The force which opposes the relative motion between different layers of liquid or gases is called
Answer: A
This is viscous force.
Adjacent layers of fluid moving at different speeds exert a tangential drag on each other: \[F = \eta A \frac{dv}{dx}\] Viscosity is in effect internal friction, and it is what makes honey pour more slowly than water.
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An air bubble in water will act like a
Answer: C
An air bubble in water behaves like a concave lens.
What matters is the refractive index of the lens relative to its surroundings. Air is optically rarer than water, so a bubble that is convex in shape diverges light instead of converging it, and acts as a diverging - concave - lens.
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Which law states, "The radiant energy emitted by a black body per unit area per unit time is directly proportional to the fourth power of its absolute temperature".
Answer: A
This is Stefan's law, or the Stefan-Boltzmann law.
\[E = \sigma T^4\] with \(\sigma = 5.67\times10^{-8}\ \mathrm{W\,m^{-2}\,K^{-4}}\). The fourth power means a small rise in temperature increases radiation sharply - doubling the absolute temperature multiplies the emission by sixteen.
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1 Joule is Equal to
Answer: C
One joule equals \(10^{7}\) ergs.
\[1\ \mathrm{J} = 1\ \mathrm{kg\,m^2\,s^{-2}} = 10^{3}\ \mathrm{g}\times(10^{2}\ \mathrm{cm})^2\,\mathrm{s^{-2}} = 10^{7}\ \mathrm{g\,cm^2\,s^{-2}} = 10^{7}\ \mathrm{erg}\]
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Radiocarbon dating technique is used to estimate the age of
Answer: D
Radiocarbon dating is used on fossils and other once-living material.
A living organism keeps its carbon-14 in equilibrium with the atmosphere; once it dies the isotope decays with a half-life of 5,730 years, so the remaining fraction gives the age. The method is useful to about 50,000 years, beyond which too little carbon-14 is left. Rocks are dated by potassium-argon or uranium-lead instead.
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Mercury is commonly used as a thermometric fluid rather than water because
Answer: C
Because mercury is more visible than water.
Mercury is opaque and shiny, so the thread can be read against the scale, whereas water is transparent. Mercury also does not wet glass, expands uniformly, conducts heat well, and stays liquid from −39 to 357 °C - a range water cannot approach.
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The nuclear reaction in which a heavy nucleus splits into two nuclei of nearly equal mass is called
Answer: C
This is nuclear fission.
\[\mathrm{^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^1_0n + energy}\] The two fragments are of comparable mass, and the small loss of mass appears as energy. Fusion is the opposite process, in which light nuclei join.
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What is unit of Astronomical distance ?
Answer: A
Astronomical distances are given in light years.
One light year is about \(9.46\times10^{15}\) m. The angstrom, \(10^{-10}\) m, measures atomic distances; the weber is magnetic flux and the lux illuminance.
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Nuclear sizes are expressed in a unit named
Answer: A
Nuclear sizes are given in fermi.
One fermi, or femtometre, is \(10^{-15}\) m. Nuclear radius follows \(R = R_0 A^{1/3}\) with \(R_0 \approx 1.2\) fm. The angstrom, \(10^{-10}\) m, is a hundred thousand times larger and suits atoms.
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Sound waves in air are
Answer: B
Sound waves in air are longitudinal.
The air molecules oscillate along the direction of travel, producing compressions and rarefactions. This is why sound cannot be polarised: polarisation requires vibration perpendicular to the direction of propagation.
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