The earliest human culture, involving villages and a sense of history, developed
A) about 10,000 years ago.
B) about one billion years ago.
C) a few million years ago.
D) 100,000 to 200,000 years ago.
E) about 1000 years ago.
A
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A long steel rod (2 cm in diameter, 2-m-long) has been heat-treated and quenched to a temperature of 100°C in an oil bath. To cool the rod further, it is necessary to remove it from the bath and expose it to room air. Will the faster cool-down result from cooling the cylinder in the vertical or horizontal position? How long will the two methods require to allow the rod to cool to 40°C in 20°C air?
GIVEN • A long steel rod in air • Diameter (D) = 2 cm = 0.02 m • Length (L) = 2 m • Initial temperature (Ts,i) = 100°C • Air temperature (T?) = 20°C FIND (a) Is it faster to cool the rod vertically or horizontally? (b) Time for rod to cool to 40°C in each position ASSUMPTIONS • Steel is 1% carbon PROPERTIES AND CONSTANTS for 1% carbon steel Thermal conductivity (ks) = 43 W/(m K) Specific heat (c) = 473 J/(kg K) Density (?) = 7801 kg/m3 for dry air at the initial mean temperature of 60°C Thermal expansion coefficient (?) = 0.003 1/K Thermal conductivity (k) = 0.0279 W/(m K) Kinematic viscosity (?) = 19.4 × 10–6 m2/s Prandtl number (Pr) = 0.71
An atom which has 4 protons and 6 neutrons will be electrically neutral if it contains
A) 4 electrons. B) 6 electrons. C) 10 electrons. D) at least one electron.
If two identical wires carrying a certain current in the same direction are placed parallel to each other, they will experience a force of repulsion
Indicate whether the statement is true or false
A car is moving in the positive direction along a straight highway and accelerates at a constant rate while going from point A to point B. If the acceleration is positive, increasing the speed of the car, when does the position where the average speed equals the instantaneous speed occur during the time interval from A to B? Assume the time interval is T
a. T/2 from the start of the interval c. after T/2 from the start of the interval b. before T/2 from the start of the interval d. It depends on the speed at the start of the time interval.