All Free Physics MCQs with Answers

391. A 0.5 kg ball moving at 6 m/s has kinetic energy

• A. 9 J
• B. 18 J
• C. 6 J
• D. 3 J

Explanation: Kinetic energy is half mv squared, so half multiplied by 0.5 multiplied by 36 gives 9 J. Forgetting the factor of one half gives 18 J, which is the commonest slip. Note that the momentum of the same ball would be 3 kg m per second, which is why 3 also appears as a distractor.

392. A fluid is flowing through a tube. To undergo transition from laminar to turbulent flow its velocity must be:

• A. Slightly less than critical velocity
• B. Equal to critical velocity
• C. Greater than critical velocity
• D. Increasing gradually but less than critical velocity

Explanation: Below the critical velocity the flow stays in orderly layers, and only once that threshold is passed does the motion break up into eddies and become turbulent. The critical velocity is set by the Reynolds number, which combines density, viscosity, speed and pipe diameter. Turbulent flow dissipates far more energy, which matters in pipelines and in blood vessels alike.

393. For two equal positive charges, the electric field is weakest:

• A. Midway between them
• B. Along the perpendicular bisector
• C. Close to either charge
• D. At infinity

Explanation: At the midpoint the two fields are equal in magnitude and opposite in direction, so they cancel exactly and the resultant is zero, which is the weakest the field can be anywhere at finite distance. Close to either charge the field is strongest. The field also tends to zero at infinity, but the question is asking for the null point produced by the pair.

394. A projectile is launched in air with certain angle; its velocity is maximum at:

• A. Point of projection
• B. Highest point
• C. Between launching and highest point
• D. At all points

Explanation: The vertical component decreases as the projectile rises and is regained on the way down, so the speed is greatest at launch and again on landing at the same height, and least at the top where only the horizontal component remains. Neglecting air resistance, the landing speed equals the launch speed. This follows directly from energy conservation, since the kinetic energy is greatest where the height is least.

395. If the distance between two charges is halved and magnitude of charges are also doubled, then the force between these charges becomes:

• A. Two times
• B. Four times
• C. Eight times
• D. Sixteen times

Explanation: Doubling both charges multiplies the numerator of Coulomb's law by four, and halving the separation divides the denominator by four since the distance is squared, so the two effects combine to give sixteen times the original force. Handling the two changes separately and then multiplying is the safe method. Forgetting that the distance is squared gives the eight times answer.

396. If a body having mass m1 (2 kg) moving with 5 m/s approaches another mass m2 (3 kg) with speed of 1 m/s in same direction, relative speed of approach is 4 m/s. Relative speed of separation after collision will be:

• A. 4 m/s
• B. 2 m/s
• C. 6 m/s
• D. Depends on masses

Explanation: For a perfectly elastic collision the relative speed of separation equals the relative speed of approach, whatever the masses, which follows from conserving both momentum and kinetic energy. So the 4 m per second closing speed becomes a 4 m per second separating speed. In an inelastic collision the separation speed would be smaller, and in a perfectly inelastic one it would be zero.