# 5. Series 32. Year

### 1. urban walk

Matěj walks across the street with constant velocity. Every 7 minutes a tram going in opposite direction passes, while every 10 minutes a tram going in his direction passes. We assume that trams ride in both directions with the same frequency. What is the frequency?

Matěj went for a walk

### 2. warm reachability into the ball

Imagine you have subcooled homogeneous metal ball that you have just taken out of a freezer set to very low temperature. You want to find out how fast the ball temperature will increase if you put it in a warm room. It would be a university-level problem. Because of that, we made it easier for you. We ask about how deep into the ball will the „warm area“ reach. You can estimate it using dimensional analysis. We know relevant parameters of the ball - its density $\left [ \rho \right ] = \jd {kg.m^{-3}}$, specific heat capacity $\left [c\right ] = \jd {J.kg^{-1}.K^{-1}}$, thermal conductivity of the ball $\left [ \lambda \right ] = \jd {W.m^{-1}.K^{-1}}$ and we are interested in dependence on time $\left [t\right ] = \jd {s}$.

Karel inspired himself by a problem from Eötvös Competition.

### 3. border

Imagine an aquarium in the shape of a cube with edge length $a = 1 \mathrm{m}$, which is separated into two parts via a vertical partition perpendicular to sides of the aquarium. Let us assume that the partition can move in the direction perpendicular to the plane of the partition, but it is fixed in other directions. Also, it can't rotate. We pour $V_1 = 200 \mathrm{\ell }$ of water (density $\rho \_v = 1 000 \mathrm{kg\cdot m^{-3}}$) into the first part and $V_2 = 230 \mathrm{\ell }$ of oil (density $\rho \_o = 900 \mathrm{kg\cdot m^{-3}}$) into the second part. In which position is the partition in mechanical equilibrium? In what height will be the surfaces of the liquids?

Bonus: Find frequency of small oscillations of the partition. Assume, that mass of the partition is $m = 10 \mathrm{oz}$ and the liquids move without friction or viscosity.

Michal cleaned an aquarium.

### 4. splash

Consider a free water droplet with radius of $R$. We start to charge the drop slowly. Find the magnitude of the charge $Q$ the drop needs to splash.

### 5. bouncing ball

We spin a rigid ball in the air with angular velocity $\omega$ high enough parallel with the ground. After that we let the ball fall from height $h_0$ onto a horizontal surface. It bounces back from the surface to height $h_1$ and falls to a slightly different spot than the initial spot of fall. Determine the distance between those two spots of fall onto ground, given the coefficient of friction $f$ between the ball and the ground is small enough.

Matej observed Fykos birds playing with a ball

### P. 1 second problems

Suggest several ways to slow down the Earth so that we would not have to add the leap second to certain years. How much would it cost?

### E. thirty centimeters tone

Everyone has ever tried out of boredom to strum on a long ruler sticking out of the edge of a school-desk. Choose the right model of frequency versus the part of the length of the ruler which is sticking out and prove it experimentally. Also, describe other properties of the ruler.

Note: Allow vibration only for outsticking part of the ruler by fixing its position above the table.

Michal K. found a ruler

### S. heavenly-mechanic

1. Consider a cosmic body with the mass of five Suns surrounded by a spherically symmetrical homogenous gas cloud with the mass of two Suns and radius $1 \mathrm{ly}$. The cloud starts to collapse into the central cosmic body. Neglect the mutual interactions of particles in the cloud (excluding gravity). Find how long it will take for the whole cloud to collapse into the central body. Do not solve this problem numerically.
2. Show that the Binet equation solves following the differential equation, which describes the motion of a mass point of mass $m$ in a spherically symmetrical central-force field. $\begin{equation*} \dot {r}^2 = \frac {2}{m} $E - V(r) - \frac {l^2}{2mr^2}$ \end {equation*}$ Where $r$ is the length of the radius vector, $E$ is the total energy, $l$ is angular momentum, and $V(r)$ is the potential energy of the mass point.
3. Set up the Lagrangian for the Sun-Earth-Moon system. Assume the Sun to be motionless. The Earth and the Moon move under the influence of both the Sun and each other. While setting up the Lagrangian, think about whether you are using an appropriate number of generalized coordinates. 