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# Newton's Law of Gravity

The last of the fundamental forces to evade unification, deep mysteries remain in our understanding of gravity. Get started at the beginning with the Newtonian description of gravity.

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**Details and Assumptions:**

- The radius of Mars is 53% the radius of Earth.
- The mass of Mars is 11% the mass of Earth.
- When climbing on Mars, he has an air supply with the same composition as the Earth's air.

Consider a small black ball of radius \(R\) and density \(\rho= 1~\frac{\mbox{g}}{\mbox{cm}^{3}}\) located at a certain distance above the surface of the Sun. For what radius \(R\) **in micrometers** is the gravitational attraction of the Sun counterbalanced by the radiation force?

Assume that the black ball absorbs all the incident light and that the total power radiated by the Sun is \(P=4\times 10^{26}~\mbox{W}\).

**Details and assumptions**

The mass of the Sun is \(M_{s}=2\times 10^{30}~\mbox{kg}\). The universal constant of gravitation is \(G=6.67\times 10^{-11}~\text{m}^{3}\text{kg}^{-1} \text{s}^{-2}\) and the speed of light \(c=3\times 10^{8}~\mbox{m/s}\).

**Details and assumptions**

Input your answer in Joules

Model the Earth as sphere of uniform volume density composed of tiny neutral pieces of mass.

The radius of the Earth is \(R_e=6371km\).

The mass of the Earth is \(M_e = 5.972\times 10^{24} kg \)

Newton's gravitational constant is \(G=6.673\times10^{-11} m^3 kg^{-1} s^{-2}\)

Ignore air resistance and take the acceleration due to gravity as \(g=-10 \text{ m/s}^2\).

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