- #106
asdofindia
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And I'm also asking if gravity is not centripetal force, what's keeping us glued to Earth on the equator where we ARE doing a curved motion. You saying friction?
asdofindia said:And I'm also asking if gravity is not centripetal force, what's keeping us glued to Earth on the equator where we ARE doing a curved motion. You saying friction?
RedX said:Not to beat a dead horse, but I read somewhere that the centrifugal force is strong enough that NASA likes to launch things near the equator. By my calculations you are lighter by 3/1000 of your weight at the equator than at the poles due to the centrifugal force.
It's ESA, not NASA, that launches from near the equator. NASA launches from the Cape (28°30′ north latitude) and from Vandenberg (34°44′ north latitude). The reason ESA does so isn't centrifugal force (which isn't a real force). It's inertia. Launch eastward from the equator and you have a velocity of about 465 m/s right off the bat.RedX said:Not to beat a dead horse, but I read somewhere that the centrifugal force is strong enough that NASA likes to launch things near the equator. By my calculations you are lighter by 3/1000 of your weight at the equator than at the poles due to the centrifugal force.
The sum of gravity (roughly downward) and the normal force (upward) is not radial if, by radial you mean directed away from the center of the Earth. What can be said is that the sum of gravity and the normal force has no z component, and this is true for any flat spot anywhere on the surface of the Earth except at the poles. (At the poles the sum of gravity and the normal force is the zero vector.)Also, the sum of gravity and the normal force is in the radial direction. The centripetal acceleration is perpendicular to the axis.
This equation is true everywhere, including the poles, with r being the distance between the point on the surface of the Earth and the Earth's rotation axis.Therefore only at the equator can:
[tex]F_g+F_N=m\omega^2r [/tex]
Friction is needed to explain why someone can stand still on an inclined ramp. It is not needed to explain why someone can stand still on a flat surface.otherwise it must either be friction
D H said:The sum of gravity (roughly downward) and the normal force (upward) is not radial if, by radial you mean directed away from the center of the Earth.
RedX said:Oh, I see what you're saying. If the Earth were a perfect sphere, then objects would flow to the equator, since the normal force and gravity are only in the radial direction (to the center of the earth), and can't add to give you a centripetal acceleration except at the equator where the radial direction to the center of the Earth coincides with the radial direction from the rotation axis.
cepheid said:I don't know what you mean. There would still be a component of the gravitational force that points towards the centre of the latitude circle that you're moving on. This component would just fall off as the cosine of your latitude.
RedX said:Right, so assuming a perfect sphere, gravity+normal force is in the radial direction. There is a component of the sum of the two in the direction towards the center of the latitude circle. However, this component cannot be used to provide centripetal acceleration, because then nothing balances the vertical component (which goes as the sine of latitude).
Anyways, it turns out most planets are spherical because of tremendous gravity. But if planets are spinning, they develop a bulge due to material flowing towards the equator.
Here is a link that explains it:
http://www-istp.gsfc.nasa.gov/stargaze/Srotfram1.htm
a quarter of the way down from the top.
This flow of material stops after the bulge is formed, because then the normal and the radial directions are not the same due to the changed shape of the planet, and the normal force+gravity can now give a force in the direction towards the center of the circle of latitude.
D H said:From the perspective of a non-rotating observer moving alongside the Earth, a person standing still on the surface of the Earth is undergoing uniform circular motion. A net force is needed to maintain that circular motion. This net force is normal to and directed towards the Earth's rotation axis. The forces acting on this person are gravitation, directed downward, and the normal force, directed upwards. Due to the Earth's non-spherical shape the angle between these forces is not quite 180 degree. The net sum of these two forces is exactly equal to the net force needed to make the person keep following that uniform circular motion.
cepheid said:I think I am beginning to understand you now. Thanks for the link.
Yech. That link oversimplifies things to the extent of being wrong.RedX said:Anyways, it turns out most planets are spherical because of tremendous gravity. But if planets are spinning, they develop a bulge due to material flowing towards the equator.
Here is a link that explains it:
http://www-istp.gsfc.nasa.gov/stargaze/Srotfram1.htm
D H said:The answer is friction. Neither the Earth nor the Moon is coated with teflon.
ashishsinghal said:That seems a bit odd to me! Imagine a vacuum space where you have levitated a charge by balancing electric and gravitational fields. Remember oil drop experiment. In that case there is no contact whatsoever and hence no friction at all. In that case according to your argument the charge will remain at rest wrt space and hence will collide with the container at great speeds. I really don't think that happened during the experiment!
It is the gravitational force of the earth.
asdofindia said: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...
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!Stcloud said: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. ''
Stcloud said:'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. ''
Stcloud said: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. ''
Drakkith said: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.)
Stcloud said: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!
Stcloud said: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!
Stcloud said: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...
Stcloud said: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!
DaveC426913 said: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.
cepheid said: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.
DaveC426913 said: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.
Stcloud said: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.
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?