A kangaroo completes 2 hops in 4 seconds. What is the period of the hops?
a) 2
b) 0.5
c) 8
d) 6
Ans: b) 0.5
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Fundamental Forces: A stable nucleus contains many protons very close to each other, all positively charged. Why do the protons not fly apart due to mutual Coulomb repulsion?
A. An attractive nuclear force in the nucleus counteracts the effect of the Coulomb forces. B. There are an equal number of electrons in the nucleus which neutralize the protons. C. The neutrons in the nucleus shield the protons from each other. D. The Coulomb force does not operate within nuclei. E. The gravity of the protons and neutrons overcomes their repulsion at such close distances.
A long wire 0.7 mm in diameter with an emissivity of 0.9 is placed in a large quiescent air space at 270 K. If the wire is at 800 K, calculate the net rate of heat loss. Discuss your assumptions.
GIVEN
• Long wire in still air
• Wire diameter (D) = 0.7 mm= 7*10-4 m
• Wire temperature (Ts) = 800 K
• Emissivity (?) = 0.9
• Air temperature (T?) = 270 K FIND
• The net rate of heat loss
ASSUMPTIONS
• The enclosure around the wire behaves as a blackbody enclosure at the temperature of the air
• The natural convection heat transfer coefficient is 18 W/(m2 K)
• Steady state conditions prevail
SKETCH
At the end of the Kelvin-Helmholtz contraction a star is hundreds of times brighter than it will be as a main sequence star
Indicate whether the statement is true or false
As more resistors are added in series to a constant voltage source (zero internal resistance), the power supplied by the source
A) does not change. B) decreases. C) increases. D) increases for a time and then starts to decrease.