# HC Verma Class XII Science Physics Chapter 11 – Gravitation

#### Exercise : Solution of Questions on page Number : 223

**Answer: 1**

A particle will be in equilibrium when the net force acting on it is equal to zero. Two particles under the action of their mutual gravitational force will be in equilibrium when they revolve around a common point under the influence of their mutual gravitational force of attraction. In this case, the gravitational pull is used up in providing the necessary centripetal force. Hence, the net force on the particles is zero and they are in equilibrium. This is also true for a three particle system.

**Answer: 2**

Weight of a body is always because of its gravitational attraction with earth.As law of gravitational attraction is universal so it applies to any two bodies (earth and sun as well).So we can define the weight of earth w.r.t a body of mass comparable or heavier than earth because of its gravitational attraction between earth and that body.

But practically no body on earth has mass comparable to earth so weight of earth will be a meaningless concept w.r.t earth frame.

**Answer: 3**

We know that acceleration due to a force on a body of mass in given by a=Fm.

If F is the gravitational force acting on a body of mass m, then a is the acceleration of a free falling body.

This force is given as F=GMmR2.

Here, M is the mass of the Earth; G is the universal gravitational constant and R is the radius of the Earth.

∴ Acceleration due to gravity, a=Fm=GMR2

From the above relation, we can see that acceleration produced in a body does not depend on the mass of the body. So, acceleration due to gravity is the same for all bodies.

**Answer: 4**

No. All practicals which have mass exert gravitational force on each other. Even massless particles experience the same gravitational force like other particles, because they do have relativistic mass.

**Answer: 5**

We know that the Earth-Moon system revolves around the Sun. The gravitational force of the Sun on the system provides the centripetal force its revolution. Therefore, the net force on the system is zero and the Moon does not experience any force from the Sun. This is the reason why the Moon revolves around the Earth and not around the Sun.

**Answer: 6**

No. Due to the revolution of the Earth around the Sun, the gravitational force of the Sun on the Earth system is almost zero. Hence, the body will not experience any force due to the Sun. Therefore, weight of the object will remain the same.

**Answer: 7**

The mutual gravitational force between the apple and the Earth is responsible for the acceleration produced in the apple falling from the tree. Although the Earth will experience the same force, it does not get attracted towards the apple because of its large mass. The insect feels that the Earth is falling towards the apple with an acceleration g because of the the relative motion.

let

vae=velocity of apple w.r.t earth

Vea=velocity of earth w.r.t apple

vae=va-ve=-(ve-va)=-vea

As the insect is in the frame of apple so he sees the earth moving with a relative velocity vea.

Any other observer on earth will see the apple moving towards earth with velocity vae.Both are opposite in direction.

**Answer: 8**

The gravitational potential due to the system is given as V=kr2.

Gravitational field due to the system:

E=-dVdr⇒E=-ddrkr2=–2kr3⇒E=2kr3

We can see that for this system, E∝1r3

This type of system is not possible because Fg is always proportional to inverse of square of distance(experimental fact).

If there were negative masses, then this type of system is possible.

This system is a dipole of two masses, i.e., two masses, one positive and the other negative, separated by a small distance.

In this case, the gradational field due to the dipole is proportional to 1r3.

**Answer: 9**

The gravitational potential energy of a two-particle system is given by U=-Gm1m2r.

This relation does not tell that the gravitational potential energy is zero at infinity. For our convenience, we choose the potential energies of the two particles to be zero when the separation between them is infinity.

No, if we suppose that the potential energy for r=∞ is 20 J, then we need to modify the formula.

Now, potential energy of the two-particle system separated by a distance r is given by

U(r,)=U(r)-U(∞)Given: U(∞)=20 J∴ U(r)=-Gm1m2r2-20

This formula should be used to calculate the gravitational potential energy at separation r.

**Answer: 10**

The weight of an object is more at the poles than that at the equator. In purchasing or selling goods, we measure the mass of the goods. The balance used to measure the mass is calibrated according to the place to give its correct reading. So, it is not beneficial to purchase goods at the equator and sell them at the poles. A beam balance measures the mass of an object, so it can be used here. For using a spring balance, we need to calibrate it according to the place to give the correct readings.

**Answer: 11**

The weight of a body at the poles is greater than that at the equator. Here, we are talking about the actual weight of the body at that particular place.

Yes. If the rotation of the Earth is taken into account, then we are discussing the apparent weight of the body.

#### Exercise : Solution of Questions on page Number : 224

**Answer: 12**

If we consider the Earth to be a perfect sphere, then the acceleration due to gravity at its surface is given by g=GMR2.

Here, M is the mass of Earth; R is the radius of the Earth and G is universal gravitational constant.

If the radius of the earth is decreased by 1%, then the new radius becomes

R’=R-R100=99100R⇒R’=0.99R

New acceleration due to gravity will be given by

g’=GMR’2=GM(0.99R)2⇒g’=1.02×GMR2=1.02g

Hence, the value of the acceleration due to gravity increases when the radius is decreased.

Percentage increase in the acceleration due to gravity is given by

g’-gg×100 =0.02gg×100 =2%

**Answer: 13**

No, it will not land on the Earth. The nut will start revolving in the orbit of the satellite with the same orbital speed as that of the satellite due to inertia of motion. An astronaut can make it land on the Earth by projecting it with some velocity toward the Earth.

**Answer: 14**

According to Kepler first law of planetary motion all planets move in elliptical orbits with sun at one of its foci. It applies to any planet and its satellite as well.This implies that plane of the satellite has to pass through the centre of planet (earth).

**Answer: 15**

T=(gR2T24π2)13-RT=4π2(h+R)3gR2=4×3.142×(36000+6400)3×1099.8×(6400×103)2=24.097 Hr

Which implies that it is a geostationary satellite with time period=24 Hrs.

**Answer: 16**

No. All geostationary orbits are concentric with the equator of the Earth.

**Answer: 17**

A person living in a house at the equator will not feel weightlessness because he is not in a free fall motion. Satellites are in free fall motion under the gravitational pull of the earth, but, due to the curved surface of the Earth, they move in a circular path. The gravitational force on the satellite due to the Sun provides the centripetal force for its revolution. Therefore, net force on the satellite is zero and, thus, a person feels weightless in a satellite orbiting the earth.

**Answer: 18**

No, both satellites will have different time periods as seen from the Earth. The satellite moving opposite (east to west) to the rotational direction of the Earth will have less time period, because its relative speed with respect to the Earth is more.

**Answer: 19**

Yes, a spacecraft consumes more fuel in going from the Earth to the Moon than it takes for the return trip. In going from the Earth to the Moon, the spacecraft has to overcome the gravitational pull of the earth. So, more fuel is consumed in going from the Earth to Moon. However, in the return trip, this gravitation pull helps the spacecraft to come back to the Earth.

**Answer: 1**

(b) 0⋅0027 m s−2

We know that the distance of the Moon from the Earth is about 60 times the radius of the earth. So, acceleration due to gravity at that distance is 0.0027 m/s2. When the Moon is stopped for an instant and then released, it will fall towards the Earth with an initial acceleration of 0.0027 m/s2.

**Answer: 2**

(c) 6⋅4 m s−2

According to the previous question, we have:

Radius of the moon, Rm=Re4=64000004=1600000 m

So, when the Moon is just about to hit the surface of the Earth, its centre of mass is at a distance of (Re + Rm) from the centre of the Earth.

Acceleration of the Moon just before hitting the surface of the earth is given by

g’=GM(Re+Rm)2=GMRe2(1+RmRe)2⇒g’=g(1+RmRe)2=10(1+14)2=10×1625⇒g’=6.4 m/s2

**Answer: 3**

(c) C

At one point between the Earth and Mars, the gravitational field intensity is zero. So, at that point, the weight of the passenger is zero. The curve C indicates that the weight of the passenger is zero at a point between the Earth and Mars.

**Answer: 4**

(d) 21/3 W at the planet

The weight of the object on the Earth is W=mGMeRe2.

Here, m is the actual mass of the object; Me is the mass of the earth and Re is the radius of the earth.

Let Rp be the radius of the planet.

Mass of the planet, Mp=2Me

If ρ is the average density of the planet then

43πRp3×ρ=2×43πRe3×ρ⇒Rp=213Re

Now, weight of the body on the planet is given by

Wp=mGMpRp2=m2GMe223Re2⇒Wp=213×mGMeRe2⇒Wp=213×W

**Answer: 5**

(a) 12mgR

Work done = -(final potential energy – initial potential energy)

⇒W=-GMm2R-GMmR⇒W=12GMmR=12mR×GMR2⇒W=12mRg

∵ g=GMR2

**Answer: 6**

The work done by the person is equal to the kinetic energy and the potential energy of the mass of 1 kg at point A.

Let VA be the potential at point A.

Now, W=12mv2+P.E.A⇒W=12mv2+VA×m⇒-3=12×1×(2)2+VA×1⇒VA=-5 J/Kg

**Answer: 7**

(c) B is correct but A is wrong.

The plot of E against r is discontinuous as gravitational field inside the spherical shell is zero (r < R). The plot of V against r is a continuous curve for a uniform spherical shell.

**Answer: 8**

(d) Both A and B are wrong.

Both the plots (i.e., V against r and E against r) are continuous curves for a uniform solid sphere.

**Answer: 9**

(b) A is correct but B is wrong.

We know that the value of acceleration due to gravity decreases when we go up from the surface of the Earth. If we take the into account the effect of bulging of the Earth due to its rotation, we can say that acceleration due to gravity is maximum at the poles and minimum at the equator

So, there are points above both the poles where the value of g is equal to its value at the equator.

#### Exercise : Solution of Questions on page Number :225

**Answer: 1**

The gravitational force of attraction between the balls is given as by

F=Gm1m2r2Given: m1=m2=10 kg and r=10 cm=0.10 m

∴ F=6.67×10-11×10×100.12 ⇒ F=6.67×10-7 N

**Answer: 2**

Force due to the particle at A, F→OA=G×m×mOA2 Let OA =r∴ F→OA=G×m×mr2 Here, r=a22+a22=a2 Force due to the particle at B, F→OB=G×m×2mr2 Force due to the particle at C, F→OC=G×m×3mr2 Force due to the particle at D, F→OD=G×m×4mr2 Now, resultant force=F→OA+F→OB+F→OC+F→OD=2 Gmma 2-i→2+j→2+4 Gmma 2i→2+j→2=6 Gmma 2i→2-j→2+8Gmma2-i→2–j→2

∴ F=44Gm2a2 j⏜

**Answer: 10**

(a) the mass of the satellite

The time period of an earth-satellite in circular orbit is independent of the mass of the satellite, but depends on the radius of the orbit.

**Answer: 11**

(b) U > K

For a system to be bound , total energy of the system should be negative.As we know that kinetic energy can never be negative.

E<0 & K>0 & E=K+U

it gives that U>K.

**Answer: 12**

(b) t1 = t2

Kepler’s second law states that the line joining a planet to the Sun sweeps out equal areas in equal intervals of time, i.e., areal velocity of a planet about the Sun is constant.

The given areas swept by the planet are equal, so t1 = t2

**Answer: 13**

(c) the normal force is zero

The gravitational pull on the satellite is used up in providing the necessary centripetal force required for its revolution around the earth. This means that there is no net force on the person sitting in a chair in the satellite. So, the normal reaction of the chair on the person is zero and he will feel weightless.

**Answer: 14**

(a) W1 = W22

Question : Solution of Questions on page Number : 225

The gravitational pull on the satellite in both cases is used up in providing the necessary centripetal force required for its revolution around the earth. This means that there is no net force acting on the body which has been suspended from a spring balance in the satellite. So, the readings of the spring balan2ce in both the cases are the same and is equal to zero.

**Answer: 15**

(c) mgR

The kinetic energy needed to project a body of mass m from the Earth’s surface to infinity is equal to the negative of the change in potential energy of the body.

i.e., kinetic energy = -(final potential energy – initial potential energy)

⇒K=-GMmR’-GMmR⇒K=-GMm∞-GMmR=GMmR⇒K=mR×GMR2⇒K=mRg ∴ g=GMR2

**Answer: 16**

(c) GMR

Potential energy of the particle at a distance R from the surface of the Earth is P.E.i=GMm(R+R)=12GMmR.

Here, M is the mass of the earth; R is the radius of the earth and m is the mass of the body.

Let the particle be projected with speed v so that it just escapes the gravitational pull of the earth.

So, kinetic energy of the body = -[change in the potential energy of the body]

Now, kinetic energy of the body = -[final potential energy – initial potential energy]

⇒12mv2=-GMm∞-GMm2R⇒v=GMmR

#### Exercise : Solution of Questions on page Number : 226

**Answer: 17**

(d) will depend on the direction of projection.For example a body projected vertically requires less escape velocity than a body projected at an angle with the vertical.

**Answer: 1**

(a) V=0 and E=0

(b) V=0 and E≠0

(c) V≠0 and E=0

(d) V≠0 and E≠0.

5All the given conditions for gravitational potential and gravitational field at a point are possible.

**Answer: 2**

(b) the gravitational field is zero

(c) the gravitational potential is same everywhere

(d) the gravitational field is same everywhere

Inside a uniform spherical shell, the gravitational field is the same everywhere and is equal to zero. The gravitational potential has a constant value inside a uniform spherical shell.

**Answer: 3**

(b) decreases

AS it is maintaining its shape so its mass will remain constant which implies that if it shrinks then its volume decreases and to keep the mass constant ,its density increases.Also the gravitational potential at the centre of a uniform spherical shell is inversely proportional to the radius of the shell with a negative sign. When a uniform spherical shell gradually shrinks, the gravitational potential at the centre decreases (because of the negative sign in the formula of potential).

**Answer: 4**

(b) is zero in some parts of the orbit

(c) is zero in one complete revolution

When a planet is moving in an elliptical orbit, at some point, the line joining the centre of the Sun and the planet is perpendicular to the velocity of the planet. For that instant, work done by the gravitational force on the planet becomes zero. As there is no net increase in the speed of the planet after one complete revolution about the Sun, the work done by the gravitational force on the planet in one complete revolution is zero.

Note:For elliptical orbits angle between force ans velocity is always 90 so there the work done is zero in any small part of the orbit.

**Answer: 5**

(a) Speeds of A and B are equal.

The orbital speed of a satellite is independent of the mass of the satellite, but it depends on the radius of the orbit. Potential energy, kinetic energy and total energy depend on the mass of the the satellite.

**Answer: 6**

(d) Angular momentum.

In planetary motion, the net external torque on the planet is zero. Therefore, angular momentum will remain constant.

#### Exercise : Solution of Questions on page Number : 226

**Answer: 3**

(a)

Consider that mass ‘m’ is placed at the midpoint O of side AB of equilateral triangle ABC.

AO = BO = a2

Then F→OA=4Gm2a2 along OA

Also, F→OB=4 Gm2a2 along OB

OC = 3a2

F→OC=4 Gm23a2=4Gm23a2 along OC

The net force on the particle at O is F→=F→OA+F→OB+F→OC.

Since equal and opposite forces cancel each other, we have:

F→=F→OC=4 Gm23a2=4Gm23a2 along OC.

(b) If the particle placed at O (centroid)

All the forces are equal in magnitude but their directions are different as shown in the figure.

Equal and opposite forces along OM and ON cancel each other.

i.e., Fcos30°=Fcos30°

∴ Resultant force =F-2Fsin30=0

**Answer: 4**

Three spheres are placed with their centres at A, B and C as shown in the figure.

Gravitational force on sphere C due to sphere B is given by

F→CB=Gm24a2cos 60° i^+Gm24a2·sin 60° j^

Gravitational force on sphere C due to sphere A is given by

F→CA=-Gm24a2 cos 60° i^+Gm24a2·sin 60°j^

∴ F →CB=F→CB+F→CA =+2Gm24a2sin 60°j^ =+2Gm24a2×32

i.e., magnitude=3 Gm24a2 along CO

**Answer: 5**

Assume that three particles are at points A, B and C on the circumference of a circle.

BC = CD = 2a

The force on the particle at C due to gravitational attraction of the particle at B is F→CB=GM22R2j^.

The force on the particle at C due to gravitational attraction of the particle at D is F→CD=-GM22R2i^.

Now, force on the particle at C due to gravitational attraction of the particle at A is given by

F→CA=-GM24R2cos 45i^+GM24R2sin 45 j^∴F→C=F→CA+F→CB+F→CD =-GM24R22+12i^+GM24R22+12 j^

So, the resultant gravitational force on C is FC=Gm24R222+1.

Let v be the velocity with which the particle is moving.

Centripetal force on the particle is given by

F=mv2R ⇒v=GMR22+14

**Answer: 6**

The acceleration due to gravity at a point at height h from the surface of the moon is given by

g=GMr2,

where M is the mass of the moon; r is the distance of point from the centre of the moon and G is universal gravitational constant.

∴ g=GMR+h2⇒g=6.67×10-11×7.4×10221740+10002×106⇒g=6.67×7.4×10111740+10002×106⇒g=6.67×7.4×10112740×2740×106⇒g=0.65 m/s2

**Answer: 7**

Consider a system of two bodies. The initial linear momentum of the system is zero as the bodies were initially at rest when they were released.

Since the gravitational force is an internal force and the net external force on the system is zero, so by the law of conservation of linear momentum, the final momentum of the system will also be zero.

So, 10×v1 = 20×v2

⇒ v1 = 2v2 …(i)

Applying the law of conservation of energy, we have:

Initial total energy = final total energy …(ii)

Initial total energy=-6.67×1011×10×201 + 0

= −13.34 × 10−9 J …(iii)

When the separation is 0.5 m, we have:

Final total energy =-13.34×10-91/2+12×10v12+12×20v22 …iv

From (iii) and (iv), we have:

−13.34 × 10−9 = 26.68 × 10−9 + 5v12+10v22

⇒ −13.34 × 10−9 = 26.68 + 10−9 + 30v22

⇒ v22=-13.34×10-930 = 4.44 × 10−10

⇒ v2 = 2.1 × 10−5 m/s

∴ v1 = 4.2 × 10−5 m/s

**Answer: 8**

Consider a small mass element of length dl subtending dθ angle at the centre.

In the semicircle, we can consider a small element dθ.

Then length of the element, dl = R dθ

Mass of the element, dm =MLR dθ

Force on the mass element is given by

dF=GmR2dm=GMRmLR2dθ

The symmetric components along AB cancel each other.

Now, net gravitational force on the particle at O is given by

F=∫2dFsin θ=∫2GMmLRsin θ dθ∴F=∫0π/2-2GMmLRsin θ dθ =2GMmLR -cos θ0π/2 =-2GMmLR-1 =2GMmLR=2GMmLL/π =2πGMmL2

**Answer: 9**

Consider a small mass element of length dx at a distance x from the centre of the rod.

Mass of the mass element, dm = (M/L) × dx

Gravitational field due to this element at point P is given by

dE=Gdm×1d2+x2

The components of the gravitational field due to the symmetrical mass element along the length of the rod cancel each other.

Now, resultant gravitational field = 2dE sin θ

=2×Gdmd2+x2×dd2+x2=2×GM×d dxLd2+x2 d2+x2

Total gravitational field due to the rod at point P is given by

E=∫0L/22Gmd dxLd2+x23/2.

On integrating the above equation, we get:

E=2GmdL2+4d2

**Answer: 10**

Consider that mass m is at a distance R1+R22 as shown in the figure.

The gravitational force of m due to the shell of M2 is zero, because the mass is inside the shell.

∴ Gravitational force due to the shell of mass M2 = GM1mR1+R222 = 4GM1mR1+R22

**Answer: 11**

Mass of the Earth, M=43πR3ρ …(i)

Consider an imaginary sphere of radius x with centre O as shown in the figure below:

Mass of the imaginary sphere, M’=43πx3ρ …(ii)From (i) and (ii),

we have:M’M=x3R3

∴ Gravitational force on the particle of mass m is given by

F=GMmx2

⇒ F=GMx3mR3x2=GMmR3x

**Answer: 12**

Let d be distance of the particle from the centre of the Earth.

Now, d2=x2+R24=4×2+R24⇒d=124×2+R2

Let M be the mass of the Earth and M’ be the mass of the sphere of radius d.

Then we have:

M=43πR3ρM1=43πd3ρ∴ M1M=d3R3

Gravitational force on the particle of mass m is given by

F=GM1md2⇒F=Gd3MmR3d2⇒F=GMmR3d

∴ Normal force exerted by the wall, FN = F cos θ=GMmdR3×R2d=GMmd2R2

**Answer: 13**

(a) Consider that the particle is placed at a distance x from O.

Here, r < x < 2r

Let us consider a thin solid sphere of radius (x – r).

Mass of the sphere, dm=m43πr3×43π(x-r)3=m(x-r)3r3

Then the gravitational force on the particle due to the solid sphere is given by

F=Gm’ dm(x-r)2 =Gm(x-r)3r3m'(x-r)2=Gmm'(x-r)r3

Force on the particle due to the shell will be zero because gravitational field intensity inside a shell is zero.

(b) If 2r < x < 2R,

Force on the body due to the shell will again be zero as particle is still inside the shell.

then F is only due to the solid sphere.

∴F=Gmm’x-r2

(c) If x > 2R, then the gravitational force is due to both the sphere and the shell.

Now, we have:

Gravitational force due to shell, F=GMm’x-R2

Gravitational force due to the sphere=Gmm’x-r2

As both the forces are acting along the same line joining the particle with the centre of the sphere and shell so both the forces can be added directly without worrying about their vector nature.

∴ Resultant force=Gmm’x-r2+GMm’x-R2

**Answer: 14**

At point P1, the gravitational field due to the sphere and the shell is given by

F=GM3a+a2+0=GM16a2

At point P2, the gravitational field due to the sphere and the shell is given by

F=GMa+4a+a2+GM4a+a2⇒F=GM36a2+GM25a2⇒F=GMa2136+125

⇒F=GMa225+36900⇒F=61900GMa2

**Answer: 15**

We know that in a thin spherical shell of uniform density, the gravitational field at its internal point is zero. So, at points A and B, the gravitational fields are equal and opposite and, thus, cancel each other. So the net field is zero.

Hence, EA = EB

**Answer: 16**

Let the mass of 0.10 kg be at a distance x from the 2 kg mass and at a distance of (2 − x) from the 4 kg mass.

Force between 0.1 kg mass and 2 kg mass = force between 0.1 kg mass and 4 kg mass

∴ 2×0.1×2=-4×0.12-x2⇒0.2×2=0.42-x2⇒1×2=22-x2⇒2-x2=2×2⇒2-x=2x⇒x2+1=2⇒x=22.414 =0.83 m

from the 2 kg mass

Now, gravitation potential energy of the system is given by

U=-G0.1×20.83+0.1×41.17+2×42⇒U=-6.67×10110.1×20.83+0.1×41.17+2×42⇒U=-3.06×10-10 J

**Answer: 17**

The work done in increasing the side of the triangle from a to 2a is equal to the difference of the potential energies of the system.

i.e., work done = final potential energy of the system – initial potential energy of the system

∴W=-3Gm22a–3Gm2a⇒W=3Gm22a

**Answer: 18**

The work done against the gravitational force to take the particle away from the sphere to infinity is equal to

the difference between the potential energy of the particle at infinity and potential energy of the particle at the surface of the sphere.

∴W=0–G×10×0.11×0.1=6.67×10-11×11×0.1=6.67×10-10 J

**Answer: 19**

Gravitational field, E→=5N/kgi^+12 N/kgj^

(a) F→=mE→

=2 kg 5N/kgi^+12 N/Kgj^=10 Ni^+24 Nj^∴ F→=100+576=676=26 N

(b) V=-E.→r→

Potential at (12 m, 0)=-60 J/Kg

Potential at (0, 5 m) = -60 J/kg

(c)

change in potential=final potential -initial potential

initial potential=potential at the origin=0

final potential=potential at (12,5)

V=-E→.r→=-(10i^+24j⏜).(12i⏜+5j⏜)=-(120+120) J=-240 J

(d)

∆V=-E→.∆r→∆r→=(12i⏜+0j⏜)-(0i⏜+5j⏜) =12i⏜-5j⏜∆V=-(10i⏜+12j⏜).(12i⏜-5j⏜) =0 J

#### Exercise : Solution of Questions on page Number : 227

**Answer: 20**

(a) V=20 Nkg x+y

GMR=MLT-2M L⇒M-1L3T-2M1L=ML2T-2M⇒ M0L2T-2=M0L2T-2∴LHS=RHS

(b) The gravitational field at the point (x, y) is given by E→x, y=-20Nkg i^-20 Nkgj^.

(c) F→=E→m

=0.5 kg -20 Nkgi^-20 Nkgj^=-10 N i^-10 N j^

∴ F→=100+100 =102 N

**Answer: 21**

The gravitational field in a region is given by E→=2i^+3j^.

Slope of the electric field, m1=tan θ1=32

The given line is 3y + 2x = 5.

Slope of the line, m2=tan θ2=-23

We can see that m1m2 = −1

Since the directions of the field and the displacement are perpendicular to earth other, no work is done by the gravitational field when a particle is moved on the given line.

**Answer: 22**

Assume that at height h, the weight of the body becomes half.

Weight of the body at the surface = mg

Weight of the body at height h above the Earth’s surface = mg’, where g’ is the acceleration due to gravity at height h

Now, g’=12g

∴ 12 GMR2=GMR+h2 ∵g=GMR2⇒ 2R2=R+h2⇒ 2R=R+h⇒h=2-1R

**Answer: 23**

Let g’ be the acceleration due to gravity on Mount Everest.

Then g’=g1-2hRhere h=8848 m =9.81-0.00276 =9.00.99724 =9.77 m/s2

∴ The acceleration due to gravity on the top of Mount Everest is 9.77 m/s2.

**Answer: 24**

Let g’ be the acceleration due to gravity in a mine of depth d.

∴ g’=g1-dR

=9.81-640640×103=9.810000-1104=9.8104×9999=9.8×0.9999=9.799 m/s2

**Answer: 25**

Let gp be the acceleration due to gravity at the poles.

Let ge be the acceleration due to gravity at the equator.

Now, acceleration due to gravity at the equator is given by

ge = gp – ω2r

= 9.81 − (7.3 × 10−5)2 × 6400 × 103

= 9.81 − (53.29 × 10−10) × 64 × 105

= 9.81 − 0.034 = 9.776 m/s2

Now, mge = 1 kg × 9.776 m/s2

= 9.776 N

∴ The body will weigh 9.776 N at the equator.

**Answer: 26**

At the equator, g’ = g − ω2R …(i)

Let h be the height above the South Pole where the body stretch the spring by the same length.

The acceleration due to gravity at this point is g’=g1-2hR.

Weight of the body at the equator = weight of the body at height h above the South Pole

∴ g-ω2r=g1-2hR⇒1-ω2R2g=1-2hR⇒h=ω2R22g =7.3×10-52×6400×10-322×9.81 =7.32×64219.62 =11125 m = 10 km approx.

**Answer: 27**

The apparent acceleration due to gravity at the equator becomes zero.

i.e., g’ = g − ω2R = 0

⇒ g = ω2R

⇒ ω=gR=9.86400×103⇒ω= 9.8×10-56.4=1.5×10-6⇒ω=1.2×10-3 rad/s

∴ T=2πω=2×3.141.2×10-3 =6.281.2×10-3 =1.41 h

**Answer: 28**

(a) Speed of the ship due to rotation of the Earth is v = ωR, where R is the radius of the Earth and ω is its angular speed.

(b) The tension in the string is given by

T0 = mg − mω2R

∴ T0 − mg = mω2R

(c) Let the ship move with a speed v.

Then the tension in the string is given by

T=mg-mω12R =T0-v-ωR2R2R=T0-v2+ω2R2-2ωRvRR

∴ T=T0+2ωvm

**Answer: 29**

According to Kepler’s laws of planetary motion, the time period of revolution of a planet about the Sun is directly proportional to the cube of the distance between their centres.

i.e., T2 ∝ R3

⇒Tm2Te2=Rm3Re3⇒RmRe3=1.8812∴RmRe=1.882/3=1.52

**Answer: 30**

Time period of rotation of the Moon around the Earth is given by

T=2πr3GM,

where r is the distance between the centres of the Earth and Moon and m is the mass of the Earth.

Now, 27.3=2×3.143.84×10536.67×10-11M⇒ 27.3×27.3=2×3.14×3.84×10536.67×10-11 M⇒ M=2×3.142×3.843×10153.335×10-11×27.32 =6.02×1024 kg

∴ The mass of the Earth is found to be 6.02 × 1024 kg.

**Answer: 31**

Time period of revolution of the satellite around the Mars is give by

T=2πr3GM,

where M is the mass of the Mars and r is the distance of the satellite from the centre of the planet.

Now, 27540=2×3.149.4×103×10336.67×10-11×M⇒ 275402=6.282×9.4×10536.67×10-11×M⇒M=6.282×9.43×10186.67×10-11×275402⇒M=6.5×1023 kg

**Answer: 32**

(a) Speed of the satellite in its orbit

v=GMr+h=gr2r+h

⇒ v=9.8×6400×1032106×6.4+2⇒ v=9.8×6.4×6.4×1068.4 ⇒v=6.9×103 m/s=6.9 km/s

(b) Kinetic energy of the satellite

K.E.=12 mv2

= 12×1000×6.9×1032=12×1000×47.6×106=2.38×1010 J

(c) Potential energy of the satellite

P.E.=-GMmR+h

= -6.67×10-11×6×1024×1036400+2000×103=40×10138400=-4.76×1010 J

(d) Time period of the satellite

T =2πr+hv = 2×3.14×8400×1036.9×103=6.28×84×1026.9=76.6×10.2 s=2.1 h

**Answer: 33**

(a) The angular speed of the Earth and the satellite will be the same.

i.e., 2πTe=2πTs⇒124×3600=12πR+h3/gh2⇒12×3600=3.14R+h3gR2⇒R+h3gR2=12×360023.142⇒6400+h3×1099.8×64002×106=12×360023.142⇒6400+h×1096272×109=432×104⇒6400+h3=6272×432×104⇒6400+h=6272×432×1041/3-6400 ⇒h=42300 km

(b) Time taken from the North Pole to the equatorial plane is given by

14T=14×6.2842300+6400310×64002×106=3.144793×106642×1011=3.14497×497×49764×64×105=6 h

**Answer: 34**

For a geostationary satellite, we have:

R = 6.4 × 103 km

h = 3.6 × 103 km

Given: mg = 10 N

The true weight of the object in the geostationary satellite is given by

mg’=mg-R2R+h2 =10-6400×10326400×103+3600×103 =10-64×10526.4×106+36×105 =10-4096×101042.42×1012 =409617980=0.227 N

**Answer: 35**

The time period of revolution of the satellite around a planet in terms of the radius of the planet and radius of the orbit of the satellite is given by T=2πR22gR12, where g is the acceleration due to gravity at the surface of the planet.

Now, T2=4π2R22gR12⇒g=4π2T2R22R12

∴ Acceleration due to gravity of the planet = 4π2T2R22R1

**Answer: 36**

Consider that B is the position of the geostationary satellite.

In the given figure, ϕ is the latitude and θ is the colatitude of a place which can directly receive a signal from a geostationary satellite.

In triangle OAB, we have:

cos ϕ=640042000 =16106=853i.e., ϕ=cos-1853 =cos-1 0.15

Now, θ=π2-ϕ⇒θ=π2-cos-1 0.15⇒θ=sin-1 0.15

**Answer: 37**

The particle attained a maximum height 6400 km, which is equal to the radius of the Earth.

Total energy of the particle on the Earth’s surface is given by

Ee=12 MV2+-gmMr ….1

Now, total energy of the particle at the maximum height is given by

E3=-GMmR+h+0⇒E3=-GMm2R …2

∴ g=R

From equations (1) and (2), we have :

-GMmR+12mv2=-GMm2R⇒12mv2=GMm-12R+1R⇒v2=GMR =6.67×10-11×6×10246400×103 =40.02+10136.4×106 =6.2×107=0.62×108∴ v = 0.62×108 =0.79×104 m/s =79 km/s

**Answer: 38**

Initial velocity of the particle, v = 15 km/s

Let its speed be v’ in interstellar space.

Applying the law of conservation of energy, we have:

12mv-v’2=∫R∞GMmx2dx

∴ 12m15×103-v’2=∫R∞GMmx2dx⇒12m15×1032-v’2=GMm-1x⇒12m225×105-v’2=GMmR⇒225×105-v’2=2×6.67×10-11×6×10246400×103⇒v’2=225×106-40.0232×108 =2.25×108-1.2×108 =108 1.05Or v’=1.01×104 m/s=10 km/s

**Answer: 39**

Mass of the sphere = 6 × 1024 kg

Escape velocity = 3 × 108 m/s

Escape velocity is given by

ve=2GMR⇒R=2GMve2 =2×6.67×10-11×6×10243×1082 =2×40.02×10139×1016 =80.029×10-3 m =8.89×10-3 m =9 mm