An atom bomb is based on the principle of
Answer: A
An atom bomb is based on nuclear fission.
A supercritical mass of uranium-235 or plutonium-239 sustains an uncontrolled chain reaction, each fission releasing neutrons that cause more. The chain reaction in a bomb is precisely not controlled; control rods are what distinguish a reactor.
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SI unit of temperature is
Answer: A
The SI unit of temperature is the kelvin.
It is one of the seven base units, and the scale starts at absolute zero, so no negative values occur. A kelvin and a degree Celsius are the same size, and \(T(\mathrm{K}) = t(^\circ\mathrm{C}) + 273.15\).
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Curie is the unit of
Answer: B
The curie is a unit of radioactivity.
One curie is \(3.7\times10^{10}\) disintegrations per second, originally the activity of one gram of radium-226. The SI unit is the becquerel, one disintegration per second.
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Distance of stars are measured in
Answer: D
Stellar distances are measured in light years.
A light year is the distance light travels in one year, about \(9.46\times10^{15}\) m. Proxima Centauri, the nearest star, is 4.24 light years away, so we see it as it was more than four years ago.
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The cover of a solar cooker is made of glass because
Answer: C
Because it retains the heat inside by the greenhouse effect.
Glass transmits the short-wavelength sunlight that heats the food, but is opaque to the long-wavelength infrared the food then re-radiates. The cover also stops the hot air escaping by convection.
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Fat can be seperated from milk in a cream seperator because of :
Answer: D
Fat separates because of the centrifugal effect.
In the rotating drum, the denser skimmed milk is thrown to the outside and the lighter fat globules are displaced inwards, so the cream collects near the axis. In the rotating frame this is described as a centrifugal force; in the ground frame it is the denser fluid's greater inertia.
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Which one of the following groups have quantities that do not have the same dimensions
Answer: C
Force and impulse do not share dimensions.
Force is \(\mathrm{MLT^{-2}}\) while impulse, being force times time, is \(\mathrm{MLT^{-1}}\). Pressure and stress are both force per unit area, velocity and speed both \(\mathrm{LT^{-1}}\), and work and energy both \(\mathrm{ML^2T^{-2}}\).
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A man stands in a lift which accelerates upwards. The resultant reaction force on the floor of the lift is
Answer: A
The reaction is greater than the weight of the man.
The floor must support him and accelerate him upward as well: \[N - mg = ma \Rightarrow N = m(g+a)\] This is why one feels heavier as a lift starts up, and lighter as it starts down.
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A series of fast moving still pictures can be made to appear as a moving picture because the eye
Answer: C
Because the eye cannot separate two images shown less than about a sixteenth of a second apart.
An image persists on the retina for roughly 1/16 second after the light is removed - the persistence of vision. Frames projected faster than that blend into continuous motion, which is why film runs at 24 frames a second.
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No current will flow between two charged bodies if they have the same
Answer: C
No current flows if the two bodies are at the same potential.
Charge flows in response to a potential difference, not to a difference in charge. A large body may hold much more charge than a small one and still sit at the same potential, since \(V = Q/C\).
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