# Exercises and problems in Physics

October 1997

## New experimental problem:

**m. 191.**
Measure the specific heat of the material of some coin.

## New exercises:

**FGy. 3089.**
Why is less force required to slice a salami if the
knife is moved forward and backward while cutting than if it is only
pressed downward?

**FGy. 3090.**
The battleship called VASA sunk near Stockholm was
lifted out by means of steel tanks. The volume of each tank was
1500 m^{3}, its mass was 300 tons. What lifting force could
such a tank exert?

**FGy. 3091.**
A perforated body can move without friction on the
vertical rod shown in the figure. When the body is carefully let onto
a sufficiently long spring, the spring is compressed by 1 cm. From
what height *h* should the body be dropped so that the largest
compression of the spring is 8 cm?

**FGy. 3092.**
A thin tube is pushed into a thicker one and fixed to it at both ends.
The tubes are loosely in contact, equally long, but their materials are
different. What is the longitudinal linear thermal expansion coefficient
of this double tube? The cross-sections of the tubes are
*A*_{1} and *A*_{2}, the linear thermal expansion
coefficients and Young's moduli are respectively
and , and *E*_{1}
and *E*_{2}.

**FGy. 3093.**
A uniformly filled wooden box of mass *M*,
width *L* and height *h* is pulled horizontally by means
of a rope fastened to the centre of its side (see the figure). A
cylindrical roller of small diameter is placed under this side of
the box. The coefficient of friction between the box and the ground is
, rolling friction is negligible. What force
*F* is required to set the box into motion?

## New problems:

**FF. 3094.**
Coulomb's law prescribes an infinitely large force
to make two point-like charges of identical sign touch. What is the
situation if the charges are uniformly distributed on two circular
plates? What is the force required to make two circular plates of
charge *Q* and radius *r* touch?

**FF. 3095.**
80 liters of gaseous oxygen at a pressure of
5^{.}10^{5} Pa and a temperature of 32 ^{o}C
are compressed adiabatically, which raises the temperature to
78 ^{o}C. What amount of work is done?

**FF. 3096.**
Estimate the maximum change in the volume of a football
with a mass of 0.4 kg and an excess pressure of
5^{.}10^{4} Pa,volleyed at an angle of 30^{o}
and landing at a distance of 50 m.

**FF. 3097.**
Three small bodies of identical masses can move along a straight
line. The central body is initially at rest, then the two other bodies
hit it nearly at the same moment and at the same speed. The collision
of the body on the left is perfectly elastic, while that of the body
on the right is perfectly inelastic. How do the bodies move after the
collisions?

## New advanced problem:

**FN. 3098.**
How high does water crawl up a smooth vertical wall if it adheres
to the wall? What is the force exerted by the wall onto the water?
What is the radius of curvature of the surface of the water at half
of the maximum height (in the plane perpendicular to the wall)?