# AY 102 Lecture Notes - Lecture 6: Tidal Locking, Tidal Force, Interstellar Cloud

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29 Jan 2018
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AY Lecture 6 1-29-18
Conservation of Angular Momentum, Effects of Gravity
Angular momentum
- A property of an object that is rotating or moving along a curved path (like an orbit)
- Example: Earth has both rotational angular momentum and orbital angular momentum.
These can be described in physical ways and are characterized by numerical values
Conservation of angular momentum
- The total angular momentum of an isolated system os conserved
- Objects in an isolated system may exchange angular momentum. If they do, one must
gain angular momentum while the other must lose angular momentum, in order to keep
the total angular momentum constant. The result could be a change in orbit properties
or rotational periods
- Only an encounter with an external object like a passing star could change the angular
momentum of a system
- In astronomy, conservation of angular momentum plays the following roles
If an object that initially is large and slowly rotating starts to shrink, it will spin
faster
If the shrinking object is tenuous (low density) due to particle collisions it will
flatten into a disk shape as the rotation rate increases (less gravitational energy
and more thermal energy)
- Many phenomena in astronomy can be linked to the conservation of angular
momentum
Rapid rotation of a neutron star (collapsed core of a dead, massive star)
Planets orbit the sun in one plane
Disk-shaped galaxies
Accretion disks in close binary systems
Neto’s las of otio
- In the absence of any external forces, an object at rest remains at rest and an object in
uniform motion remains in uniform motion
No change in velocity = no force
- Forces accelerate objects. Amount of acceleration depends on force strength
- For every force, there is always an equal and opposite reaction force
Example of conservation of energy and angular momentum at work
- Celestial objects are believed to form by gravitational collapse of a large interstellar
cloud of gas and dust
- Gravitational collapse
A natural process whereby the inward pull of gravity within an interstellar cloud
overcomes the outward pressure in the cloud due to random thermal motions
The hotter the cloud, the greater the outward pressure, and less likely collapse
would occur
Gravitational collapse will occur only in the densest, coldest parts of the cloud
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