Assertion & Reason - JEE physics STD 12 Science Questions - Vidyadip

Questions

Assertion & Reason

Take a timed test

33 questions · self-marked practice — reveal the answer and mark yourself.

Question 11 Mark

Assertion : For the planets orbiting around the sun, angular speed, linear speed, K.E. changes with time, but angular momentum remains constant.
Reason : No torque is acting on the rotating planet. So its angular momentum is constant.

Answer

(a) The torque on a body is given by $\vec{\tau}=\frac{d \vec{L}}{d t}$In case of planet orbiting around sun no torque is acting on it. $\frac{d \vec{L}}{d t}=0 \Rightarrow \vec{L}=$ constant.

View full question & answer→

Question 21 Mark

Assertion : Gravitational field is zero both at centre and infinity.
Reason : The dimensions of gravitational field is $\left[L T^{-2}\right]$.

Answer

(b)

View full question & answer→

Question 31 Mark

Assertion : The speed of revolution of an artificial satellite revolving very near the earth is $8 \mathrm{kms}^{-1}$.
Reason : Orbital velocity of a satellite, become independent of height of near satellite.

Answer

(a)$v_0=R_e \sqrt{\frac{g}{R_e+h}}$For satellite revolving very near to earth $R_e+h=R_e$As $(h<<)$$v_o=\sqrt{R_e g} \simeq \sqrt{64 \times 10^5 \times 10}=8 \times 10^3 \mathrm{~m} / \mathrm{s}=8 \mathrm{kms}^{-1}$Which is independent of height of a satellite.

View full question & answer→

Question 41 Mark

Assertion : The speed of satellite always remains constant in an orbit.
Reason : The speed of a satellite depends on its path.

Answer

(e) If the orbital path of a satellite is circular, then its speed is constant and if the orbital path of a satellite is elliptical, then its speed in its orbit is not constant. In that case its areal velocity is constant.

View full question & answer→

Question 51 Mark

Assertion : A person sitting in an artificial satellite revolving around the earth feels weightless.
Reason : There is no gravitational force on the satellite.

Answer

(c) A person feels his weight only when the surface on which he is standing exerts a reactionary force on him. Because the acceleration of the person and that of the satellite revolving round the earth are equal $(=g)$, hence acceleration of the person with respect to the satellite is zero.Therefore person feels weightless on satellite, although the gravitational force is acting on a satellite.

View full question & answer→

Question 61 Mark

Assertion : For a satellite revolving very near to earth's surface the time period of revolution is given by 1 hour 24 minutes.
Reason : The period of revolution of a satellite depends only upon its height above the earth's surface.

Answer

(a) The time period of satellite which is very near to earth is given by$T=2 \pi \sqrt{\frac{R}{g}} \cong 84 \mathrm{~min}=1 \mathrm{hr} .24 \mathrm{~min}$

View full question & answer→

Question 71 Mark

Assertion : If earth were a hollow sphere, gravitational field intensity at any point inside the earth would be zero.
Reason : Net force on a body inside the sphere is zero.

Answer

(a) Intensity inside a hollow sphere is zero, so force is also equal to zero. $\vec{F}=m \vec{E}$

View full question & answer→

Question 81 Mark

Assertion : We can not move even a finger without disturbing all the stars.
Reason : Every body in this universe attracts every other body with a force which is inversely proportional to the square of distance between them.

Answer

(a) According to Newton's law of gravitation, every body in this universe attracts every other body with a force which is inversely proportional to the square of the distance between them. When we move our finger, the distance of the objects with respect to finger changes, hence the force of attraction changes, disturbing the entire universe, including stars.

View full question & answer→

Question 91 Mark

Assertion : The binding energy of a satellite does not depend upon the mass of the satellite.
Reason : Binding energy is the negative value of total energy of satellite.

Answer

(e) Binding energy $=\frac{G M m}{R}=-[$ Total energy of satellite $]$ and it is clear that it depends upon the mass of the satellite.

View full question & answer→

Question 101 Mark

Assertion : Space rockets are usually launched in the equatorial line from west to east.
Reason : The acceleration due to gravity is minimum at the equator.

Answer

(b) We know that earth revolves from west to east about its polar axis. Therefore, all the particles on the earth have velocity from west to east.This velocity is maximum in the equatorial line, as $v=R \omega$, where $R$ is the radius of earth and $\omega$ is the angular velocity of revolution of earth about its polar axis.When a rocket is launched from west to east in equatorial plane, the maximum linear velocity is added to the launching velocity of the rocket, due to which launching becomes easier.

View full question & answer→

Question 111 Mark

Assertion : A body becomes weightless at the centre of earth.
Reason : As the distance from centre of earth decreases, acceleration due to gravity increases.

Answer

(c) As the distance from centre of earth decreases, acceleration due to gravity decreases and at the centre of earth it becomes zero.$g^{\prime}=g\left(1-\frac{d}{R}\right) \text {. If } d=R \text { then } g^{\prime}=0$

View full question & answer→

Question 121 Mark

Assertion : Generally the path of a projectile from the earth is parabolic but it is elliptical for projectiles going to a very large height.
Reason : The path of a projectile is independent of the gravitational force of earth.

Answer

(c) Upto ordinary heights, the change in the distance of a projectile from the centre of earth is negligible compared to the radius of earth. Hence the projectile moves under a nearly uniform gravitational force and the path is parabolic. But for the projectiles moving to a large height, the gravitational force decreases quite decreasing variable force, the path of the projectile becomes elliptical.

View full question & answer→

Question 131 Mark

Assertion : When distance between two bodies is doubled and also mass of each body is also doubled, gravitational force between them remains the same.
Reason : According to Newton's law of gravitation, force is directly proportional to mass of bodies and inversely proportional to square of distance between them.

Answer

(a) According to Newton's law of gravitation,$F=\frac{G m_1 m_2}{r^2}$. When $m_1, m_2$ and $r_2$ all are doubled,$F=\frac{G\left(2 m_1\right)\left(2 m_2\right)}{(2 r)^2}=\frac{G m_1 m_2}{r^2}$, i.e. $F$ remains the same.

View full question & answer→

Question 141 Mark

Assertion : The time period of revolution of a satellite close to surface of earth is smaller than that revolving away from surface of earth.
Reason : The square of time period of revolution of a satellite is directly proportional to cube of its orbital radius.

Answer

(a) According to kepler's law $T^2 \propto r^3 \propto(R+h)^3$i.e. if distance of satellite is more then its time period will be more.

View full question & answer→

Question 151 Mark

Assertion : Two different planets have same escape velocity.
Reason : Value of escape velocity is a universal constant.

Answer

(d) As, escape velocity $=\sqrt{\frac{2 G M}{R}}$, so its value depends on mass of planet and radius of the planet. The two different planets have same escape velocity, when these quantities (mass and radius) are equal.

View full question & answer→

Question 161 Mark

Assertion : The principle of superposition is not valid for gravitational force.
Reason : Gravitational force is a conservative force.

Answer

(e) The total gravitational force on one particle due to number of particles is the resultant force of attraction (or gravitational force) exerted on the given particle due to individual particles. i.e., $\quad \vec{F}=\overrightarrow{F_1}+\overrightarrow{F_2}+\overrightarrow{F_3}+\ldots$ it means the principle of superposition is valid.

View full question & answer→

Question 171 Mark

Assertion : The time period of geostationary satellite is 24 hours.
Reason : Geostationary satellite must have the same time period as the time taken by the earth to complete one revolution about its axis.

Answer

(b) As the geostationary satellite is established in an orbit in the plane of the equator at a particular place, so it move in the same sense as the earth and hence its time period of revolution is equal to 24 hours, which is equal to time period of revolution of earth about its axis.

View full question & answer→

Question 181 Mark

Assertion : Earth has an atmosphere but the moon does not.
Reason : Moon is very small in comparison to earth.

Answer

(b) If root mean square velocity of the gas molecules is less than escape velocity from that planet (or satellite) then atmosphere will remain attached with that planet and if $v_{r m s}>v_{\text {escape }}$ then there will be no atmosphere on the planet. This is the reason for no atmosphere at moon.

View full question & answer→

Question 191 Mark

Assertion : If an earth satellite moves to a lower orbit, there is some dissipation of energy but the satellite speed increases.
Reason : The speed of satellite is a constant quantity.

Answer

(c) Due to resistance force of atmosphere, the satellite revolving around the earth losses kinetic energy. Therefore in a particular orbit the gravitational attraction of earth on satellite becomes greater than that required for circular orbit there. Therefore satellite moves down to a lower orbit. In the lower orbit as the potential energy $(U=-G M m / r)$ becomes more negative,Hence kinetic energy $\left(E_K=G M m / 2 r\right)$ increases, and hence speed of satellite increases.

View full question & answer→

Question 201 Mark

Assertion : Orbital velocity of a satellite is greater than its escape velocity.
Reason : Orbit of a satellite is within the gravitational field of earth whereas escaping is beyond the gravitational field of earth.

Answer

(e) Escape velocity $=\sqrt{2} \times$ orbital velocity.

View full question & answer→

Question 211 Mark

Assertion : A planet moves faster, when it is closer to the sun in its orbit and vice versa.
Reason : Orbital velocity in orbital of planet is constant.

Answer

(c) According to Kepler's law of planetary motion, a planet revolves around the sun in such a way that its areal velocity is constant. i.e., it move faster, when it is closer the sun and vice-versa.

View full question & answer→

Question 221 Mark

Assertion : Even when orbit of a satellite is elliptical, its plane of rotation passes through the centre of earth.
Reason : According to law of conservation of angular momentum plane of rotation of satellite always remain same.

Answer

(a) As no torque is acting on the planet, its angular momentum must remain constant in magnitude as well as direction. Therefore, plane of rotation must pass through the centre of earth.

View full question & answer→

Question 231 Mark

Assertion : Gravitational potential of earth at every place on it is negative.
Reason : Every body on earth is bound by the attraction of earth.

Answer

(a) Because gravitational force is always attractive in nature and every body is bound by this gravitational force of attraction of earth.

View full question & answer→

Question 241 Mark

Assertion : The ratio of inertial mass to gravitational mass is equal to one.
Reason : The inertial mass and gravitational mass of a body are equivalent.

Answer

(a) Inertial mass and gravitational mass are equivalent. Both are scalar quantities and measured in the same unit. They are quite different in the method of their measurement. Also gravitational mass of a body is affected by the presence of other bodies near it where as internal mass remain unaffected.

View full question & answer→

Question 251 Mark

Assertion : A force act upon the earth revolving in a circular orbit about the sun. Hence work should be done on the earth.
Reason : The necessary centripetal force for circular motion of earth comes from the gravitational force between earth and sun.

Answer

(e) Earth revolves around the sun in circular path and required centripetal force is provided by gravitational force between earth and sun but the work done by this centripetal force is zero.

View full question & answer→

Question 261 Mark

Assertion : There is no effect of rotation of earth on acceleration due to gravity at poles.
Reason : Rotation of earth is about polar axis.

Answer

(a) As the rotation of earth takes place about polar axis therefore body placed at poles will not feel any centrifugal force and its weight or acceleration due to gravity remains unaffected.

View full question & answer→

Question 271 Mark

Assertion : The difference in the value of acceleration due to gravity at pole and equator is proportional to square of angular velocity of earth.
Reason : The value of acceleration due to gravity is minimum at the equator and maximum at the pole.

Answer

(b) Acceleration due to gravity,$g^{\prime}=g-R \omega^2 \cos ^2 \lambda$At equator, $\lambda=0^{\circ}$ i.e. $\cos 0^{\circ}=1 \therefore g_e=g=R \omega^2$At poles, $\lambda=90^{\circ}$ i.e. $\cos 90^{\circ}=0 \therefore g_p=g$Thus, $g_p-g_e=g-g+R \omega^2=R \omega^2$Also, the value of $g$ is maximum at poles and minimum at equators.

View full question & answer→

Question 281 Mark

Assertion : If earth suddenly stops rotating about its axis, then the value of acceleration due to gravity will become same at all the places.
Reason : The value of acceleration due to gravity is independent of rotation of earth.

Answer

(c) The value of $g$ at any place is given by the relation, $g^{\prime}=g-\omega^2 R_e \cos ^2 \lambda$ When $\lambda$ is angle of latitude and $\omega$ is the angular velocity of earth. If earth suddenly stops rotating, then $\omega=0$$\therefore g^{\prime}=g$ i.e., the value of $g$ will be same at all places.

View full question & answer→

Question 291 Mark

Assertion : If a pendulum is suspended in a lift and lift is falling freely, then its time period becomes infinite.
Reason : Free falling body has acceleration equal to acceleration due to gravity.

Answer

(a) If a pendulum is suspended in a lift and lift is moving downward with some acceleration $a$, then time period of pendulum is given by, $T=2 \pi \sqrt{\frac{l}{g-a}}$.In the case of free fall, $a=g$ then $T=\infty$ i.e., the time period of pendulum becomes infinite.

View full question & answer→

Question 301 Mark

Assertion : The value of acceleration due to gravity does not depend upon mass of the body on which force is applied.
Reason : Acceleration due to gravity is a constant quantity.

Answer

(c) Acceleration due to gravity is given by $g=\frac{G M}{R^2}$. Thus it does not depend on mass of body on which it is acting. Also it is not a constant quantity it changes with change in value of both $M$ and $R$ (distance between two bodies).

View full question & answer→

Question 311 Mark

Assertion : The universal gravitational constant is same as acceleration due to gravity.
Reason : Gravitational constant and acceleration due to gravity have same dimensional formula.

Answer

(d) The universal gravitational constant $G$ is totally different from $g$.$G=\frac{F R^2}{M m}$The constant $G$ is scalar and posses the dimensions $\left[M^{-1} L^3 T^{-2}\right]$.$g=\frac{G M}{R^2}$$g$ is a vector and has got the dimensions $\left[M^0 L T^{-2}\right]$.It is not a universal constant.

View full question & answer→

Question 321 Mark

Assertion : Gravitational force between two particles is negligibly small compared to the electrical force.
Reason : The electrical force is experienced by charged particles only.

Answer

(b) For two electron $\frac{F_g}{F_e}=10^{-43}$ i.e. gravitational force is negligible in comparison to electrostatic force of attraction.

View full question & answer→

Question 331 Mark

Assertion : Smaller the orbit of the planet around the sun, shorter is the time it takes to complete one revolution.
Reason : According to Kepler's third law of planetary motion, square of time period is proportional to cube of mean distance from sun.

Answer

(a) According to Kepler's third law $T^2 \propto r^3$If $r$ is small then $T$ will also be small.

View full question & answer→

Generate Paper Free All Gravitation topics