All Free Physics MCQs with Answers

361. If P is the momentum of an object and m is its mass, then its kinetic energy is:

• A. P/2m
• B. P2/2m
• C. 1/2 Pm2
• D. 1/2 P2m

Explanation: Kinetic energy is half mv squared and momentum is mv, so substituting v equal to P over m gives half m multiplied by P squared over m squared, which simplifies to P squared over 2m. A quick check is that energy must rise with the square of momentum, which only the second option does. This relation is used constantly in collision and de Broglie problems.

362. The electric field at a point due to two equal and opposite charges is 100 N/C. If the magnitude of each charge is doubled then the electric field at that point becomes:

• A. 50 N/C
• B. 100 N/C
• C. 200 N/C
• D. 400 N/C

Explanation: This is the option marked correct in the official UHS answer key. Field intensity is directly proportional to the charge producing it, so doubling both charges doubles each contribution and doubles the resultant, which by that reasoning gives 200 N/C. The key's figure of 400 corresponds to treating the effect as multiplying by four, so this is a question where the official answer and the standard proportional argument diverge.

363. If a plastic sheet of relative permittivity 2.5 is inserted between two point charges placed in vacuum, then the electrostatic force between them

• A. Increases by a factor of 2.5
• B. Decreases by a factor of 2.5
• C. Increases by a factor of 5
• D. Decreases by a factor of 5

Explanation: Relative permittivity appears in the denominator of Coulomb's law, so inserting a medium of relative permittivity 2.5 reduces the force to 1 over 2.5 of its vacuum value. Water, with a relative permittivity of about 80, weakens ionic attraction eightyfold, which is why ionic compounds dissociate so readily in it. Air is close enough to 1 that the vacuum formula is used for it.

364. After 3 half-lives, the remaining fraction of a radioactive sample is:

• A. 1/2
• B. 1/4
• C. 1/8
• D. 1/16

Explanation: Each half life halves the number of undecayed nuclei, so three of them give a half multiplied by a half multiplied by a half, which is one eighth. Decay is exponential, so the fractions multiply rather than the half lives adding in any simple way. After four half lives one sixteenth would remain.

365. If the horizontal range of a projectile becomes half of its maximum possible horizontal range, the probable angle of projection is:

• A. 15 degrees
• B. 30 degrees
• C. 45 degrees
• D. 60 degrees

Explanation: Range is proportional to sin of twice the angle and is greatest at 45 degrees, where sin 90 equals 1, so half the maximum range needs sin 2 theta equal to 0.5, giving 2 theta equal to 30 degrees and theta equal to 15 degrees. The other solution is 75 degrees, since the two angles are complementary. Answering 30 degrees comes from halving the angle instead of the sine.

366. In an ideal transformer, if the primary voltage is doubled and the turns ratio remains the same, what happens to the secondary current?

• A. Doubles
• B. Halves
• C. Remains the same
• D. Becomes four times

Explanation: With the turns ratio fixed, doubling the primary voltage doubles the secondary voltage, and for a given load resistance the secondary current rises in proportion by Ohm's law. The power drawn from the primary rises fourfold to match. The halving answer would apply to comparing primary with secondary current at a fixed input, which is a different question.

367. An angular displacement of 90 degrees is equal to:

• A. One-fourth revolution
• B. One-third revolution
• C. One half revolution
• D. One complete revolution

Explanation: A full revolution is 360 degrees, so 90 degrees is a quarter of it, equivalent to pi over 2 radians. Half a revolution would be 180 degrees or pi radians. Converting between degrees, radians and revolutions is worth doing fluently, since rotational formulae require radians.

368. The time in which half of the given number of radioactive nuclei decay is known as

• A. 1/2 life of radioactive element
• B. 2/3 life of radioactive element
• C. 1/4 life of radioactive element
• D. 2/5 life of radioactive element

Explanation: The half life is the interval in which half the undecayed nuclei present decay, and because decay is random with a constant probability per nucleus, that interval stays the same however much material remains. Carbon 14 has a half life of about 5730 years and iodine 131 about 8 days. The quantity is a property of the nuclide and cannot be altered by temperature or pressure.

369. Earth receives large amount of energy directly from:

• A. Wind
• B. Water
• C. Sun
• D. Moon

Explanation: Solar radiation is the primary energy input to the Earth, and wind and flowing water are themselves driven by the uneven solar heating of the atmosphere and oceans, which makes them secondary rather than direct sources. The Sun's energy comes from the fusion of hydrogen into helium in its core. Tidal energy is the one significant contribution that traces back to the Moon instead.

370. A phase difference of 90 degrees is equal to:

• A. pi radians
• B. pi/2 radians
• C. 2 pi radians
• D. pi/4 radians

Explanation: A complete cycle is 360 degrees or 2 pi radians, so 90 degrees is a quarter of a cycle, that is pi over 2 radians. This is the phase difference between current and voltage in a purely capacitive or purely inductive AC circuit, which is why no net power is dissipated in either. The Greek letter did not print cleanly in the original paper.