# Fisika Study Center

## IPhO VI 1972 Bucharest Problem 3

This sample is taken from Problems of the 6th International Physics Olympiad (Bucharest, Romania in 1972) problem 3.

Problem 3 (Electricity)
A plane capacitor with rectangular plates is fixed in a vertical position having the lower part in contact with a dielectric liquid (fig. 3.1) Determine the height, h, of the liquid between the plates and explain the phenomenon. The capillarity effects are neglected. It is supposed that the distance between the plates is much smaller than the linear dimensions of the plates. ## IPhO V 1971 Sofia Problem 1

Theoretical problems
Question 1.
A triangular prism of mass M is placed one side on a frictionless horizontal plane as shown in Fig. 1. The other two sides are inclined with respect to the plane at angles α1 and α2 respectively. Two blocks of masses m2 and m2, connected by an inextensible thread, can slide without friction on the surface of the prism. The mass of the pulley, which supports the thread, is negligible.
• Express the acceleration a of the blocks relative to the prism in terms of the acceleration a0 of the prism.
• Find the acceleration ao of the prism in terms of quantities given and the acceleration g due to gravity.
• At what ratio m1/m2 the prism will be in equilibrium? ## IPhO IV 1970 Moscow Problem 2

The 4th IPhO took place in Moscow, Soviet Union in 1970. Here is a sample problem from the 4th IPhO Theoretical Problems.

Problem 2
A unit cell of a crystal of natrium chloride (common salt- NaCl) is a cube with the edge length a = 5.6⋅10-10 m (Fig.2). The black circles in the figure stand for the position of natrium atoms whereas the white ones are chlorine atoms. The entire crystal of common salt turns out to be a repetition of such unit cells. The relative atomic mass of natrium is 23 and that of chlorine is 35,5. The density of the common salt ρ = 2.22⋅103 kg/m3 . Find the mass of a hydrogen atom. ## IPhO 1969 Brno Problem 1

This problem sample below is taken from theoretical problems of third IphO that's conducted in Brno, 1967, problem 1.

Problem 1
1. Figure 1 shows a mechanical system consisting of three carts A, B and C of masses m1 =0.3 kg, m2 =0.2 kg and m 3 = 1.5 kg respectively. Carts B and A are connected by a light taut inelastic string which passes over a light smooth pulley attaches to the cart C as shown. For this problem, all resistive and frictional forces may be ignored as may the moments of inertia of the pulley and of the wheels of all three carts. Take the acceleration due to gravity g to be 9.81 m s−2. 1. A horizontal force is now applied to cart C as shown. The size of is such that carts A and B remain at rest relative to cart C.
a) Find the tension in the string connecting carts A and B.
b) Determine the magnitude of .

2. Later cart C is held stationary, while carts A and B are released from rest.
a) Determine the accelerations of carts A and B.
b) Calculate also the tension in the string.

Solution
Case 1. The force has so big magnitude that the carts A and B remain at the rest with respect to the cart C, i.e. they are moving with the same acceleration as the cart C is. Let , and denote forces acting on particular carts as shown in the Figure 2 and let us write the equations of motion for the carts A and B and also for whole mechanical system. Note that certain internal forces (viz. normal reactions) are not shown. The cart B is moving in the coordinate system Oxy with an acceleration ax. The only force acting on the cart B is the force , thus

T2 = m2 ax (1)

Since and denote tensions in the same cord, their magnitudes satisfy

T1 = T2

The forces and act on the cart A in the direction of the y-axis. Since, according to condition 1, the carts A and B are at rest with respect to the cart C, the acceleration in the direction of the y-axis equals to zero, ay = 0, which yields

Consequently

T2 = m2 g . (2)

So the motion of the whole mechanical system is described by the equation

F = ( m1 + m2 + m3 ) ax , (3)

because forces between the carts A and C and also between the carts B and C are internal forces with respect to the systemof all three bodies. Let us remark here that also the tension is the internal force with respect to the system of all bodies, as can be easily seen from the analysis of forces acting on the pulley. From equations (1) and (2) we obtain Substituting the last result to (3) we arrive at Numerical solution: Case 2. If the cart C is immovable then the cart Amoves with an accelera- tion ay and the cart Bwith an acceleration ax. Since the cord is inextensible (i.e. it cannot lengthen), the equality

ax =−ay =a

holds true. Then the equations of motion for the carts A, respectively B, can be written in following form

T1 =G1−m1a, (4)
T2 =m2a. (5)

The magnitudes of the tensions in the cord again satisfy

T1 =T2 . (6)

The equalities (4), (5) and (6) immediately yield

(m1+m2)a=m1 g .

Using the last result we can calculate Numerical results: Sources/Literatures :
-The Olympic home page www.jyu.fi/ipho

## IPhO 1968 Budapest Problem 3

This problem is taken from the second IphO Budapest, Hungary, Theoretical problems, conducted in 1968.

Problem 3
Parallel light rays are falling on the plane surface of a semi-cylinder made of glass, at an angle of 45°, in such a plane which is perpendicular to the axis of the semi-cylinder (Fig. 4). (Index of refraction is √2.) Where are the rays emerging out of the cylindrical surface?  ### More Articles...

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