Spinning Motion Around Centre of Gravity

Click For Summary

Discussion Overview

The discussion revolves around the mechanics of spinning motion, specifically why objects like bottles spin around their center of gravity and the implications of forces acting through this point in the context of rotational dynamics. The scope includes theoretical explanations and conceptual clarifications related to physics principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why objects thrown with a twist spin around their center of gravity, suggesting a relationship between the axis of spin and the center of gravity.
  • One participant asserts that any spinning object must have an axis of spin that passes through the center of gravity to comply with Newton's laws, using the example of twirling a bat to illustrate the necessity of applying force to achieve spin around a different point.
  • Another participant recommends consulting a physics book on rotational and translational momentum for further understanding.
  • A later reply distinguishes between the center of mass and the center of gravity, stating that while they often coincide, they are not the same and can differ under certain conditions, such as non-uniform gravitational fields.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the center of gravity and the center of mass, indicating that there is no consensus on the definitions and implications of these concepts in the context of spinning motion.

Contextual Notes

The discussion highlights potential limitations in understanding the distinction between center of mass and center of gravity, particularly in non-uniform gravitational fields, and the need for mathematical derivations to clarify the principles involved.

aerobilal
Messages
10
Reaction score
0
why do bottles or any other things thrown up with a twist always spin about their centre of gravity? Also if all forces pass through centre of gravity of a rocket in space, it will not rotate , even though there is a net moment about any other point except the cg. why
 
Physics news on Phys.org
Any spinning object has to have some axis around which it spins, just by the nature of what it means to spin.

The axis of spin has to pass through the center of gravity in order to satisfy neewton's laws. Imagine twirling a big long bat around the endpoint, rather than the middle. You can make it spin like that, but you have to apply a force to do it.

A more detailed explanation is possible, but requires some math. You can usually see a derivation in terms of the sum of the forces in most books on dynamics.
 
consult a good physics boom on rotational/transational momentum.
 
aerobilal said:
why do bottles or any other things thrown up with a twist always spin about their centre of gravity? Also if all forces pass through centre of gravity of a rocket in space, it will not rotate , even though there is a net moment about any other point except the cg. why

An objects spins around its center of mass, not it's center of gravity. These are distinct points that often coincide, but not necessarily. The center of mass is the point that is fix, no matter what the axis of rotation is.

The center of gravity is the point which would have the same trajectory in a gravity field as a the entire body.

They differ when gravity is not constant throughout the object (such as a mountain or a very very tall post).
 

Similar threads

Undergrad Effects of the centre of gravity on aircraft's stability
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Technique to find CG seems like should be for C.M. instead
  • · Replies 2 ·
Replies
2
Views
1K
Undergrad Finding the Axis of Rotation with Given CoG and Force Point
  • · Replies 32 ·
2
Replies
32
Views
3K
Undergrad Gyroscopic precession of a spin-stabilized bullet
  • · Replies 10 ·
Replies
10
Views
5K
High School Sympathetic motion generated by cleaning Rubik's cube
  • · Replies 10 ·
Replies
10
Views
1K
High School Strapped to a quadcopter in zero gravity
  • · Replies 3 ·
Replies
3
Views
1K
Undergrad Rotating plate thrown parallel to ground
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Writing an equation for torque around an arbitrary point on a body
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad Which inertial frame is used when resolving toward the centre
  • · Replies 15 ·
Replies
15
Views
2K
Undergrad Why do objects always rotate about their centre of mass?
  • · Replies 133 ·
5
Replies
133
Views
13K