Conservation of angular momentum and energy

In summary, when a spinning body changes its shape, the moment of inertia changes which affects its angular velocity. If angular velocity is conserved, the body will spin at a different speed but in the same direction. The rotational kinetic energy may also change and the excess energy may take the form of translational motion. The symmetry of the shape change may also impact the angular velocity. In terms of conservation, both energy and angular momentum will remain constant. In a simple example with two weights connected by a cord, pulling the weights in or letting them go out will require or generate work and both energy and angular momentum will be conserved.
  • #1
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if you have a spinning body, and then its shape changes so that the moment of inertia changes, what happens? If angular velocity is conserved, since there's no net torque, then it spins a different speed, but the same direction. But then has the rotational kinetic energy changed? And if so, does this mean the excess kinetic energy must take the form of translational motion? Does it matter if the change in shape was symmetrical?
 
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  • #2
Consider a very simple example - two weights connected by a cord spinning around their common center.

If you pull the weights in, it will require work to do so. (The easiest way to see this is to go to a non-inertial frame that's corotating with the weights, and consider the centrifugal force). Similarly, if you let the weights go out, work will be generated and must be dissipated (perhaps by friction).

Energy and angular momentum will both be conserved.
 
  • #3
The (instantaneous) angular velocity may well change its direction, since (for one) we cannot assume that the shapechange does not affect the inertial tensor (with respect to the C.M).
The angular momentum, remains, however, constant.
 

Related to Conservation of angular momentum and energy

1. What is conservation of angular momentum and energy?

Conservation of angular momentum and energy is a fundamental principle in physics that states that the total angular momentum and energy of a system remains constant in the absence of external forces. This means that in a closed system, where no external forces act, the total angular momentum and energy cannot be created or destroyed, but can only be transferred or transformed between different forms.

2. How does conservation of angular momentum and energy apply in real-life situations?

Conservation of angular momentum and energy can be observed in various real-life situations, such as the motion of planets in the solar system, the spinning of a top, or the movement of a figure skater. In all of these cases, the total angular momentum and energy of the system remain constant, even as individual components may change.

3. What is the relationship between angular momentum and energy?

Angular momentum and energy are closely related in the sense that they are both conserved quantities in a closed system. In other words, any changes in angular momentum will result in corresponding changes in energy, and vice versa. This relationship is described by the law of conservation of energy.

4. How is conservation of angular momentum and energy calculated?

In order to calculate the conservation of angular momentum and energy, you need to know the initial and final values of angular momentum and energy, as well as any external forces acting on the system. Using the principles of conservation, you can then determine how these quantities are transferred or transformed within the system.

5. What happens if conservation of angular momentum and energy is violated?

In cases where conservation of angular momentum and energy is violated, it usually means that there are external forces acting on the system that are not accounted for. This can result in unexpected changes in the system, such as a change in the direction of rotation or a change in the energy state. Violations of conservation laws are extremely rare and are usually only observed in extreme conditions, such as in quantum mechanics or in the presence of strong gravitational fields.

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