# Do objects in circular motion also *have* to undergo rotation?

• Likith D
In summary: Say that suddenly, i have gravity pulling it to its center elsewhere in space, will the object be like image 1 or image 2 ?The way we usually accomplish these things, (e.g. swinging a rock on a rope) it almost always winds up as you show in the second picture. The conservation of angular momentum as planets accrete from the dust around a star have them spinning much faster. Tidal locking eventually slows the moon and the planets down to the second situation. You are almost never shown a case where the object isn't spinning as it orbits either in lock step like case 2 or faster in the same direction.
Likith D
1.

2.

Well, these are two pics on circular motion, i found on net...
If the objects had a zero rotational motion before circular motion, and the next moment suddenly a force acts ( perpendicular to their linear momentum ) such that they move in circular motion,... which of these pictures would it resemble?... 1 or 2?

Circular motion requires constant acceleration. What force is causing that acceleration?

Likith D
DrClaude said:
Circular motion requires constant acceleration. What force is causing that acceleration?
Yeah... constant force say,.. gravity towards a point in space,... so should the motion resemble 1 or 2

Likith D said:
Yeah... constant force say,.. gravity towards a point in space,... so should the motion resemble 1 or 2
Pictures 1 and 2 are interchangeable. They depict the exact same scenario. There is no difference other than the fact that one object is rectangular and one is round and that one object has an acceleration vector drawn in and the other has a velocity vector drawn in.

What difference do you see between the two pictures that you wish us to comment on?

Likith D said:
Yeah... constant force say,.. gravity towards a point in space,... so should the motion resemble 1 or 2
It will depend very much on the way the force is impacted, but the general case will be closer to picture 1. Only in the case where the rotational period is the same as the orbital period will the result be picture 2.

jbriggs444 said:
What difference do you see between the two pictures that you wish us to comment on?
I take picture 1 to show no rotation of the object.

Likith D
jbriggs444 said:
Pictures 1 and 2 are interchangeable. They depict the exact same scenario. There is no difference other than the fact that one object is square and one is round and that one object has an acceleration vector drawn in and the other has a velocity vector drawn in.

What difference do you see between the two pictures that you wish us to comment on?
Well,... the squared object undergoes 1 rotation about its axis for every i rotation around a fixed point in space
the round object does not rotate about its axis...
so they are different...

DrClaude said:
I take picture 1 to show no rotation of the object.
So, it doesn't rotate about its own axis... thanks for the clarification

Likith D said:
Well,... the squared object undergoes 1 rotation about its axis for every i rotation around a fixed point in space
the round object does not rotate about its axis...
so they are different...
Thanks, I missed seeing the three holes on the bowling ball.

Likith D
jbriggs444 said:
Thanks, I missed seeing the three holes on the bowling ball.
Yeah... OK !
Say, i have an object which is in linear motion and is also not in any kind of rotational motion...
Say that suddenly, i have gravity pulling it to its center elsewhere in space, will the object be like image 1 or image 2 ?

The way we usually accomplish these things, (e.g. swinging a rock on a rope) it almost always winds up as you show in the second picture. The conservation of angular momentum as planets accrete from the dust around a star have them spinning much faster. Tidal locking eventually slows the moon and the planets down to the second situation. You are almost never shown a case where the object isn't spinning as it orbits either in lock step like case 2 or faster in the same direction.

So it is with no small bit of evidence that one might expect this is necessary. It is not. If we could plop a planet down and just shove in the orbital direction without spinning it, it would orbit without spinning as in picture 1. The force it takes to put the object in orbit need not impart any torque.

The easiest place to see this is with a playground tether ball.

Likith D
Likith D said:
Well, these are two pics on circular motion, i found on net...
If the objects had a zero rotational motion before circular motion, and the next moment suddenly a force acts ( perpendicular to their linear momentum ) such that they move in circular motion,... which of these pictures would it resemble?... 1 or 2?

Interesting question... never thought of this before, it's probably useful to incorporate into my Intro class. Note that there are 2 reference frames involved- the stationary lab frame, and the rotating frame of the velocity (and/or acceleration) vector. In the upper case, the object is not rotating in the lab frame but is rotating with respect to the acceleration vector; the opposite occurs for the lower case.

As an analogy, the Hubble Space Telescope orbits the Earth like the top diagram, while AFAIK the International Space Station (and Space Shuttle, back in the day) orbit like the lower diagram [LHLV (local horizontal-local vertical) attitude].

russ_watters and Likith D
Likith D said:
Say, i have an object which is in linear motion and is also not in any kind of rotational motion...
Say that suddenly, i have gravity pulling it to its center elsewhere in space, will the object be like image 1 or image 2 ?
It will change from 1 to 2 over time, and increase the distance:
https://en.wikipedia.org/wiki/Tidal_locking

Likith D said:
If the objects had a zero rotational motion before circular motion, and the next moment suddenly a force acts ( perpendicular to their linear momentum ) such that they move in circular motion,... which of these pictures would it resemble?... 1 or 2?
Depends on how the force is applied. If that force creates no torque around the objects center of mass, then it will stay in state 1 forever.

Last edited:
Likith D
I recently found out that for an object to undergo rotation ;
1. All it's particles should be moving in a 2D circular path or a part of a 2D circular path
3. All the center of circles of the moving particles should be on a line
2. This line should be perpendicular to all the planes of motion of the particles ( axis of rotation )
using these conditions, the first picture is not undergoing rotation as the 3rd condition is not satisfied
the 2nd picture is undergoing rotation since it satisfies all the 3 conditions
hope that helps !

There are two separate issues here. If the 'object' is spherical and the force is applied at the CM, then there is nothing to make the object rotate about its own axis. IF the force is applied elsewhere then there is a couple to make the object rotate about the point of application. Its rotation on the end of a string would cause an angular oscillation about the mean radial direction of the string.

## 1. Do all objects in circular motion also undergo rotation?

Yes, all objects in circular motion also undergo rotation. This is because circular motion is defined as an object moving along a circular path, which requires the object to constantly change its direction of motion. This change in direction is caused by rotation.

## 2. Can an object be in circular motion without undergoing rotation?

No, it is not possible for an object to be in circular motion without undergoing rotation. As mentioned before, rotation is necessary for circular motion to occur. Without rotation, the object would simply move in a straight line and not follow a circular path.

## 3. What is the difference between circular motion and rotation?

The main difference between circular motion and rotation is the point of reference. In circular motion, the object is moving along a circular path relative to an external point of reference. In rotation, the object is spinning on its own axis without any external reference point.

## 4. Do all objects rotating also have circular motion?

No, not all objects rotating also have circular motion. An object can rotate on its own axis without moving along a circular path. For example, a spinning top rotates but does not have circular motion. Circular motion requires a change in direction of motion, while rotation does not.

## 5. Is it possible for an object to have circular motion and rotation at the same time?

Yes, it is possible for an object to have circular motion and rotation at the same time. This occurs when an object is both moving along a circular path and spinning on its own axis. An example of this is a spinning ball rolling along a curved surface.

• Mechanics
Replies
37
Views
3K
• Mechanics
Replies
15
Views
2K
• Mechanics
Replies
4
Views
337
• Mechanics
Replies
6
Views
4K
• Mechanics
Replies
24
Views
2K
• Mechanics
Replies
6
Views
1K
• Classical Physics
Replies
67
Views
3K
• Mechanics
Replies
15
Views
3K
• Mechanics
Replies
3
Views
1K
• Mechanics
Replies
7
Views
6K