# 5. Series 21. Year

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### 1. caution, do not suffocate

In this set of questions we will visit the world of science fiction. Space is filled with many space ships – one of them is Rama (A. C. Clarke: Rendezvous with Rama, Rama II, The Garden of Rama, Rama Revealed). Rama is large intergalactic ship constructed by extra-terrestrial civilisation and came to Solar system. Lets discuss some challenges, which are facing us.

Rama has a shape of cylinder of length 54 km and internal diameter 16 km. It is filled inside by air. Rama has its own gravity, which comes from rotating around its axis once per every four minutes. On inside surface the pressure of air is 1 bar.

The entrance to Rama is an orifice in the centre of one of its basis. Before removing the space suit, lets think, if on axis is breathable air, what is its density comparing to density on cylinder surface. Assume the temperature is the same everywhere.

Martin Formánek a Kuba Benda

### 2. question of survival

The entrance and the internal surface are connected by a ladder. You have already descended one kilometer, when you slipped and are falling down. What will be the speed you will fall down on Rama surface? How long it will take? Is there a chance to survive?

Martin Formánek

### 3. staircase from the sky

The ladder is only 2 km long going to the platform, from which you have to take a stairs, which goes with long arch above the countryside. The staircase has very special shape: for each step you need the same amount of mechanical work. Calculate, how the step height depends on the distance from the Rama axis, if the length of stairs is constant. What shape is the arch?

Martin Formánek

### 4. Sun can

Rama travels between the stars in such way, that one half of time is constantly accelerating and second half of time is slowing down. Currently the Rama is on parabolic trajectory around the Sun with peak on Earth orbit. It gets energy from Sun light. Its surface absorbs 80 % of incident energy. Will it get enough energy to get to Sirius, which is in distance of 12 light years in less than 24 years?

Nadhodil Jakub Benda

### P. Rama-quake

Finally you got onto the surface of Rama. Suddenly it started to shake and you think the rotation speed has changed. How you can measure the rotation speed? Can you find more than one experiment?

Martin Formánek

### E. milestones of life in Rama

Will have the Rama (brand name for margarine) another physical properties after you will melt it and let it solidify back? We suggest to measure density, viscosity and colour.

Vytlačil Marek Pechal.

### S. sequence, hot orifice and white dwarf

- Derive Taylor expansion of exponential and for $x=1$ graphically show sequence of partial sums of series \sum_{$k=1}^{∞}1⁄k!$ with series { ( 1 − 1 ⁄ $n)^{n}}_{n=1,2,\ldots}$.

Using the same method compare series { ( 1 − 1 ⁄ $n)^{n}}_{n=1,2,\ldots}$ and series of partial sums of series \sum_{$k=1}^{∞}x^{k}⁄k!$, therefore series {\sum_{$k=1}^{n}x^{k}⁄k!}_{n=1,2,\ldots}$, now for $x=-1$.

- The second task is to find concentration of electrons and positrons on temperature with total charge $Q=0$ in empty and closed cavity (you can choose value of $Q.)$ Further calculate dependence of ration of internal energy $U_{e}$ of electrons and positrons to the total internal energy of the system $U$ (e.g. the sum of energy of electromagnetic radiation and particles) on temperature and find value of temperature related to some prominent temperature and ratios (e.g. 3 ⁄ 4, 1 ⁄ 2, 1 ⁄ 4, …; can this ratio be of all values?).

Put your results into a graph – you can try also in 3-dimensions.

To get the calculation simplified, it could help to take some unit-less entity (e.g. $βE_{0}$ instead of $β$ etc.).

- Solve the system of differential equations for $M(r)$ and $ρ(r)$ in model of white dwarf for several well chosen values of $ρ(0)$ and for every value find the value which it get close $M(r)$ at

$r→∞$. This is probably equal to the mass of the whole star. Try to find the dependence of total weight on $ρ(0)$ and find its upper limit. Compare the result with the upper limit of mass for white dwarf (you will find it in literature or internet). Assume, that the star consists from helium.

Zadali autoři seriálu Marek Pechal a Lukáš Stříteský.