Which one of the following conservation laws is a consequence of the Newton's third law of motion?
- A.Conservation of energy
- B.Conservation of momentum
- C.Conservation of charge
- D.Conservation of mass
Correct answer
Explanation
The correct answer is B, Conservation of momentum. Equal and opposite forces acting for the same time give equal and opposite changes in momentum, so the total stays the same.
Newton's third law says that when body A pushes body B, B pushes A with a force equal in size and opposite in direction. Both forces act for exactly the same time, so the impulse each body receives, force multiplied by time, is equal and opposite. Impulse is the change in momentum, so what one body gains the other loses. For an isolated system with no outside force, total momentum therefore stays constant. This explains the recoil of a gun and rocket propulsion.
A is wrong: conservation of energy is a separate principle, linked to the first law of thermodynamics, not derived from the third law.
C is wrong: conservation of electric charge is a law of electricity, unrelated to Newton's laws.
D is wrong: conservation of mass is a principle of chemical reactions, stated by Lavoisier.
Exam tip: Newton's third law leads to conservation of linear momentum — gun recoil and rockets are the classic examples.
Practice Questions
View allA snowboard pulled up by a tow rope travels at the rate of 5 m/s up a mountain. If 3000 watt of power is used, what force was applied to it ?
- A.50 N
- B.100 N
- C.600 N
- D.15000 N
Show answer
Power equals force times velocity, so the force is 3,000 W divided by 5 m/s, which is 600 N. Power is the rate of doing work: P = W/t. Work is force times distance, W = F × d, so P = F × d/t, and since d/t is the…
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- A.material of the medium only
- B.energy only
- C.both material of the medium and energy
- D.none of the above
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Read the full explanation →Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?
- A.Ionization energy
- B.Electron affinity
- C.Electronegativity
- D.Atomic radius
Show answer
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Read the full explanation →A sound wave of frequency of 2 kHz has a wavelength of 35 cm in a given medium. How long will it take to travel a distance of 2.1 km through the medium?
- A.30 s
- B.2.1 s
- C.3.0 s
- D.4.1 s
Show answer
The speed of the wave is frequency times wavelength, 700 m/s, and 2,100 m at 700 m/s takes 3 seconds.
Read the full explanation →Which one of the following structure is NOT found in prokaryotes ?
- A.Nuclear envelope
- B.Cell membrane
- C.Ribosome
- D.Cell wall
Show answer
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Read the full explanation →