# Series 4, Year 34

* Upload deadline: 23rd February 2021 11:59:59 PM, CET (local time in Czech Republic)*

### (3 points)1. two water drops

Suppose two water drops fall in quick succession from a water tap. How will their respective distance change with time? Neglect air resistance. **Bonus:** Do not neglect air resistance, make an estimate of all relevant parameters and find the distance of the water drops after a very long time.

Karel was hypnotized by water.

### (3 points)2. there is always another spring

Find the work needed to twist a spring from equilibrium position to an angular displacement of $\alpha =60\dg $. We are holding the spring in the twisted position with a torque $M=1{,} \mathrm{N\cdot m}$.

Dodo was hanging laundry on a string.

### (5 points)3. curved optics

Let's have a point source of light and a planar glass panel with a refractive index $n = 1,50$. In the foot of the perpendicular from the source to the panel there are wavefronts with a radius of curvature $R = 5,00 \mathrm{m}$ inside the glass. What is the real distance between the source and the panel?

### (8 points)4. ants

The ants have a peculiar way of keeping the anthill warm – they crawl out, let the sunlight heat them up, and then crawl back in, where the heat is transferred to the anthill. The anthill can be approximated as a cone of height $H=0{,}8 \mathrm{m}$ with base radius of $R_0=1,5 \mathrm{m}$. The walls are made of cellulose with heat conductivity $\lambda = 0{,}039 \mathrm{W\cdot m^{-1}\cdot K^{-1}}$ and are $2 \mathrm{cm}$ thick.

Assume that the entire heat exchange between the anthill and its surroundings (which have temperature $T\_o = 10 \mathrm{\C }$) is only mediated by the ants and by the conduction of heat through the walls, i.e. neglect the heat exchange with the ground. An ant weighs $m =5 \mathrm{mg}$ and has a specific heat capacity of approximately $4~000 J.kg^{-1}.K^{-1}$. How many ants, heated up to $T\_m = 37 \mathrm{\C }$, have to enter the anthill every second in order to keep the inner volume of the anthill at constant temperature of $T\_M = 20 \mathrm{\C }$?

Káťa missed biology classes.

### (8 points)5. Efchári-Goiteía

Efchári and Goiteía are two components of a double planet around recently arisen stellar system. They orbit around a common centre of mass on circular trajectories in the distance $a = 250 \cdot 10^{3} \mathrm{km}$. Efchári has the radius $R_1 = 4~300 km$, density $\rho _1 = 4~100 kg.m^{-3}$ and siderial period $T_1 = 14 \mathrm{h}$. Goiteía is smaller – it has the radius $R_2 = 3~800 km$, but it has a higher density $\rho _2 = 4~500 kg.m^{-3}$ and a shorter period $T_2 = 11 \mathrm{h}$. Rotation axes of both planets and the system are parallel. After several hundred years, the system transfers due to tidal forces into so-called tidal locking. Find the resulting difference in the period of the system, assuming that both bodies are homogeneous and roughly spherical.

Dodo keeps confusing Phobos and Deimos.

### (10 points)P. Fykos bird on vacation

How would aviation work on other planets (with atmosphere)? Consider mainly jet aircraft. Which planetary parameters would influence the aviation positively and which negatively, compared to Earth's?

Karel visited the museum of aviation in Košice.

### (13 points)E. breathtaking syringes

Find the magnitude of the friction force between the plunger and the barrel of a syringe.

Dano remembered his trip to Russia.

### (10 points)S. Oscillations of carbon dioxide

We will model the oscillations in the molecule of carbon dioxide. Carbon dioxide is a linear molecule, where carbon is placed in between the two oxygen atoms, with all three atoms lying on the same line. We will only consider oscillations along this line. Assume that the small displacements can be modelled by two springs, both with the spring constant $k$, each connecting the carbon atom to one of the oxygen atoms. Let mass of the carbon atom be $M$, and mass of the oxygen atom $m$.

Construct the set of equations describing the forces acting on the atoms for small displacements along the axis of the molecule. The molecule is symmetric under the exchange of certain atoms. Express this symmetry as a matrix acting on a vector of displacements, which you also need to define. Furthermore, determine the eigenvectors and eigenvalues of this symmetry matrix. The symmetry of the molecule is not complete – explain which degrees of freedom are not taken into account in this symmetry.

Continue by constructing a matrix equation describing the oscillations of the system. By introduction of the eigenvectors of the symmetry matrix, which are extended so that they include the degrees of freedom not constrained by the symmetry, determine the normal modes of the system. Determine frequency of these normal modes and sketch the directions of motion. What other modes could be present (still only consider motion along the axis of the molecule)? If there are any other modes you can think of, determine their frequency and direction.

Štěpán was thinking about molecules