never faces earth. It therefore can not be spinning around on its own axis.. because it does not do other than 'keep facing the same side of itself toward Earth'.. with which claim you all who want to say the moon IS spinning on its own axis agree!

I can't quite understand your wording but it seems to me you are saying that it is unanimously accepted that the moon does not spin on its axis because it always faces the same side in relation to us. On the contrary, it is a well known fact that the moon's rate of rotation and its orbital period are equal so that it always faces the same side.

 

[Drakkith]

Huh? If the body (Earth in this case) stopped spinning around as it orbited the sun the people on Earth would see stars that 'never changed'? You mean always see the same stars? All the time? Of course they would see all the stars surrounding them, as the moved along their circular path around the sun!... And if people lived on the far side of the moon from the sun, they wouldn't ever see the sun, if the Earth stopped spinning... Just as the case with the moon.. if the moon stopped spinning on its own axis, any people on the far side of the moon from earth, would never see Earth... their 'other side of the moon' would always face earth.

They would see the same sky all year long. Throughout the year the sky in the night changes and we can see the full celestial sphere over the course of a year. If the Earth stopped spinning this would not be the case. We would only see about 50% of the sky. The same constellations and stars would never change their positions in the sky relative to the time. If Arcturus were directly above me when the Earth stopped spinning, it would stay there forever. (Arcturus is a star that is currently overhead at night. In a few months it will not be.)

I think it is time to use logic, pure logic.. The moon IF it spun around on its own axis MUST at some time during its spin face earth... Which is to say.. if the moon faced all of itself toward a central object or point.. it is spinning. Regardless as to what is or may at that central point.

No. This is 100% wrong for the reasons already explained to you. I'm sorry you cannot understand or accept why.

THIS is unequivocally understood : The far side of the moon is aways the same side. It never faces earth. It therefore can not be spinning around on its own axis.. because it does not do other than 'keep facing the same side of itself toward Earth'.. with which claim you all who want to say the moon IS spinning on its own axis agree!

Correction: The far side of the moon is always the same side FROM THE POINT OF VIEW OF THE EARTH. From the suns point of view it sees every side of the moon every 27 days. (Also from the POV of distant stars, galaxies, ETC.)

 

[cepheid]

I think it is time to use logic, pure logic.. The moon IF it spun around on its own axis MUST at some time during its spin face earth...

Your wording is not entirely clear here, but it seems like you're saying that if the moon spins on its axis, then all parts of it must eventually turn to face towards Earth. This is simply not the case. It is possible for the moon to spin and yet for there to be parts of it that never see Earth. Did you take a look at the diagrams that I posted in my previous post? The one on the right, entitled, "What actually happens", explains how this can be so. So as to attract more attention to it, I will embed it directly into this post below, rather than as an attachment:

http://img811.imageshack.us/img811/3373/moon2l.png

Which is to say.. if the moon faced all of itself toward a central object or point.. it is spinning. Regardless as to what is or may at that central point.

THIS is unequivocally understood : The far side of the moon is aways the same side. It never faces earth. It therefore can not be spinning around on its own axis.. because it does not do other than 'keep facing the same side of itself toward Earth'.. with which claim you all who want to say the moon IS spinning on its own axis agree!

Again, it is possible for the moon to spin and yet for there to be one side of it that always faces toward Earth, and one that never does. The diagram above shows this. If you do not follow it, I posted an explanation of it in my previous post.

 

[DaveC426913]

The issue here is about choosing a frame of reference.

If I hold a tennis ball at arm's length, and turn on the balls of my feet, I will see the same side of the tennis ball at all times. From my reference point, the tennis ball is not rotating. From the reference point of my brother, on the porch, the tennis ball is rotating at the same rate it is revolving.

As with the tennis ball, so it is with the Moon. In the Earth's frame of reference, the Moon is not rotating. From an external frame of reference it is.

 

[cjl]

The issue here is about choosing a frame of reference.

If I hold a tennis ball at arm's length, and turn on the balls of my feet, I will see the same side of the tennis ball at all times. From my reference point, the tennis ball is not rotating. From the reference point of my brother, on the porch, the tennis ball is rotating at the same rate it is revolving.

As with the tennis ball, so it is with the Moon. In the Earth's frame of reference, the Moon is not rotating. From an external frame of reference it is.

However, if an object appears not to be rotating when viewed from a rotating frame of reference, the object must in fact be rotating.

 

[Stcloud]

Your wording is not entirely clear here, but it seems like you're saying that if the moon spins on its axis, then all parts of it must eventually turn to face towards Earth. This is simply not the case. It is possible for the moon to spin and yet for there to be parts of it that never see Earth. Did you take a look at the diagrams that I posted in my previous post? The one on the right, entitled, "What actually happens", explains how this can be so. So as to attract more attention to it, I will embed it directly into this post below, rather than as an attachment:

http://img811.imageshack.us/img811/3373/moon2l.png



Again, it is possible for the moon to spin and yet for there to be one side of it that always faces toward Earth, and one that never does. The diagram above shows this. If you do not follow it, I posted an explanation of it in my previous post.

What your diagram shows is that, the moon is orbiting a central point. And that is ALL it shows. Whether the moon spins on its own axis has nothing to do with any other object!.. The moon either spins or it does not. The diagram by the way needs more arrows that those that show the moon is traveling in a circular direction - if the moon is spinning it needs arrow around the moon showing the moon is spinning around its own axis.

By the way re : 'Your wording is not entirely clear here, but it seems like you're saying that if the moon spins on its axis, then all parts of it must eventually turn to face towards Earth.'. I said that the far side of the moon must at some time during its spin, face Earth.. I was specific, by the way, to ensure it was understood that the astronaut on the far side of the moon be landed on the surface of the CENTRE of the far side from Earth. Thus he WOULD see 'all of Earth' if the moon spun itself around, and he with it.

 

[Stcloud]

The issue here is about choosing a frame of reference.

If I hold a tennis ball at arm's length, and turn on the balls of my feet, I will see the same side of the tennis ball at all times. From my reference point, the tennis ball is not rotating. From the reference point of my brother, on the porch, the tennis ball is rotating at the same rate it is revolving.

As with the tennis ball, so it is with the Moon. In the Earth's frame of reference, the Moon is not rotating. From an external frame of reference it is.

As I said, it is not a question of 'frame of reference'.. from far out in space the moon always faces the same side of itself toward a central point, around which it is circling. From Earth (that 'central point) the moon always faces the same side of itself toward a central point..

Now, that ball you are holding at arm's length while YOU spin around on your own axis - Does that ball miraculously start to spin itself around its own axis in your clasping hand? uh huh?

 

[Drakkith]

What your diagram shows is that, the moon is orbiting a central point. And that is ALL it shows. Whether the moon spins on its own axis has nothing to do with any other object!.. The moon either spins or it does not. The diagram by the way needs more arrows that those that show the moon is traveling in a circular direction - if the moon is spinning it needs arrow around the moon showing the moon is spinning around its own axis.

Not only are you wrong, you are also argumentative. The issue has been explained multiple times to you. We aren't making this up, this is actually how it happens.

As I said, it is not a question of 'frame of reference'.. from far out in space the moon always faces the same side of itself toward a central point, around which it is circling. From Earth (that 'central point) the moon always faces the same side of itself toward a central point..

Now, that ball you are holding at arm's length while YOU spin around on your own axis - Does that ball miraculously start to spin itself around its own axis in your clasping hand? uh huh?

See above. Again you are wrong, unwilling to listen, and just plain rude.

 

[Doc Al]

This thread is going in circles. (But is it rotating as well? )

Closed.