'Black hole' is a
- A.huge black star which has zero acceleration due to gravity on its surface
- B.star which has moderate acceleration due to gravity on its surface
- C.star which has collapsed into itself and has large acceleration due to gravity on its surface
- D.star which has collapsed into itself and has zero acceleration due to gravity on its surface
Correct answer
Explanation
The correct answer is C, star which has collapsed into itself and has large acceleration due to gravity on its surface. A black hole forms when a very massive star collapses under its own gravity, and its pull becomes so strong that not even light can escape.
A star shines by nuclear fusion, whose outward push balances gravity. When a star many times heavier than the Sun runs out of fuel, nothing can stop gravity, and the core collapses into a tiny region of enormous density. The boundary from which nothing returns is called the event horizon. Because no light leaves it, a black hole is found by its effect on nearby stars and gas.
A is wrong because a black hole's gravity is extreme, not zero. B is wrong because moderate gravity describes ordinary stars like the Sun. D is wrong because a collapsed star has a very large, not zero, pull of gravity.
Exam tip: small star ends as a white dwarf, heavier star as a neutron star, the heaviest as a black hole.
Practice Questions
View allWhich of the following statements about electromagnetic waves, sound waves and water waves is/are correct ? 1. They exhibit reflection 2. They carry energy 3. They exert pressure 4. They can travel in vacuum Select the correct answer using the code given below :
- A.1,2 and 3
- B.2 and 4
- C.1 and 3 only
- D.1 only
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All three kinds of wave reflect, carry energy and exert pressure, but only electromagnetic waves can travel through a vacuum.
Read the full explanation →Which one of the following is NOT the unit of energy?
- A.Joule
- B.Watt-hr
- C.Newton-metre
- D.kg-metre/sec²
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A kilogram-metre per second squared is mass times acceleration, which is force; it is the newton, not a unit of energy.
Read the full explanation →To protect steel and iron from rusting, a thin layer of which one of the following metals is applied?
- A.Magnesium
- B.Zinc
- C.Aluminium
- D.Lead
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Coating iron or steel with a thin layer of zinc is called galvanisation, and it is the most common way to protect them from rusting.
Read the full explanation →In an electric circuit, a wire of resistance 10 Ω is used. If this wire is stretched to a length double of its original value, the current in the circuit would become :
- A.half of its original value.
- B.double of its original value.
- C.one-fourth of its original value.
- D.four times of its original value.
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Stretching the wire to twice its length makes its resistance four times larger, so at the same voltage the current falls to a quarter.
Read the full explanation →Which one of the following scientists mentioned that an electron in an atom could revolve in certain stable orbits without the emission of radiant energy ?
- A.Ernest Rutherford
- B.J.J. Thomson
- C.Niels Bohr
- D.Albert Einstein
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In his 1913 model of the atom, Bohr proposed that electrons move in certain fixed orbits, called stationary states, in which they do not radiate energy.
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