Bukhovtsev-et-al-Problems-in-Elementary-Physics

80 PROBLEMS
angle will be different, and therefore the horizontal component
of the force of surface tension F will also be different on each
side of an arm (Fig. 139). Will this cause the cross to rotate?
364. Light bodies wetted by water (for example, two matches)
and floating on its surface are mutually attracted. This also
occurs if bodies are not wetted (matches coated with a thin
layer of paraffine). If one body is wetted and the other not,
they will be repulsed. How can these phenomena be explained?
2-5. Mutual Conversion of Liquids and Solids
365. Water in a glass freezes at O°C. If this water is separated
into fine drops, the water in them can be overcooled
to -40 0 C. For example, water drops which clouds are composed
of usually begin to freeze at a temperature below -17° C. How
can these facts be explained?
366. A vessel with 100 g. of water at a temperature of 0°C
is suspended in the middle of a room. In 15 minutes the temperature
of the water rises to 2°C. When ice equal in weight
to the water is placed into the vessel, it melts during ten hours.
May these data be used to appraise the specific heat of fusion
of ice H?
367. Two identical pieces of ice fly toward each other with
equal velocities and are converted into vapour upon impact.
Find the minimum possible velocities of the
pieces before the
impact if their temperature is -12°C.
368. A calorimeter contains ice. Determine the heat capacity
of the calorimeter if Q 1
= 500 cal are required to heat it together
with its contents from 270 to 27T K, and Q, = 16,600 cal from
272 to 274° K.
369. A calorimeter contains 400 g of water at a temperature
of +5°C. Then, 200 g of water at a temperature of +IO°C are
added and 400 g of ice at a temperature of -60° C are put in.
What is the temperature in the calorimeter?
370. Ice with a mass of m t = 600 g and at a temperature of
t,=-IO°C is placed into a copper vessel heated to t 1=350°C.
As a result, the vessel now contains m a
=::; 550 g of ice mixed
with water. Find the mass of the vessel. The specific heat of
copper C 1
= 0.1 cal/deg-g.
371. When a small ice crystal is placed into overcooled water
it begins to freeze instantaneously.