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dalvyx [7]
2 years ago
13

(28 points) In a little over 5 billion years, our star will slough off ~20% of its mass and collapse to a white dwarf star of ra

dius 8,000 km. We will model it as a sphere. What will its angular momentum be in terms of its current angular momentum? What will its rotation period be in terms of its current period? b) (12 points) What will its rotational kinetic energy be in terms of its current rotational kinetic energy?
Physics
1 answer:
Sedbober [7]2 years ago
8 0

Answer:

The angular momentum is same as it was before.

The rotation period is 1.058*10^{-4} times the original period.

The rotational kinetic energy is 9452 times greater.

Explanation:

The angular momentum L of a rigid body is

L = I\omega,

where I is the moment of inertia and \omega is the angular velocity.

Now, the law of conservation of momentum demands that

I_1\omega_1 = I_2\omega_2,

in words this means the angular momentum before must equal the angular momentum after.

Let us call M the mass, R the radius, and \omega_1 the angular velocity of the sun before it becomes a white dwarf, then its linear momentum is

I_1\omega_1 = \dfrac{2}{5}MR^2 \omega_1     (Remember for a solid sphere I = \dfrac{2}{5} MR^2)

After it has become a white dwarf, the suns mass is 80% of what it had before <em>(went off by 20%),</em> and its radius has become 0.0115% its initial value <em>(8000 km is 0.0115% of the radius of the sun ); </em>therefore, the angular momentum is

I_2\omega_2 = \dfrac{2}{5} (0.8M)(0.0115R)^2 \omega_2

which must be equal to the angular momentum it had before; therefore

\dfrac{2}{5}MR^2 \omega_1 = \dfrac{2}{5} (0.8M)(0.0115R)^2 \omega_2

which we solve for \omega_2:

MR^2 \omega_1 = (0.8M)(0.0115R)^2 \omega_2

MR^2 \omega_1 = (0.8)(0.0115)^2 MR^2\omega_2

\omega_1 = (0.8)(0.0115)^2 \omega_2

\omega_2= \dfrac{\omega_1}{(0.8)(0.0115)^2 }

\boxed{ \omega_2 = 9452\omega_1.}

which is about whopping 9500 times larger than initial angular velocity!!

Now the rotation period T is

T_2 = \dfrac{2\pi}{\omega_2}

T = \dfrac{2\pi}{ 9452\omega_1}= 1.058*10^{-4} (\dfrac{2\pi}{ \omega_1})

since \dfrac{2\pi}{ \omega_1} =T_1

\boxed{T_2 = 1.058*10^{-4} T_1}

Similarly, the rotation kinetic energy will be

K_2 = \dfrac{1}{2}I_2\omega_2^2

K_2 = \dfrac{1}{2}*\dfrac{2}{5} (0.8M)(0.0115R)^2 ( 9452\omega_1})^2

K_2 =0.8*0.0115^2*9452^2 [\dfrac{1}{2}*\dfrac{2}{5} mR^2w_1^2]

\boxed{K_2 =9452 K_1}

which is about 9500 times larger than initial rotational kinetic energy!

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2. A student places an object with a mass of m on a disk at a position r from the center of the disk. The student starts rotatin
ycow [4]

Answer:

\frac{0.065}{r}

Explanation:

The maximum velocity of an object moving in a curve beyond which it will slide off the curve is given by the relationship in equation (1);

v=\sqrt{\mu gr}....................(1)

where \mu is the coefficient of friction between the object and the surface of the curve, g is acceleration due to gravity and r is the radius of the curve.

Given;

v = 0.8m/s

g = 9.81m/s^2

r = ?

\mu=?

In order to solve for \mu, we can simply make it the subject of formula from equation (1) as follows;

v^2=\mu gr\\hence\\\mu=\frac{v^2}{gr}.................(2)

since we were not given the value of r, we can just substitute other known values, then solve and leave the answer in terms of r.

Therefore;

\mu=\frac{0.8^2}{9.81r}

\mu=\frac{0.64}{9.81r}\\\\\mu=\frac{0.065}{r}

8 0
3 years ago
Which of the following is not part of the role of a forester?
Anastasy [175]
<span>the one that is not part of the role of a forester is: B/ prevent removal of timber Actually, it is okay to remove timbers and use it for production as long as we do it in a responsible manner. The role of a forester is to make sure that all the people that use the timbers do it within the pre-determined guideline</span>
5 0
3 years ago
Read 2 more answers
(i) Calculate the energy, in electron volts, of a photon whose frequency is (a) 620 THz}, (b) 3.10GHz , and(c) 46.0 MHz
DedPeter [7]

The energy in electron volts of the photons that has the following frequencies is as follows:

  1. 620 THz = 2.564eV
  2. 3.10GHz = 1.28 × 10-⁵eV
  3. 46.0 MHz = 1.902 × 10-⁷eV

<h3>How to calculate energy?</h3>

The energy of a photon can be calculated using the following formula:

E = hf

Where;

  • E = energy
  • h = Planck's constant (6.626 × 10-³⁴ J/s)
  • f = frequency

First, we convert the frequencies to hertz as follows;

  • 620THz = 6.2 × 10¹⁴Hz
  • 3.10GHz = 3.1 × 10⁹Hz
  • 46.0MHz = 4.6 × 10⁷Hz

  1. E = 6.626 × 10-³⁴ × 6.2 × 10¹⁴ = 2.564eV
  2. E = 6.626 × 10-³⁴ × 3.1 × 10⁹ = 1.28 × 10-⁵eV
  3. E = 6.626 × 10-³⁴ × 4.6 × 10⁷ = 1.902 × 10-⁷eV

Learn more about energy of a photon at: brainly.com/question/2393994

#SPJ1

6 0
1 year ago
If the unit for force is f, the unit for velocity is v and the unit for time t, then the unit for momentum is
ASHA 777 [7]
The answer is m I believe
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Amy counts the wave crests traveling down a stretched string.Five wave crests pass Amy in two seconds.What is the frequency of t
Verdich [7]
The frequency of a wave is a measure of the number of waves that passes through a point per unit of time. It has SI units of s^-1. It is also equivalent to Hertz (Hz). We calculate the frequency of the wave described as follows:

Frequency = 5 waves / 2 s = 2.5 / s or 2.5 Hz
4 0
3 years ago
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