Tasha has mass 20 kg and wants to use a 6.0-m board of mass 10 kg as a seesaw. Her friends are busy, so Tasha seesaws by herself by putting the support at the system's center of gravity when she sits on one end of the board. How far is she from the support point?
a. 2.0 m
c. 0.67 m
b. 1.0 m
d. 0.33 m
B
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What do the size and shape of Widmanstätten patters indicate?
a. the age of the meteorite b. that the molten metal of the meteorite cooled very slowly c. the metallic content of the core of the meteorite d. the origination of the meteorite e. the number of organic compounds found within the meteorite
Suppose we look at two distant galaxies: Galaxy 1 is twice as far away as Galaxy 2. In this case,
A) Galaxy 1 must be twice as big as Galaxy 2. B) we are seeing Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2. C) we are seeing Galaxy 1 as it looked at a later time in the history of the universe than Galaxy 2. D) Galaxy 2 must be twice as old as Galaxy 1.
A simple solar heater consists of a flat plate of glass below which is located a shallow pan filled with water, so that the water is in contact with the glass plate above it. Solar radiation is passing through the glass at the rate of 490 W/m2. The water is at 92°C and the surrounding air is 27°C. If the heat transfer coefficients between the water and the glass and the glass and the air are 28 W/(m2 K), and 7 W/(m2 K), respectively, determine the time required to transfer 1.1 MJ/m2 of surface to the water in the pan. The lower surface of the pan may be assumed to be insulated.
GIVEN
• A simple solar heater: shallow pan of water below glass, the water touches the glass
• Solar radiation passing through glass (qr/A) = 490 W/m2
• Water temperature (Tw) = 92°C
• Surrounding air temperature (T?) = 27°C
• Heat transfer coefficients
? Between water and glass ( cw h ) = 28 W/(m2 K)
? Between glass and air ( ca h ) = 7 W/(m2 K)
FIND
• The time (t) required to transfer 1.1 MJ/m2 to the water
ASSUMPTIONS
• One dimensional, steady state heat transfer prevails
• The heat loss from the bottom of the pan is negligible
• The radiative loss from the top of the glass is negligible
• The thermal resistance of the glass is negligible
SKETCH
Calculate the radius that a loop should have so that the magnetic energy density at its center is equal to 6 mJ/ m3 when a current I = 20A passes through the loop?
A) 26 cm B) 10 cm C) 7 cm D) 18 cm E) 15 cm