Class 10 SELINA Solutions Physics Chapter 13 - Assertion-Reasoning Type Questions
Assertion-Reasoning Type Questions Exercise 326
Solution 1
(a) Both A and R are true.
The assertion is true because, according to the principle of equilibrium,
A sum of anticlockwise moments = sum of clockwise moments
In beam balance, when the beam is balanced in horizontal position, the clockwise moment of force due to object on its right pan balances the anticlockwise moment of force due to the weights in left pan.
Thus, the reason statement is also correct.
Thus, both A and R are true.
Solution 2
(c) A is false but R is true.
Assertion (A) is false because the moment of force depends on both the force and the distance. A heavier object placed farther from the pivot point will have a greater moment than a lighter object placed closer to the pivot point. This is because the greater distance produces a larger turning effect, even if the force itself is smaller due to the heavier object. Even though a heavier object might exert a larger force, if the distance from the pivot point is much smaller for the heavier object compared to the lighter object, the moment of force for the lighter object could be greater due to the larger lever arm.
While the given reason statement is true. The moment of force is indeed determined by the magnitude of the force (F) and its perpendicular distance (d) from the pivot point.
It is calculated by the formula: Moment of force (τ) = F × d.
Solution 3
(c) A is false but R is true.
The assertion is false. The center of gravity of an irregularly shaped object does not always lie at its geometric center. Irregular shapes can have their mass distributed unevenly, causing the center of gravity to be offset from the geometric center.
This reason is true. The center of gravity depends on the distribution of mass within an object. Wherever the majority of the mass is concentrated, the center of gravity will be located along a line directly below that point.
Solution 12
(c) A is false but R is true.
Work done is calculated by multiplying the force by the component of the displacement in the direction of the force (W = Fd cos θ). When a force retards the motion (acts opposite to the direction of movement), the angle between the force and displacement becomes greater than 90 degrees, and the cosine of angles is negative. A negative value multiplied by the force and displacement results in negative work done.
Thus, the assertion is false, and the reason statement is true.
Solution 13
(a) Both A and R are true
Consider that the original momentum of body is ‘p’ and 50% increase makes momentum as,
p + (50/100) = 1.5p … (1)
The relation between momentum and kinetic energy is,
K = p2/2m …(2)
Thus, new kinetic energy will be,
K’ = (1.5p)2/2m = 2.25 (p2 /2m) = 2.25 K
Thus, an increase in the percentage of kinetic energy will be,
% increase = [(K’ – K)/K] × 100 = [(2.25 K – K)/K] ×100 = 1.25 ×100 =125%
A 50% increase in momentum translates to a 125% increase in kinetic energy.
We know,
K.E. = ½ mv2
Kinetic energy depends on the square of velocity.
Thus, Both A and R are true.
Solution 4
(d) A is true but R is false
A crack in a glass vessel often shines like a mirror because cracks in glass can act like tiny mirrors due to a phenomenon called total internal reflection. Thus, the assertion is true.
The reason statement is false because refraction is the bending of light as it passes from one medium to another. While refraction does play a role in how light interacts with glass, it's not the reason for the mirror-like shine of cracks. The actual reason for the shine is total internal reflection.
Solution 6
(a) Both A and R are true.
The correct option is (d): A is true but R is false.
The assertion statement is true. When light passes through a rectangular glass slab with parallel faces, it undergoes refraction at both surfaces. However, since the faces are parallel, the emergent ray remains parallel to the incident ray, even if it shifts sideways. Thus, there is no dispersion of light.
Also, the reason statement is true. Dispersion of light is indeed is the phenomenon of splitting of white light into its constituent colours.
Solution 8
(a) Both A and R are true.
If the angles of the base of a prism are equal, then in the position of minimum deviation, the refracted ray does pass parallel to the base of the prism. This is because the the prism causes the incident ray to bend an equal amount at each surface when the deviation is minimum. Thus, the assertion is true.
Also, it is observed that the minimum angle of deviation is minimum when the angle of incidence is equal to the angle of emergence.
Solution 9
(a) Both A and R are true.
The distance of stars from Earth is not the direct cause of twinkling. Stars twinkle due to the Earth's atmosphere. When starlight enters the atmosphere, it gets bent due to variations in air density and temperature. This bending causes the star to appear to twinkle. While planets are much closer to Earth than the stars. Also, the twinkling effect averages out over the planet and thus, they appear to shine steadily. Thus, planets do not twinkle.
Thus, Both A and R are true statements.
Solution 10
(d) A is true but R is false.
Assertion is true because when an empty test tube is partially submerged in water at an angle and viewed from a specific direction, it can appear silvery. And it not due to refraction it is due to a phenomenon called total internal reflection.
Thus, the assertion is true and the reason is false.
Solution 5
(c) A is false but R is true.
The assertion statement is false. The focal length of a lens actually does change when different colors of light pass through it. Light with shorter wavelengths like blue bends more than light with longer wavelengths like red when passing through a lens. As a result, the focal point for blue light is closer to the lens compared to the focal point for red light. This phenomenon is called chromatic aberration.
The reason statement is true. The focal length of a lens depends on the refractive index of the lens material. The refractive index determines how much light bends as it travels through the material.
(Note that the refractive index is not constant for different colors of light. It varies with the wavelength which causes the chromatic aberration.)
Solution 7
(c) A is false but R is true.
The assertion statement is false. A beam of white light does not give a spectrum on passing through a hollow prism. White light is a comprised of seven colors, which have different wavelengths. A prism separates white light into a spectrum because different colors have different refractive indices. In a hollow prism, there's just air inside. Since light travels through air before and after entering the prism, there's no change in refractive index and thus, the spectrum is not formed in this case.
Light travels at a constant speed in a specific medium. The reason is true because the speed of light in air present outside the prism is the same as the speed of light in air present inside a hollow prism.
Solution 11
(b) Both A and R are false.
Sound waves cannot travel through vacuum but light waves can. Light waves are electromagnetic waves while sound waves are mechanical waves.
Thus, both A and R are false.
Solution 14
(a) Both A and R are true.
The speed of sound in a gas increase with an increase in humidity. This is because water vapor molecules are lighter than the nitrogen and oxygen molecules that make up most of dry air. So, when humid air replaces some of the dry air, the overall density of the air also decreases.
Thus, both A and R are true.
Solution 15
(d) A is true but R is false.
The flash of lightening is seen before the sound of thunder is heard. This is because the speed of light is greater than the speed of sound.
Thus, A is true but R is false.
Solution 16
(c) A is false but R is true.
When we fill an empty pitcher with water the amount of air inside the pitcher decreases as a result, the pitch of the sound will increase, as frequency or pitch of sound is inversely proportional to the length of vibrating air column.
As we know, a pitch of sound is directly related to frequency, so as the frequency increases, the pitch also increases.
Hence the given assertion in false reason is true.
Assertion-Reasoning Type Questions Exercise 327
Solution 23
(d) A is true but R is false.
At the highest position, the potential energy of the hammer is maximum, whereas as it starts falling, the potential energy transforms to kinetic energy.
When a hammer is made to fall on a nail fixed upright on a wooden piece, the nail begins to penetrate the wood because of the work done by the hammer on the nail.
Hence, the given assertion is true, but the reason is false.
Solution 24
(d) A is true but R is false.
As we know, M.A of class III lever is always less than 1, this is because the effort arm is always shorter than the load arm.
Examples of third-class levers are:
Sugar tongs, forceps, a spade used to lift coal or soil, a fishing rod, a hockey stick, and tweezers.
Solution 17
(c) A is false but R is true.
A ray of light when travelling from rarer medium to denser medium, it bends towards normal not away from normal.
Whereas the speed of light decreases as it travels from rarer medium to denser medium.
Hence assertion is false, but reason is true.
Solution 18
(a) Both A and R are true.
When freely suspended, a current-carrying solenoid aligns itself in the north-south direction like a bar magnet. The end of the solenoid, where the current direction is anti-clockwise, acts as a north pole. In contrast, the end with a clockwise current direction behaves as a south pole. This can be proven using the right-hand thumb rule.
Solution 19
(d) A is true but R is false.
As we know, magnetic field strength is directly proportional to the current flowing in a coil.
i.e., The strength of the magnetic field produced by a current-carrying circular coil increases with the strength of the current.
Hence for the given case, assertion is true and reason given is false.
Solution 21
(c) A is false but R is true.
Terminal voltage is equal to the emf of cell when cell is not in use, while less than the emf when cell is in use.
Whereas terminal voltage depends on the amount of current drawn from the cell. The more the current is drawn from the cell, less is the terminal voltage.
Hence the given assertion if false and reason is true.
Solution 30
(d) A is true but R is false.
The filament of a bulb is made of high resistance and high melting point.
As a result, although same current flows through the live wires and the filament of a bulb, but the heat produced in the filament is much higher due to higher resistivity of wire.
Solution 25
(d) A is true but R is false.
A fuse is a short piece of wire of high resistance and of material with a low melting point.
A fuse of different thicknesses can be used for both small as well as large current flow.
Hence the assertion is true, but reason is false.
Solution 28
(d) A is true but R is false.
A transformer is a device by which the amplitude of an alternating e.m.f. can be increased or decreased.
Now to increase the output voltage, the number of turns in secondary coil must be greater than the number of turns in the primary coil.
Hence given reason is false, whereas the assertion is true.
Solution 29
(a) Both A and R are true.
A transformer can only be used with AC, not DC.
And laminated core can minimize eddy current losses.
Solution 31
(a) Both A and R are true.
The magnitude of induced e.m.f. becomes maximum when the magnetic flux linked with the coil reduces to zero from its maximum value. This happens in an A.C generator when the plane of coil lies in the direction of the magnetic field.
Solution 26
(b) Both A and R are false.
The specific heat capacity of a substance is its characteristic property, and it is defined as the amount of heat energy required to raise the temperature of the unit mass of that substance by 1°C (or 1 K).
Solution 27
(a) Both A and R are true.
According to the principle of the mixture method, the heat energy lost by a hot body is equal to the heat energy gained by a cold body.
i.e., 
This is principle based on law of conservation of energy and it involves mixing substances at different temperatures to find the final temperature.
Solution 20
(c) A is false but R is true.
As we know, the penetrating power of γ radiation is maximum and α particle is minimum, out of α, β and γ radiation.
i.e., a < β < γ
Whereas mass of α particle is maximum among α, β and γ radiation.
Hence given assertion is false, and reason given is true
Solution 22
(a) Both A and R are true.
Isobars are atoms of different elements that have the same mass number A but differ in their atomic number Z.
Example: 
and 
or 
and 
And for the given case, has 6 protons and 8 neutrons in its nucleus, whereas has 7 protons and 7 neutrons in its nucleus.
Hence both assertion and reason are true.
