# Problems in 4th round

Deadline for posting: February 27, 2017

A booklet with all the problems and the actual chapter of the series (in Czech):

## Problem IV . 1 … princess Point on point (3 points)

It is not easy for story characters if they want to know when they appear on screen. Fortunately, today's technology makes it easier, take for example princess Point from a story with six chapters. All chapters have the same length with the height of every chapter on Karel's display being 1200 pixels (but the display itself can only show 900 pixels at any one time). When reading Karel scrolls continuously and reads with constant speed. After three minutes of reading, Point passed the first end of the slider in the scrollbar and after seven minutes she passed the other end. Which chapter does Point appear in?

Note: The ratio of the height of the slider and the height of the display is the same as the ratio of the height of the display to the height of the entire story.

Solution:

## Problem IV . 2 … jerky pendulum (3 points)

It is well known fact that to make a train ride as comfortable as possible, when accelerating or braking, the acceleration needs to change as little as possible. It is therefore good practice when a train starts with small, constant change of acceleration. The change of acceleration is called a jerk. Determine how does the equilibrium position of a pendulum (the angle with the vertical φ). Denoting the length of the pendulum l, the train starts with a constant jerk k ( k = Δa ⁄ Δt , where a denotes acceleration) and the train is on Earth with acceleration due to gravity g.

Bonus: Derive the equations of motion and solve them numerically for φ(0) = 0 and dt(0)= 0 for various values of k.

Solution:

## Problem IV . 3 … bicone (8 points)

Consider a wooden construction (shown in the picture) with its foundation in the shape of an isosceles triangle with a base of length c = 35 cm. The height of the legs of this triangle increases towards the base with an angle α = 2 ° with the flat ground. A bicone with opening angle φ = 40 ° and height 2h = 40 cm is placed in the vertex opposite the base, where the triangle has angle β = 70 °. When released, the bicone starts rolling „uphill“, that is in the direction of the increasing height of the legs.

• Explain why the bicone can roll uphill.
• Find the dependence of the position of the centre of mass on the distance covered by the bicone?
• What is the velocity of the bicone in the instant before it hits the base.
• How many turns will the bicone make before it hits the base?
The bicone is initially placed horizontally on the construction in such a way that its centre of mass is exactly above the vertex opposite the base.

Solution:

## Problem IV . 4 … heat engine (8 points)

Consider a heat engine filled with a diatomic gas. This engine works thanks to a cycle ABCDEFA as shown in the picture. The 6 processes that make up the cycle are

• A  →  B - isobaric heating from a state 4p0 and V0 (let us denote temperature at A 4T0) to a state with volume 3V0,
• B  →  C - isothermic expansion to volume 4V0,
• C  →  D - isochoric cooling to pressure p0,
• D  →  E - isobaric cooling to volume 2V0,
• E  →  F - isothermic compression to volume V0,
• F  →  A - isochoric heating to pressure 4p0.

Determine the remaining state variables in B, C, D, E, and F, the maximal and the minimal temperature of the ideal gas during the process (as a multiple of T0), heat received and lost by the gas in each process, and the overall efficiency of the engine. Compare this efficiency with that of a Carnot engine working between the same minimal and maximal temperatures. Assume for simplicity that the molar amount of the gas does not change and there are no chemical changes during the cycle. A sketch can be seen in figure.

Bonus: Do the same for a much simpler „square“ cycle, ABCDA, where the gas starts in a state p0, V0 and T0 and izochorically heats up to 4p0, isobarically heats up and expands to 4V0, isochorically cools down to p0 and isobarically cools down to V0. Compare the efficiency of these two heat engines and suggest which one is better.

Solution:

## Problem IV . 5 … weird atmosphere (9 points)

Have you ever seen such a weird atmosphere? Up to a certain height the speed of light inside it is constant, v0, but from that certain height the speed of light starts increasing linearly as v ( Δh ) = v0 + kΔh. At one point, exactly at the height where the speed of light starts changing, light beams are sent upward in all directions. Show that all these beams move along circular arcs and determine the radii of these arcs. Also find out the distance between the place where the the light was emitted and the point where the beams return to the original height.

Solution:

## Problem IV . P … statistician's daily bread (9 points)

We've all been there, you spread some honey or some preserve on a slice of bread, take a bite, and suddenly, the spread drips through a hole and lands right on your hand. Determine how does the probability that there is a hole straight through a slice of bread depend on its thickness. The model of how does the dough rise is left up to you. (For example, evenly distributed bubbles with an exponential distribution of radii is a good model).

Solution:

## Problem IV . Exp … Mikulas's egg (11 points)

Measure the surface area of a bird's (e.g. chicken's) egg.

Solution: