# 1. Series 30. Year

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### (3 points)1. With rum, or without?

Three substances: water, steel and rum are put in a pot, which effectively doesn't convect any heat. The water has mass $m_{v}=0,5\;\mathrm{kg}$, temperature $t_{v}=90°F$ and specific heat capacity $c_{v}=1kcal\cdot \;\mathrm{kg}^{-1}\cdot K^{-1}$. The steel is in form of a cylinder, that has mass $m_{o}=200g$, temperature $t_{o}=60\;\mathrm{°C}$ and specific heat capacity $c_{o}=0,260kJ\cdot \;\mathrm{kg}^{-1}\cdot °F^{-1}$. Rum has mass $m_{r}=100000mg$, temperature $t_{r}=270K$ and specific heat capacity $c_{r}=3,5J\cdot g^{-1}\cdot \;\mathrm{°C}^{-1}$. What will be the temperature (in degrees centigrade) of the system when it reaches balance?

Lukáš Mirkovi sděloval svoje zkušenosti s alkoholem.

### (3 points)2. Breaking

Petr likes to ride a bike on a flat road with a speed $v=10\;\mathrm{m}\cdot \mathrm{s}^{-1}$, and his smart bike tells him that his average power is $P=100W$. After an accident, his breaks are bent and they now persistently act on a wheel with a friction force $F_{t}=20N$ near the circumference. For how long ($t′)$ he needs to cycle now (with the same speed $v)$, to do the same amount of work as before, in time $t?$

Petr si uvědomil výhody zaseknuté brzdy.

### (5 points)3. Bouncy bounce

Let's have an ideal bouncy ball (with coefficient of restitution equal to one and negligible dimensions). We throw this bouncy ball down an infinitely long staircase, where a step has height $h$ and length $l$. The bounces happen without any influence from friction. Describe the relation between the maximum height reached (measured from the first step) after $n-th$ bounce and the initial parameters.

Lubošek potkal v městské dopravě Mikuláše.

### (6 points)4. The world is tilted

An observer is on a ship in the open sea, in the height $h$ above the sea surface. There is a horizontal railing in the distance $d$ from him, in such a position, that when he looks directly, perpendicularly at it, the bottom edge of the railings touches the horizon. However, when he looks at the part of the railing that is distance $l$ to the left of the original point, the horizon appears to be $s±s_{s}$ lower. What is the radius of the Earth.

Lubošek trpí mořskou nemocí.

### (7 points)5. On a walk

Katka decided to go for a walk with her pet rat. They arrived on a flat meadow and when the rat was at a distance $x_{1}=50\;\mathrm{m}$ from Katka, she threw him a ball with the speed $v_{0}=25\;\mathrm{m}\cdot \mathrm{s}^{-1}$ and an angle of elevation $α_{0}$. In that moment, he started running towards her with the speed $v_{1}=5\;\mathrm{m}\cdot \mathrm{s}^{-1}$. Find a general formula for an angle $φ$ as a function of time, where the angle $φ(t)$ is the angle between the horizontal plane and the line between the rat and the ball. Draw this function into a graph and, based on the graph, determine, whether it's possible for the ball to obscure the Sun for the rat, when the Sun is situated $φ_{0}=50°$ above the horizon in the direction of the running rat. Use the acceleration due to gravity $g=9.81\;\mathrm{m}\cdot \mathrm{s}^{-2}$ and for simplicity imagine we are throwing the ball from a zero height.

Mirek pozoroval, co se děje v trávě.

### (8 points)P. The sky is falling

Did you ever think about, why the clouds simply don't fall down, when they consist of water, which is much denser than air? The raindrops fall to the ground in minutes, so why not clouds? Try to physically explain this. Support all of your claims with calculations.

Mirek se zadíval na nebe a dostal strach.

### (12 points)E. Pechschnitte

Does bread always falls on the side that has the spread on it? Explore this Murphy's law experimentally with emphasis on statistics! Does it depend on the dimensions of the slice, or the composition and the thickness of the spread? Try to explain the experimental results with a theory. Use a sandwich bread.

Terka má stůl ve špatné výšce.