Which one of the following statements about gravitational force is NOT correct?
- A.It is experienced by all bodies in the universe
- B.It is a dominant force between celestial bodies
- C.It is a negligible force for atoms
- D.It is same for all pairs of bodies in our universe
Correct answer
Explanation
The correct answer is D, It is same for all pairs of bodies in our universe. This statement is wrong: the gravitational force between two bodies depends on their masses and on the distance between them.
Newton's law of gravitation gives F = G m₁m₂ / r². The constant G, about 6.67 × 10⁻¹¹ N m² kg⁻², is the same everywhere, but the force itself changes from pair to pair: doubling one mass doubles the force, and doubling the distance makes it one-fourth. That is why the pull between the Earth and the Moon is enormous, while the pull between two people standing side by side is far too weak to notice.
A is not the answer: every body in the universe attracts every other body, so all bodies experience gravity.
B is not the answer: between planets, stars and galaxies, gravity is the dominant force that holds their orbits together.
C is not the answer: between atoms, gravity is negligible compared with electrical and nuclear forces.
Exam tip: G is universal and constant; the force F changes with the masses and falls as 1/r².
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
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The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
Read the full explanation →The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
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Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing. Humans hear sound between about 20 Hz and 20 kHz.
Read the full explanation →Which one of the following is an example of a clean fuel ?
- A.Coke
- B.Propane
- C.Petrol
- D.Wax
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Propane is a light gaseous hydrocarbon that burns completely with a blue flame, leaving no smoke, soot or ash. A clean fuel burns completely, gives high heat and releases few pollutants.
Read the full explanation →A sample of 'soft soap' contains
- A.Caesium
- B.Potassium
- C.Calcium
- D.Magnesium
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Soft soaps are potassium salts of fatty acids, while ordinary hard bar soaps are sodium salts. Soap is made by saponification, heating an oil or fat with an alkali.
Read the full explanation →Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
Show answer
Magnesium, calcium and barium are all alkaline earth metals of group 2, and each has a valency of 2. Elements in the same group of the periodic table have the same number of electrons in their outermost shell, and so the same valency.
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