Quiz: Class XI Physics Mechanics and Modern Physics — 13 questions

1. Which technique uses the dimensions of physical quantities to test whether an equation is dimensionally consistent?

A. Vector resolution

B. Dimensional analysis

C. Graphical interpolation

D. Significant-figure rounding

Answer: (B)

Explanation

Dimensional analysis compares the dimensions on both sides of an equation to check consistency. Significant figures address numerical precision, whereas vector resolution concerns components of vectors.

2. Which vector construction represents the sum of two vectors acting from a common point?

A. The parallelogram law

B. The scalar product rule

C. The unit-vector convention

D. The dimensional-analysis method

Answer: (A)

Explanation

The parallelogram law finds the resultant of two vectors drawn from the same point by using the diagonal of the parallelogram. The scalar product produces a scalar, while unit vectors describe direction and dimensional analysis checks equations.

3. A ray entering a second medium bends toward the normal when its refractive index is greater than that of the first medium; what does this indicate about its speed?

A. The ray keeps the same speed in both media

B. The ray speeds up in the second medium

C. The ray stops briefly at the boundary

D. The ray slows down in the second medium

Answer: (D)

Explanation

A higher refractive index corresponds to a lower speed of light in the medium, so the ray bends toward the normal as it slows down. Bending away from the normal would indicate entry into a medium with a lower refractive index.

4. What is the defining relationship between a particle and its antiparticle?

A. They have opposite relevant charges while sharing the same mass

B. They have different masses while sharing every charge

C. They are composite particles formed from three identical quarks

D. They are two particles with identical charge and different spin

Answer: (A)

Explanation

A particle and its antiparticle have the same mass but opposite values of relevant charges, and they can annihilate when they meet. They are not defined by having different masses or by being three-quark composites.

5. Why does nuclear fission release energy according to the mass-energy relation?

A. The nucleus gains charge, which creates additional mass

B. The products have slightly less mass, converted into energy

C. The products have slightly more mass, converted into energy

D. The released neutrons permanently increase the nuclear mass

Answer: (B)

Explanation

In fission, the total mass of the products is slightly smaller than the original nucleus, and the mass defect appears as energy through Einstein’s relation. An increase in product mass would not account for released energy.

6. A projectile is launched near Earth with no air resistance. Which feature characterizes its motion after launch?

A. Its horizontal acceleration increases while its vertical velocity remains constant

B. Its horizontal velocity remains constant while its vertical acceleration is downward

C. Its horizontal velocity decreases while its vertical acceleration becomes zero

D. Its horizontal and vertical velocities remain constant throughout the flight

Answer: (B)

Explanation

Projectile motion can be treated as constant horizontal velocity combined with downward gravitational acceleration in the vertical direction. Treating both velocity components as constant neglects the effect of gravity.

7. Why does heat flow spontaneously between two bodies that are at different temperatures?

A. Both bodies exchange energy without any preferred direction

B. Matter flows from the denser body to the less-dense body

C. Energy flows from the lower-temperature body to the higher-temperature body

D. Energy flows from the higher-temperature body to the lower-temperature body

Answer: (D)

Explanation

A temperature difference drives spontaneous heat transfer from the hotter body toward the colder body until thermal equilibrium is reached. The reverse direction does not occur spontaneously, and heat transfer has a preferred direction when temperatures differ.

8. Which statement correctly distinguishes baryons from mesons?

A. Baryons are made of three quarks, whereas mesons consist of a quark and an antiquark

B. Baryons are made of leptons, whereas mesons consist of three neutrinos

C. Baryons are antiparticles, whereas mesons are particles without internal constituents

D. Baryons consist of a quark and an antiquark, whereas mesons contain three quarks

Answer: (A)

Explanation

In the quark model, baryons contain three quarks, while mesons contain a quark-antiquark pair. Leptons and neutrinos are not the constituent particles used to define these hadrons.

9. For an isotropic solid with linear expansion coefficient $\alpha$, what is the cubical expansion coefficient?

A. $\alpha^2$

B. $\alpha$

C. $3\alpha$

D. $\frac{\alpha}{3}$

Answer: (C)

Explanation

For an isotropic solid, expansion occurs independently along three perpendicular directions, so the cubical expansion coefficient is approximately $3\alpha$. The coefficient $\alpha$ describes linear expansion, not volume expansion.

10. Which quantity identifies the total number of protons and neutrons in a nucleus?

A. Atomic number

B. Packing fraction

C. Atomic mass

D. Mass number

Answer: (D)

Explanation

The mass number is the sum of the protons and neutrons in a nucleus. The atomic number counts protons, while atomic mass represents the measured mass of an atom.

11. According to the kinetic-molecular model of an ideal gas, what physical quantity is directly related to the gas pressure?

A. The color of the gas molecules

B. The average translational kinetic energy of its molecules

C. The gravitational potential energy of the container

D. The total mass of the container walls

Answer: (B)

Explanation

Gas pressure results from molecular collisions with the container walls and is related to the molecules' average translational kinetic energy. Wall mass, molecular color, and container gravitational energy do not determine the kinetic pressure relation.

12. Which condition allows total internal reflection to occur at a boundary?

A. Light travels from a lower refractive index to a higher one and exceeds the critical angle

B. Light travels from a higher refractive index to a lower one and exceeds the critical angle

C. Light travels from a higher refractive index to a lower one at normal incidence

D. Light travels between equal refractive indices at an angle below the critical angle

Answer: (B)

Explanation

Total internal reflection requires light to move from an optically denser medium to a less dense medium and strike the boundary at an angle greater than the critical angle. Refraction from a lower to a higher refractive index does not produce this condition.

13. What does instantaneous acceleration describe for a moving particle at a particular instant?

A. The total distance traveled during the entire motion

B. The rate of change of velocity at that instant

C. The displacement divided by the final time only

D. The average speed measured over a finite interval

Answer: (B)

Explanation

Instantaneous acceleration is the rate at which velocity changes at a specific moment. Average speed or displacement over an interval describes motion over a finite time rather than the acceleration at one instant.