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Very Challenging Problem For Physics Prodigies Only!!?

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Old 07-26-2008, 10:01 AM
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Default Very Challenging Problem For Physics Prodigies Only!!?

1) A star with the mass (M = 2.0 x 10^30 kg) and size (R = 3.5 x 10^8 m) of our sun rotates once every 35.0 days. After undergoing gravitational collapse, the star forms a pulsar that is observed by astronomers to emit radio pulses every 0.200 s. By treating the neutron star as a solid sphere, deduce its radius.

2) What is the speed of a point on the equator of the neutron star? Your answer will be somewhat too large because a star cannot be accurately modeled as a solid sphere.
Some problem background information:

During most of its lifetime, a star maintains an equilibrium size in which the inward force of gravity on each atom is balanced by an outward pressure force due to the heat of the nuclear reactions in the core. But after all the hydrogen "fuel" is consumed by nuclear fusion, the pressure force drops and the star undergoes a gravitational collapse until it becomes a neutron star. In a neutron star, the electrons and protons of the atoms are squeezed together by gravity until they fuse into neutrons. Neutron stars spin very rapidly and emit intense pulses of radio and light waves, one pulse per rotation. These "pulsing stars" were discovered in the 1960s and are called pulsars.

I've been working on this problem for a while and still can't get it. I think you have to use the equation:

I1*w1 = I2*w2, with

I1-->inertia of star 2*(MR^2)/5
w1-->angular veloc. of star
I2--> inertia of neutron star
w2 --> angular veloc. of neutron star
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Old 07-26-2008, 10:44 AM
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r = 3.5 x 10^8 m x (35 day/.2 sec) = 556 m
circ = 2?r = 3,491 m
equatorial speed = 17,453 m/sec
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Old 07-26-2008, 10:44 AM
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1) L? = I??? = I? = L
L? = [(2/5)mR?²][2?/T?]
L = [(2/5)mR²][2?/T] = [(2/5)mR?²][2?/T?]
R²/T = R?²/T?
R = R??(T/T?) = (3.5x10^8m)?(0.2s / 3.02x10^6s)
= 9.0x10^4 m

2) v = (2?R)/T = 2?(9.0x10^4m)/(0.2s)
=2.83x10^6 m/s
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