
Exercises and problems in Physics
December 1998 
New experimental problem:
m. 202. Measure the fractions of incident light reflected,
transmitted and absorbed by a plastic ruler as a function of the angle of
incidence. (6 points)
New exercises:
FGy. 3204. The area of the point of a drawingpin is
approximately 0.1 mm^{2}, while that of its
head is 1.2 cm^{2}. What pressure has to be
exerted onto the head of the pin so that its point exerts a pressure one
thousand times greater than the atmospheric pressure onto the drawingboard?
What force is then applied? (3 points)
FGy. 3205. A consumer is connected to an accumulator across a series
resistor. When the resistance of the series resistor is increased from
100 to 150 , the power output of the accumulator decreases by 25 %. By
how many percents does the power consumption of the consumer with unchanged
resistance decrease? (4 points)
FGy. 3206. A floating areometer is calibrated for densities between
800 kg/m^{3} and 2000 kg/m^{3}, the distance between the signs corresponding to these
is 28 cm. What is the distance between the bottom sign and the sign
corresponding to 1400 kg/m^{3}? (4 points)
FGy. 3207. There is a small, closed, evacuated tube with heatinsulating walls in a large container containing He gas. The pressure of the He is 10^{5} Pa, its temperature is 300 K. The end of the small tube is opened for a little while and some He enters. What is the temperature of the He in the tube? (4 points)
New problems:
FF. 3208. Estimate the masses of the atmospheres of the
Earth and Venus. The pressure at the surface of Venus is 90 times greater than
that at the surface of the Earth, the mean diameter of Venus is 5 % smaller,
its density 10 % smaller than the respective data of the Earth. (4 points)
FF. 3209. There is 1 mole of He gas at a temperature of
0 ^{o}C in a vertically standing,
5 mhigh, heatinsulated cylinder with a crosssection of
0.01 m^{2}. A heatinsulating piston with a
mass of 4 kg, moving without friction, is held at the top of the
cylinder. The atmospheric pressure is 10^{5} Pa. How deep does the piston sink when released? (4 points)
FF. 3210. In principle, both the Earth and the Moon could be
charged electrically by shooting electrons from them into space. What would be
the minimum work required to form an electrostatic repulsion strong enough to
finally separate the Moon from the Earth? What would the majority of this work
be used for? (5 points)
FF. 3211. A rod of mass m and resistance R is placed
perpendicularly onto a pair of sufficiently long, parallel bars of negligible
resistance, at distance l from each other in a horizontal plane. The
rod can slide without friction parallelly to itself on the bars. One end of
the pair of bars is open, the other is connected to a capacitor of capacitance
C charged to p.d. U. The bars are in a homogeneous, vertical
magnetic field of induction B. After connecting the capacitor to the
bars, what is the final speed of the rod and what is the final p.d. across the
capacitor? What should the mass of the rod be to allow the maximum kinetic
energy? (5 points)
FF. 3212. White light is shone onto a 250 nmthick soap
membrane at an angle of incidence of 30^{o}. What
is the colour of the light reflected from the membrane? (5 points)
New advanced problem:
FN. 3213. What shape is the image made of a sphere of radius
r by a condensing lens of focal length f if the centre of the
sphere is the focus of the lens? (6 points)
