Two circus clowns (each having a mass of 50 kg) swing on two flying trapezes (negligible mass, length 25 m) shown in the figure. At the peak of the swing, one grabs the other, and the two swing back to one platform. The time for the forward and return motion is
?
?
a.
10 s.
b.
50 s.
c.
15 s.
d.
20 s.
e.
25 s.
a
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Wearing layers of clothing in cold weather is often recommended because dead-air spaces between the layers keep the body warm. The explanation for this is that the heat loss from the body is less. Compare the rate of heat loss for single 2 cm thick layer of wool [k = 0.04 W/(m K)] with three 0.67 cm layers separated by 1.5 mm air gaps. The thermal conductivity of air is 0.024 W/(m K).
GIVEN
• Wool insulation
• Thermal conductivities
? Wool (kw) = 0.04 W/(m K
? air (ka) = 0.024 W/(m K)
FIND
• Compare the rate of heat loss for a single 2 cm thick layer of wool to that of three 0.67 cm layers separated by 1.5 mm layers of air gaps.
ASSUMPTIONS
• Heat transfer can be approximated as one dimensional, steady state conduction
SKETCH
A 30-year-old astronaut goes off on a long term mission in a spacecraft that travels at speeds close to that of light. The mission lasts exactly 20 years as measured on Earth. Biologically speaking, the astronaut's age would be
A) more than 50 years. B) exactly 50 years. C) less than 50 years. D) exactly 30 years. E) exactly 25 years.
The acceleration of a ball as it rolls down a ramp is 4 m/s2 . The ball is referred to 2 s after it starts to roll. The acceleration in m/s2 after starting from rest is
a. 0. b. 2. c. 4. d. 8. e. 16.
Water comes down the spillway of a dam from an initial vertical height of 100 m. What is the highest possible speed of the water at the end of the spillway?
a. 15 m/s c. 58 m/s b. 44 m/s d. 1370 m/s