Magyar Information Contest Journal Articles

# KöMaL Problems in Physics, March 2005

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## Problems with sign 'M'

Deadline expired on 11 April 2005.

M. 259. Prepare sodium chloride solutions of various mass percentages and immerse different metal electrodes in them (Fe, Cu, Al etc.). Measure the initial electromotive forces and their temporal variations.

(6 pont)

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## Problems with sign 'P'

Deadline expired on 11 April 2005.

P. 3782. In a movie we see that the hero jumps off a bridge into a small boat proceeding with a constant velocity. The horizontal velocity of the boat does not change when the hero jumps in it. Is this a trick picture or could it be a real jump?

(3 pont)

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P. 3783. We throw a solid iron ball of a mass of 0.2 kg previously heated in blue flame, into 0.15 l water of a temperature of 13 oC contained in a copper calorimeter. The common temperature settles at 55 oC. What color was the iron ball before it was thrown into the water.

(4 pont)

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P. 3784. In each of two big tanks there is 100 l of water of a temperature of 10 oC in one and 30 oC in the other. From the warmer tank we pour 1 l of water into the colder one. After blending we pour 1 l of water back. How many times are we to repeat this process to reach at least 5 oC temperature change in both tanks? (Neglect losses.)

(5 pont)

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P. 3785. We cool down ideal gas of molar heat capacity of CV=20 J/(mol.K) at a constant pressure of 100 kPa. The initial volume of 1 m3 of the gas is halved in the course of cooling. How much heat is emitted by the gas during cooling? What kind of gas can it be?

(4 pont)

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P. 3786. J. F. Jolly (1809-1884), a Munich teacher of physics, measured with his very sensitive self-made balance that at a depth of 5.3 m the weight of a body is greater by one and a half millionth than that on the surface. How can that be reconciled with Newton's gravitational law?

(5 pont)

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P. 3787. We lean a cubic shaped box against the wall as shown in the figure so that it just does not roll over. The box is filled with water exactly to its half and a wooden ball is floating on the water surface. The ball is started from the wall and is approaching the other edge. What will happen?

(4 pont)

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P. 3788. Two uniform disks can slide frictionless on the horizontal ground. The friction coefficient of the disks between each other is . One of the disks is at rest, the other is pushed against it with a velocity of v. What can the velocities of the disks be after the collision?

(5 pont)

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P. 3789. In the heroic age of radio every night at the end of the broadcast the announcer warned the audience: Please don't forget to earth your aerials.'' Why was it important and how could that be done?

(4 pont)

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P. 3790. An idealized model of a light source radiates uniformly between wavelengths of 400 nm and 800 nm with a power of 100 W. Estimate how many photons a second are emitted from it.

(4 pont)

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P. 3791. Can a reference frame be chosen in a way that only either the magnetic or the electric field of an electron beam of constant velocity can be detected?

(4 pont)

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