# 4th Round of 23th year

Series

### 1. ... Green or Blue?( points)

The main source of atmospheric oxygen is photosynthesising plants. Imagine they become extinct. How long would the world’s oxygen stock last given that the consumption of the planet stays unchanged? The pieces of information that you may need can be found on the internet.

### 2. ... Fever( points)

Returning home from an observatory, watching the sunrise, Janap discovered an easy way to calculate the temperature of the Sun. We do give away that the Earth is an absolute black body with a temperature of 0° C.

### 3. ... Lethal Carousel(0 points)

There is a weight of mass $m$ tied by a cord of length $l$ to a stake of radius $r$ put in the ground. The cord is taut and the weight lies on the ground. Lukas kicks the weight orthogonal to the cord so that it has a velocity of $v$. The cord starts to wrap around the stake. Calculate the necessary change in the coefficient of Coulomb friction between the weight and the ground in function of weight’s distance from the stake so that the velocity remains constant.

### 4. ... Terka Jumps( points)

Terka jumps from one-metre high wall. At the beginning, she has her arms stretched and raised above her head. She lowers her arms during the fall. How much does it change her velocity at the moment of hitting the ground? First, make a qualified guess as to mass, acceleration, velocity of Terka’s arms as well as other body parameters. Second, solve the problem.

### P. ... Resistance( points)

Do think of resistance of a flowing electrolyte. Is it dependent on the direction of the flow of the electric current? Try to evaluate the difference if there is any.

### E. ... MacGyver and Thermometer( points)

Construct a thermometer using the materials available at home. Base the calibration of the degree scale on well-known temperatures. Do not forget to attach a photograph of the end result of your efforts.

### S. ... Maxwell( points)

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