53.
A bicyclist consumes 9.0 L of water over the span of 3.5 hours
during a race. Making the approximation that 80
%
of the
cyclist’s energy goes into evaporating this water (at 20°C)
as sweat, how much energy in kcal did the rider use during
the ride? [
Hint
: See page 399.]
54.If coal gives off when burned, how much coal isneeded to heat a house requiring for the wholewinter? Assume that 30%of the heat is lost up the chimney.
55.
A 15-g lead bullet is tested by firing it into a fixed block of
wood with a mass of 35 kg. The block and imbedded bullet
together absorb all the heat generated. After thermal equi-
librium has been reached, the system has a temperature rise
measured as 0.020 C°. Estimate the bullet’s entering speed.
56.
A 310-kg marble boulder rolls off the top of a cliff and
falls a vertical height of 120 m before striking the ground.
Estimate the temperature rise of the rock if 50
%
of the
heat generated remains in the rock.
57.A 2.3-kg lead ball is placed in a 2.5-L insulated pail ofwater initially at 20.0°C. If the final temperature of thewater–lead combination is 32.0°C, what was the initialtemperature of the lead ball?
58.
A microwave oven is used to heat 250 g of water. On its maxi-
mum setting, the oven can raise the temperature of the liquid
water from 20°C to 100°C in
(
a
) At
what rate does the oven put energy into the liquid water?
(
b
) If the power input from the oven to the water remains
constant, determine how many grams of water will boil away
if the oven is operated for 2 min (rather than just 1 min 45 s).
1
min 45
s
A
=
105
s
B
.
2.0
*
10
5
MJ
30
MJ kg
1.5
m
2
.
General Problems
39.
(II) How long does it take the Sun to melt a block of ice at
0°C with a flat horizontal area
and thickness 1.0 cm?
Assume that the Sun’s rays make an angle of 35° with the
vertical and that the emissivity of ice is 0.050.
40.
(II)
Heat conduction to skin
. Suppose 150 W of heat flows by
conduction from the blood capillaries beneath the skin to
the body’s surface area of
If the temperature differ-
ence is 0.50 C°, estimate the average distance of capillaries
below the skin surface.
41.(II) Two rooms, each a cube 4.0 m per side, share a 14-cm-thick brick wall. Because of a number of 100-W lightbulbsin one room, the air is at 30°C, while in the other room it isat 10°C. How many of the 100-W bulbs are needed tomaintain the temperature difference across the wall?

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