Class 9 SELINA Solutions Physics Chapter 11: Assertion-Reasoning Type Questions

(b) Both A and R are false.

Derived units are those which are created by combining the fundamental units. For example, a unit of speed is metres per second and it is derived from a unit of fundamental quantities length (metre) and time (seconds). So, A is false.

Also, these units can be changed and are related to other fundamental units. Thus, R is also false.

Thus, the correct option is (b). Both A and R are false.

(a) Both A and R are true

Least count is the smallest value an instrument can measure with certainty. If an instrument has a smaller least count, it can distinguish between finer gradations in measurement, leading to more precise measurements. Thus, the assertion statement is true.

The least count directly relates to the accuracy of the measurement. An instrument cannot provide a measurement value smaller than its least count. Thus, the reason statement is also true.

Thus, both A and R are true.

(c) A is false but R is true.

The least count of a screw gauge can be decreased by decreasing the pitch and increasing the total number of divisions on a circular scale.

Thus, the assertion statement is false.

From the above equation, it is clear that the least count of screw gauge is directly proportional to the pitch of the screw gauge.

Thus, the reason statement is true.

Thus, the correct option is option (c). A is false but R is true.

(d) A is true but R is false.

As we go deeper into a mine, the distance from the Earth's centre decreases slightly. However, the overall mass of the Earth remains constant. Thus, there is a slight decrease in the acceleration due to gravity (g) at deeper depths.

Thus, reason statement is false.

The time period (T) of a pendulum is related to its length (l) and the acceleration due to gravity (g) by the formula:

T = 2π√(l/g)

Here, we can see that the time period (T) is proportional to the square root of the length (l) and inversely proportional to the square root of gravity (g).

Since the length of the pendulum is constant, a decrease in g in mines will lead to an increase in the time period (T). Thus, the pendulum will take longer to swing back and forth.

Thus, the pendulum clock will run slower when it is taken to mines.

Thus, the assertion statement is true.

So, the correct option is (d) A is true but R is false.

(a) Both A and R are true.

The displacement of a body can be zero even if the distance travelled by it is not zero. It happens when during its path travelled it returns to its initial point. Thus, A is true.

Displacement is the shortest distance from the initial to the final position of body. Thus, R is also true.

So, the correct option is (a) Both A and R are true.

(a) Both A and R are true.

Average speed is the total distance travelled divided by the total time taken for the journey.

Average velocity is the straight-line displacement divided by the total time taken.

For example, when a person walks around a rectangular park. He starts and ends at the same corner.

Therefore, even though he walked for a certain time, the average velocity (displacement/time) would be zero, while the average speed (distance/time) would be a non-zero value.

Thus, A is true.

Speed is a scalar quantity because for its measurement only magnitude is considered. Velocity is vector quantity because while measuring it both magnitude as well as direction is considered.

Thus, R is also true.

So, the correct option is (a) Both A and R are true.

(d) A is true, but R is false.

The slope of displacement -time graph represents the velocity. In the displacement-time graph, displacement is the dependent variable and is represented on the y-axis, while time is the independent variable and is represented on the x-axis. Thus, A is true.

Velocity is the ratio of displacement and time and not the product. Thus, R is false.

So, the correct option is (d) A is true, but R is false.

(b) Both A and R are false.

When a body is in motion with uniform velocity, the velocity-time graph is a straight line parallel to the time axis and not the velocity axis. Here velocity is the dependent variable and thus it is represented on the y-axis and time is the independent variable which is represented on the x-axis. The straight line will be parallel to the time axis. Thus, the assertion statement is false and as the reason statement is dependent upon the assertion statement it is also false.

Thus, Both A and R are false.

Note: If a body is in motion with uniform velocity, the velocity–time graph is a straight line parallel to the time axis.

(a) Both A and R are true. 

If the velocity of the body in motion increases uniformly with time, the acceleration-time graph is a straight line parallel to the time axis on the positive side. This type of graph indicates that the body is uniformly accelerated. Thus, the assertion statement is true. When the body is freely falling under gravity, it is moving with uniform acceleration. Thus, the reason statement is also true.

Thus, the correct option is (a) Both A and R are true.

(c) A is false, but R is true.

Acceleration of a moving body can be positive, negative and zero. Thus, the assertion statement is false. But, acceleration is the rate of change of velocity with respect to the time is the true statement. Thus, the correct option is option (c) A is false, but R is true.

Both A and R are true.

The magnitude of non–contact forces like gravitational force, magnetic force, electrostatic force etc. decreases with an increase in the distance of separation between them.

The non-contact forces follow inverse square law, which means the force between the two bodies weakens as the force is inversely proportional to the square of the distance between the two bodies.

(c) A is false but R is true.

Inertia is a tendency of a body by virtue of which it resists the change in a state of rest as well as a state of motion.  Thus, the assertion statement is false.

Force is indeed an external influence that can change the state of rest or state of motion of an object.

Thus, the correct option is (c) A is false but R is true.

(a) Both A and R are true.

When a force is applied to an object, it causes acceleration. According to Newton’s second law of motion, acceleration depends on the force applied and the mass of the object. A cricket ball has a larger mass than a tennis ball. Thus, with the same magnitude of force applied, the cricket ball will have a smaller acceleration and therefore a lower final velocity compared to the tennis ball. Thus, A is true.

Mass is a measure of inertia. The more the mass of an object, more is the inertia.

Thus, the correct option is (a) Both A and R are true.

(d) A is true but R is false.

According to Newton’s third law of motion, to every action, there is always an equal and opposite reaction. The action and reaction are always in opposite directions and act on different bodies. Thus, the assertion is true, but reason is false.

Thus, the correct option is (d) A is true but R is false.

(c) A is false but R is true.

Momentum is the product of mass and velocity and not acceleration. Momentum has both magnitude as well as direction thus, it is a vector quantity.

Thus, A is false but R is true.

(d) A is true but R is false.

When we stop pedalling a bicycle, it does slow down due to the absence of the force in the forward direction. Thus, the assertion statement is true.

The force of friction opposes the motion of an object. Thus, the frictional force generally acts in the direction opposite to the motion. Thus, the reason is false.

Thus, the correct option is (d) A is true but R is false.

(b) Both A and R are false.

A person will feel lighter on the Moon compared to Earth. This is because the Moon's gravity is about 1/6^th of Earth's gravity.

Gravitational force is exerted by the Moon, not by the man. The person experiences the gravitational force of the moon.

Thus, both the assertion and reason statement are false.

(a) Both A and R are true.

If the distance between two bodies increases by a factor of 4, the gravitational force reduces by 1/16. This is because the gravitational force is inversely proportional to the square of the distance between two bodies.

Thus, both assertion and reason are true. Thus, the correct option is (a).

(b) Both A and R are false.

SI unit of weight is Newton while the SI unit of mass is kilogram. Weight is the product of mass and acceleration due to gravity. It is force exerted on an object due to gravity. Mass is a measure of the amount of matter in an object, independent of its location or the influence of gravity. It remains constant regardless of where the object is placed.

Thus, both assertion and reason are false. Thus, the correct option is (b).

(d) A is true but R is false.

Nose bleeding occurs at higher altitudes because, at higher altitudes the air pressure decreases.

At higher altitudes, the blood pressure inside our body is higher than the atmospheric pressure. This pressure difference can cause delicate blood vessels in the nose to rupture and this leads to nosebleeds.

Thus, the correct option is (d) A is true but R is false.

(c) A is false but R is true.

Water does stick to the glass surface due to which it can affect the barometric readings. Thus, the assertion statement is false. Barometric liquid should be such so that it does not stick to the walls of the barometer.

Thus, correct option is (c) A is false, but R is true.

(a) Both A and R are true.

When the brick is placed with its longest side vertical, the area it touches on the ground is less. We know that pressure (P) is force (F) divided by area (A), and P = F/A. Thus, the pressure exerted by the smaller area is maximum.

Thus, the assertion statement is true.

Also, the larger the area on which a given thrust acts, the lesser is the pressure exerted by it.

Thus, the correct option is (a) Both A and R are true.

(d) A is true but R is false.

The walls of a dam is made thicker at the bottom because the pressure exerted by a liquid increases with its depth.

Thus, assertion is true but reason is false.

(a) Both A and R are true. 

A hydraulic press is used for squeezing oil out of linseed and cotton seeds. Hydraulic machines like hydraulic press, hydraulic jack and hydraulic brakes are based on the principle of Pascal’s law of transmission of pressure in liquids.

Thus, the correct option is (a) Both A and R are true.

(a) Both A and R are true. 

A body weighs slightly less in air than in vacuum because there is absence of air in vacuum. The buoyant force lessens some of the weight of object, making the reading on the spring balance slightly less than the object's actual weight.

Thus, the correct option is (a) Both A and R are true.

(b) Both A and R are false.

When a bucket of water is immersed in water, the buoyant force acting on it causes it to become lighter, reducing its apparent weight. Hence the given assertion is false.

According to the given reason, the effect of upthrust is that the weight of body immersed in liquid appears to be heavier than its actual weight. This is incorrect since upthrust, also known as buoyancy, causes a body immersed in a liquid to appear lighter rather than heavier.

(d) A is true but R is false.

As we know, the density of mercury is greater than the density of iron. Subsequently, when an iron nail is placed over mercury, it floats because its density is lower than that of mercury.

However, when the same nail is placed over water, it sinks because the density of iron is greater than that of water.

(a) Both A and R are true.

The given assertion states that a ship carrying a load will be more submerged than an unloaded ship. This is true because a loaded ship displaces more water, causing it to sit lower in the water.

The reason statement that a loaded ship displaces more water than an unloaded ship in order to float is also true. This is because the buoyant force acting on the ship is equal to the weight of the water displaced. Therefore, a heavier, loaded ship must displace more water in order to stay afloat.

(a) Both A and R are true.

In winter, a pond's surface water cools and sinks until it reaches 4°C. Further cooling causes the top layers to expand and become less dense, preventing them from sinking. When the air temperature falls below 0°C, the surface water freezes into ice, while the water beneath it remains at 4°C.

As a result, fish survive because the ice layer acts as an insulator, preventing heat transfer.

(a) Both A and R are true.

As we know, according to the law of reflection, the angle of incidence (i) is equal to the angle of reflection (r).

i.e., ∠i = ∠r

Thus, according to the given assertion, if the angle of incidence is 0° then the angle of reflection will also be 0°.

Hence, both A and R are true.

(c) A is false but R is true.

As shown in the figure below, an erect and virtual image is obtained from a concave mirror when an object is placed between pole P and the focal point of the mirror. Hence, the assertion that the image of an object placed close to a concave mirror is a real image is incorrect.

The reason is true since a real image is one that can be obtained on a screen and formed due to the actual intersection of the reflected rays.

Hence, A is false but R is true.

(c) A is false but R is true.

Lateral inversion means the apparent reversal of the mirror image's left and right when compared with the object.

The given assertion is false since the letter A appears as A in its mirror image due to a phenomenon know as lateral inversion.

Hence, A is false but R is true.

(d) A is true but R is false.

If is odd.

The number of images is n when the object is placed asymmetrically between the mirrors.

The number of images is n – 1, when the object is placed symmetrically between the mirrors.

Hence, the given reason is incorrect.

Now, when an object is placed symmetrically when the angle θ between the mirrors is 72°, the number of images formed is 4.

Then,

∴ Number of images = n -1 = 4

(a) Both A and R are true. 

The assertion that concave mirrors are used as doctor's head mirrors is true, as the concave shape of the mirror allows it to focus light onto the area being examined, providing better illumination and magnification.

Also, the reason is true since a concave mirror focuses a parallel beam of light to a point. This is a fundamental property of concave mirrors that enables their use in medical examinations and procedures.

(b) Both A and R are false.

The assertion states that a student identifies a spherical mirror with a focal length of -10 cm as a convex mirror, which is false. The negative focal length indicates that the mirror is concave, not convex.

Also, the reason given is false since the focal length of a concave mirror is negative, while for a convex mirror, the focal length is positive.

(a) Both A and R are true.

Sound waves need a medium to travel and can't pass through vacuum due to the lack of a material medium. They are longitudinal waves with vibrations in the same direction as their propagation, which is important to understand their travel through different mediums.

(c) A is false but R is true.

As we know, the speed of sound depends on the density of the medium, which is greater in solids, lower in liquids, and least in gases. Therefore, the given assertion is false, and the reason is true.

(a) Both A and R are true.

At standard temperature, the density of water vapour is about 5/8^th times that of dry air. Thus, the increase in moisture in the air tends to decrease its density.

Hence sound travels faster in humid air than in dry air.

(b) Both A and R are false.

The phenomenon of sound propagation is dependent on the presence of a medium that is capable of transmitting vibrations. Therefore, due to the lack of such a medium on the moon, it is not possible for two individuals to communicate through sound waves.

Hence, both assertion and reason are false.

(c) A is false but R is true.

Sound with a frequency above 20 kHz is referred to as ultrasound or ultrasonic waves, while sound with a frequency between 20 Hz and 20 kHz is known as sonic waves. Ultrasound (or ultrasonics) is the term used for sound with a frequency greater than 20 kHz.

Supersonic waves travel at the highest speed followed by sonic waves, and ultrasonic waves travel at the lowest speed. Therefore, the given statement is false, and the reason for it is true.

(a) Both A and R are true.

In the primary cell, current is produced due to irreversible chemical reactions, which are of the use-and-throw type.

Hence both assertion and reason statements are true.

(a) Both A and R are true.

The assertion is true since the current is defined as the rate of charge flow across a cross-section normal to the direction of the flow of current.

This reason is true. The conventional direction of current flow is defined as the direction of the flow of positive charges, which is opposite to the actual direction of the flow of electrons in a circuit.

(b) Both A and R are false.

The given assertion is false since the ammeter is used to determine the amount of current flowing through a circuit, whereas the voltmeter measures the potential difference between two points.

Whereas an ammeter is designed to have a very low resistance, so that it does not significantly affect the current flowing through the circuit when connected in series.

Hence, both assertion and reason are false.

(d) A is true but R is false.

Insulators are materials that do not allow electric current to flow easily through them due to tightly bound electrons, leading to high resistance.

These properties of materials such as cotton and rubber make them good insulators.

Hence, the assertion is true, but the reason is false.

(b) Both A and R are false.

A thin wire offers more resistance than a thick wire since the resistance of a wire is directly proportional to its length and inversely proportional to its area of cross-section.

Hence both assertion and reason statements are false.

(a) Both A and R are true.

A conductor having an excess of electrons is said to be at a lower or negative potential, while a conductor having a deficit of electrons is said to be at a higher or positive potential.

Current always flows from a higher potential to a lower potential.

i.e., In the direction opposite to the flow of electrons.

Hence, both assertion and reason statements are true.

(d) A is true but R is false.

A longer wire offers more resistance than a smaller wire since the resistance of a wire is directly proportional to its length and inversely proportional to its area of cross-section.

Hence, assertion is true, but reason is false.

(c) A is false but R is true.

As we know, unlike pole attract each other and like pole repel each other.

Hence, assertion is false, but reason is true.

(a) Both A and R are true.

The magnetic field strength of an electromagnet can be increased by:

1. Increasing the number of turns of winding in the solenoid.

2. Increasing the current flowing through the solenoid.

Hence, both assertion and reason statements are true.

(a) Both A and R are true. 

A magnetic field line is a continuous curve in a magnetic field such that tangent at any point of the curve gives the direction of the magnetic field at that point.

Thus, they never intersect each other.

(b) Both A and R are false.

Neutral points are the points at which two magnetic fields are equal in magnitude but opposite in direction. The net magnetic field at a neutral point is zero.

A compass needle if placed at the neutral point will rest in any direction.

Hence both assertion and reason are false.