Register to reply 
Rotation of Heavenly Bodies 
Share this thread: 
#1
Mar914, 02:31 PM

P: 10

Why do celestial bodies follow different laws of physics than terrestrial bodies?
A nonrotating object has a point on its axis, or axle, continually aligned with a point on the object. An axis is virtual, or imaginary; an axle is real and we live in a real physical world. In a real physical world, there are two ways a nonrotating object can move in a circle: 1. A point on the axle is continually aligned with the direction of motion and a point on the object. This is similar to a horse on a merrygoround (MGR). The horse is rotating about the center of the MGR, not about its pole. Observer at the center only sees one side of the horse. Distant observer sees all sides of the horse once/orbit. 2. A point on the axle continually faces the same direction and is always aligned with a point on the object. This is similar to a nonrotating wheel on a vertical axle continually facing the same direction while moving in a circle. Observer at the center sees all sides of the wheel once/orbit. Distant observer only sees one side of the wheel. In both scenarios the object is orbiting the center of the circle; not rotating on its axle. In both scenarios, if the object is rotating on its axle and orbiting the center of a circle, a point on the axle is aligned with a point on the object once per orbit. Rotating Object moving in a circle: 1. With the axle moving in the direction of motion and the object rotating once per orbit, the observer at the center sees all sides of the object once. A distant observer sees all sides twice. 2. With the axle continually facing the same direction and the object rotating once per orbit, the observer at the center only sees one side of the object. A distant observer sees all sides once. Now compare tidally locked celestial bodies with a plane flying in a circle, a train moving on a circular track, and a horse on a MGR. Every object's axis is imaginary. None have a real axle about which to rotate. All are orbiting the center of a circle. When the forward motion of the plane, train, or MGR is stopped, the objects are not rotating. Why do tidally locked bodies continue to rotate? Also, in the real world, it's impossible to fit the nonrotating plane, train, or horse into the scenario with an axis always facing the same direction. This can only be done virtually in your imagination. 


#2
Mar914, 03:00 PM

P: 697

All bodies, be they celestial or not, follow the same physcial laws.
The short answer for the question stated in the penultimate paragraph is: Friction The situations are not equivalent for that reason. Without friction, a plane, a train, and anything else for that matter, with a nonzero angular velocity would keep on rotating with constant angular velocity unless a net torque is applied(Netwon's 1st law for rotational motion)  just like planets do. This is actually no different than asking why planets keep on going on and on around the sun when objects on Earth(a train, a plane, a MGR) will come to a halt if you turn off the engine. 


#3
Mar914, 07:08 PM

Mentor
P: 11,605




#4
Mar914, 10:12 PM

P: 10

Rotation of Heavenly Bodies
A tidally locked celestial body is rotating once per orbit, with a point on its axis always aligned in the same direction. The observer at the center only sees one side of the object. The only terrestrial object that fits the above scenario is one with a real axle. A plane, a train, a horse, or a celestial body does not have a real axle. 


#5
Mar914, 11:50 PM

Mentor
P: 11,605




#6
Mar1014, 08:01 AM

Engineering
Sci Advisor
HW Helper
Thanks
P: 6,964




#7
Mar1114, 09:46 PM

P: 10

Imagine a horse on an oval race track. An observer at the center of the track only sees one side of the horse. An observer in the stands sees all sides of the horse once. If at some point on the track the rider makes the horse go in a circle and then continues to the end of the track, the observer in the stands would see all sides of the horse twice; once orbiting the center of the track and once rotating in a circle. The observer at the center would now see all sides of the horse once; right side, head, left side, butt, and right side again. If the rider made the horse go in 2 circles, the observer in the stands would see all sides of the horse 3 times: once orbiting the center of the track and twice rotating in a circle. The observer at the center would only see all sides of the horse when it was rotating. From the perspective of a stationary sun, the observer would see all sides of Earth 365 ¼ times a year. From the perspective of a stationary distant observer: 366 ¼ times a year; once as a result of orbiting the sun and 365 ¼ times as a result of actual rotation. Earth in its orbit is not rotating. It is merely changing direction. An axis could be real or imaginary. A real axis is called an axle. Celestial bodies are treated as though their axis is real. The MGR horse, plane flying in a circle, train on a circular track, and orbiting celestial body all have an imaginary axle (axis). The real axis (axle) is their barry center. 


#8
Mar1214, 01:05 PM

Mentor
P: 22,248

Your error here is that you are mixing and matching frames of reference inconsistently. You can say that with respect to you the horse is not rotating and with respect to the spectators it is and that isn't a contradiction. But is is an error to say the rotation is an "illusion". Neither is an illusion: they are direct observations. So if you want to hold a consistent frame of reference (and you should) based on the point of view of the spectators, then you should say the horse is rotating while revolving  and of course, the rider is also rotating with the horse.
Similarly, the moon is revolving about the earth and rotating about its axis (once a month for both) with respect to the fixed stars. 


#9
Mar1214, 06:59 PM

Mentor
P: 11,605

The Earth is rotating about an axis that passes through its center and it is also rotating around the Sun (its orbit). 


#10
Mar1314, 05:06 AM

P: 697




#11
Mar1314, 06:00 AM

Mentor
P: 22,248

An actual problem might help:
You are on the edge of a 5 meter diameter merrygoround, rotating at 120 rpm. You release a ball. What is its speed and rotation rate? 


#12
Mar1314, 10:26 PM

P: 10

Thanks for the correction about my use of the word illusion.
From the reference frame of the spectator, the horse is rotating once per orbit. But then, a nonrotating orbiting horse is impossible. The spectator would only see one side of the horse and the observer at the center would see all sides of the horse once: an impossibility. Also, the observer at the center would not be able to distinguish between a nonrotating horse and one rotating twice per orbit: all sides of the object would be visible once in both instances. Again an impossibility. Summary: If we start with a nonrotating, orbiting object, center sees all sides once; spectator sees only one side. 1 rotation/orbit: center, only sees one side; spectator, all sides visible once. 2 rotations/orbit: center, sees all sides once; spectator, all sides visible twice. ~365 rotations/orbit: center, sees ~365 rotations; spectator, ~366 rotations. End So, lets start again by considering orbiting objects continually facing the center as nonrotating. Observer at the center sees only one side; spectator sees all sides once. 1 rotation/orbit: center, sees all sides once; spectator, sees all sides twice. 2 rotations/orbit: center, sees all sides twice; spectator, sees all sides three times. ~365 rotations/orbit, center, sees all sides ~365 times; spectator sees all sides ~366 times. However, with this scenario, there is no instance where the spectator would only see one side of an orbiting object. The object would have to be mounted on a real axle and bearing. 


#13
Mar1314, 11:21 PM

Mentor
P: 11,605




#14
Mar1414, 05:41 AM

Mentor
P: 22,248

Maybe you should try demonstrating this with objects on a table that you move with your hands. 


#15
Mar1414, 08:42 AM

P: 10




#16
Mar1414, 09:18 AM

Mentor
P: 11,605




#17
Mar1414, 11:29 AM

P: 10

An axis always facing the same direction would be a point on the MGR pole always pointing east. 


#18
Mar1414, 12:49 PM

Mentor
P: 22,248

On a Ferris Wheel, the cars revolve about the center axis without rorating.
Can I ask what the point of all of this is? Do you agree now that the first sentence of your first post was wrong? 


Register to reply 
Related Discussions  
Curved trails by motion of heavenly bodies in space  Special & General Relativity  3  
Rotation of Rigid bodies  Introductory Physics Homework  3  
Rotation of heavenly bodies  Astronomy & Astrophysics  9  
Rotation of rigid bodies  Introductory Physics Homework  3  
Rotation of rigid bodies: yoyo  Introductory Physics Homework  1 