Physics, 12.11.2019 04:31 mathscience9301

Suppose a star the size of our sun, but of mass 8.0 times as great, were rotating at a speed of 1.0 revolution every 12 days. if it were to undergo gravitational collapse to a neutron star of radius 12 km, losing 3/4 of its mass in the process, what would its rotation speed be? assume the star is a uniform sphere at all times.

(a) assume that the thrown-off mass carries off no angular momentum, in rev/s

(b) assume that the thrown-off mass carries off its proportional share (3/4) of the initial angular momentum, in rev/s

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You have two different phases of the star: 1 the Sun-like phase and 2 the neutron star phase. The given quantities are:
r₁ = r(Sun) = 695700km
m₁ = 8 Msun
f₁ = 1 rev / 12 days
m₂ = $\frac{1}{4}$ ·m₁

First thing, you need to transform the frequency in units of revolution/seconds
f₁ = 1 rev / (12·24·60·60) = 1 rev / 1036800 s

and then into angular velocity through the formula
ω₁ = 2πf = 6.06E-6 rad/s

a) If the angular momentum stays the same: L₁ = L₂
where L = I·ω
and the momentum of inertia I is given by I = m·r²

Therefore, substituting we have:
m₁·r₁²·ω₁ = m₂·r₂²·ω₂

And we can find:

ω₂ = $\frac{r1²w1}{0.25r2²}$

(remember m₂= $\frac{1}{4}$ ·m₁ so we can cancel out the two m₁)

We obtain:
ω₂ = (695700²·6.06E-6)/(0.25·12²) = 81473.1 rad/s

we can transform it back into frequency:
f₂ = ω₂/2π = 1 rev / 12967 s = 7.7e-5 rev/s

b) If L₂ = $\frac{1}{4}$ ·L₁
we do expect an angular velocity 4 times smaller.
Using the same formulas as above:

$\frac{1}{4}$ ·m₁·r₁²·ω₁ = $\frac{1}{4}$ ·m₁·r₂²·ω₂

ω₂ = $\frac{r1²w1}{r2²}$ =
= [(695700²·6.06E-6)(12²) = 20368 rad/s

and

f₂ = 1 rev / 3242 s = 3.08e-4 rev/s

Answer from: Quest

another example of kinetic energy would be,

kinetic energy > moving energy

answer > letter choice (c), a hammer falling off a table.

hope that ! : )

Answer from: Quest

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

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