Q: What happens to the gravitational force between two objects, if(i) the mass of one object is doubled?(ii) the distance between the objects is doubled and tripled?(iii) the masses of both objects are doubled?

\$F=\dfrac{GMm}{R^2}\$\$(i)\$ If one mass say, \$m\$ is doubled, then force is\$F'=\dfrac{2GMm}{R^2}=2F\$ the force gets doubled.\$(ii)\$ If the distance is doubled and tripled-\$F'=\dfrac{2GMm}{(2R)^2}=\dfrac{F}{4}\$ and,\$F'=\dfrac{2GMm}{(3R)^2}=\dfrac{F}{9}\$\$(iii)\$ If both masses are doubled, then force is\$F'=\dfrac{4GMm}{R^2}=4F\$ the force gets quadrupled.

Q: Two particles of equal mass m go around a circle of radius R under the action of their mutual gravitational attraction. The speed of each particle is

Given,Masses of the two particles are \$m_1=m_2=m\$And radius of the two particles are \$r_1=r_2=R\$So ,The Gravitational force of attraction between the particles \$F=\dfrac{G\times{m}\times{m}}{R^2}--------(1)\$Ans, centripital force \$F=\dfrac{m\times{v^2}}{R}------(2)\$From given condition (1) and (2)\$\dfrac{Gmm}{(2R)^2}=\dfrac{mv^2}{R}\$, \$v=\dfrac{Gm}{4R}=\dfrac{1}{2}\sqrt{\dfrac{Gm}{R}}\$So the speed of the each particle is \$v=\dfrac{1}{2}\sqrt{\dfrac{Gm}{R}}\$

Q: A stone is released from the top of a tower of height \$19.6\$m. Calculate its final velocity just before touching the ground.

Initial Velocity \$ u = 0\$Fianl velocity \$ v = ?\$Height, \$s = 19.6 m\$By third equation of motion\$v^{2} = u^{2} + 2 gs\$\$v^{2} = 0 + 2 \times 9.8\times19.6\$\$v^{2} = 384.16\$\$\Rightarrow v = 19.6 m/s\$

Q: What happens to the gravitational force between two objects, if \$(i)\$ The mass of one object is doubled?\$(ii)\$ The distance between the objects is doubled and tripled?\$(iii)\$ The masses of both objects are doubled?Give reasons in each case

Gravitational force \$F = \dfrac{Gm_1m_2}{r^2}\$Case \$(i)\$ : Mass of one object is doubled i.e \$m'_1 = 2m_1\$\$\therefore \$ \$F' = \dfrac{G (2m_1) m_2}{r^2}\$ \$\implies F' = 2F\$ Thus gravitational force gets also doubledCase \$(ii)\$ : Distance between the objects is doubled i.e \$r' = 2r\$ \$\therefore \$ \$F' = \dfrac{G m_1 m_2}{(2r)^2}\$ \$\implies F' = \dfrac{F}{4}\$ Thus gravitational force gets reduced by \$4\$ times. Distance between the objects is tripled i.e \$r' = 3r\$ \$\therefore \$ \$F' = \dfrac{Gm_1 m_2}{(3r)^2}\$ \$\implies F' = \dfrac{F}{9}\$ Thus gravitational force gets reduced by \$9\$ times.Case \$(iii)\$ : Mass of both the objects are doubled i.e \$m'_1 = 2m_1\$ and \$m_2' = 2m_2\$ \$\therefore \$ \$F' = \dfrac{G (2m_1) (2m_2)}{r^2}\$ \$\implies F' = 4F\$ Thus gravitational force becomes \$4\$ times.

Q: Derive an expression for acceleration due to gravity at a depth 'd' below the earth's earth surface.

Let \$M\$ be mass of the earth\$,\$ \$R\$ be the radius of the earth\$g_d\$ be gravitational acceleration at depth \$'d'\$ from the earth surface\$g\$ be gravitational acceleration on the earth surfaces\$.\$\$p\$ be the density of the earth\$.\$\$'p'\$ be the point inside the earth at depth \$'d'\$ from earth surfaces\$.\$\$\therefore CS - CP = d,\ \, \, \,\therefore CP = R - d\$-----------\$(1)\$ \$(\$since CS=R\$)\$\$g = \dfrac{{GM}}{{{R^2}}}\$\$\therefore g = \dfrac{{G\frac{4}{3}\pi {R^3}p}}{{{R^2}}}\$\$\therefore g = \dfrac{{4G\pi Rp}}{3}\$--------------\$(2)\$\$gd=\$ acceleration due to gravity at depth \$'d'\$\$g_d=\dfrac{{G \times Mass\,of\,the\,sphere\,with\,radius\,CP}}{{C{p^2}}}\$\$\therefore g_d = \dfrac{{G\frac{4}{3}\pi C{P^3}\rho }}{{C{P^2}}}\$\$\therefore g_d = \dfrac{{4G\pi CP\rho }}{3}\$-----------\$(3)\$Dividing eq\$.\$ \$(3)\$ by eq\$(2)\$\$\dfrac{{g_d}}{g} = \dfrac{{CP}}{R} = \dfrac{{R - d}}{R}\$\$\therefore g_d = g\left( {1 - \dfrac{d}{R}} \right)\$

Q: The time period of oscillation of a simple pendulum is \$T = 2\pi\sqrt{(l/g)}\$ .measured value of length is 20cm known to have 1mm accuracy and time for 100 oscillation of the pendulum is found to be 90s using wrist watch of 1s resolution.what is the accuracy in determining the value of acceleration due to gravity?

\$T=\$ Time period of one oscillation\$t=\$ Time period of 100 oscillationFor 100 oscillation, \$t=100\,T\$Take ln and differentiate.\$\dfrac{\Delta t}{t}=\dfrac{\Delta T}{T}=\dfrac{2}{90}\,\,\,.....\,\,(1)\,\$Time period of one oscillation\$T=2\pi \sqrt{\dfrac{L}{g}}\$\$g=4{{\pi }^{2}}\dfrac{L}{{{T}^{2}}}\$Take ln on both side and differentiate\$ \dfrac{\Delta g}{g}=\pm \left( \dfrac{\Delta L}{L}+2\dfrac{\Delta T}{T} \right) \$\$ \dfrac{\Delta g}{g}=\pm \left( \dfrac{0.1}{20}+\dfrac{2}{90} \right)==\pm 2.7% \$Hence, error in gravity is \$\pm 2.7\,\$%

Q: A body weighs \$63 \$N on the surface of the earth. What is the gravitational force (in N) on it due to the earth at a height equal to half the radius of the earth ?

Weight of the body,\$W=63\ N\$Acceleration due to gravity at height \$h\$ from the Earths surface is given by the relation:\$g'=\dfrac{g}{(1+\dfrac{h}{R_e})^2}\$Substituting \$h=R_e/2\$,\$g'=4g/9\$Weight of body of mass m at height h is given as:\$W'=mg'\$ \$=4/9\ mg\$ \$=4/9\times 63=28\ N \$

Q: If \$v_e\$ and \$v_0\$ represent the escape velocity and orbital velocity of a satellite corresponding to circular orbit of radius \$R\$, then

escape velocity \$v_e=\sqrt{GM/R}\$Orbital velocity \$v_o=\sqrt{GM/2R}\$so, \$v_e=\sqrt 2v_o\$

Question 1

What happens to the gravitational force between two objects, if(i) the mass of one object is doubled?(ii) the distance between the objects is doubled and tripled?(iii) the masses of both objects are doubled?

Accepted Answer

$F=\dfrac{GMm}{R^2}$$(i)$ If one mass say, $m$ is doubled, then force is$F'=\dfrac{2GMm}{R^2}=2F$ the force gets doubled.$(ii)$ If the distance is doubled and tripled-$F'=\dfrac{2GMm}{(2R)^2}=\dfrac{F}{4}$  and,$F'=\dfrac{2GMm}{(3R)^2}=\dfrac{F}{9}$$(iii)$ If both masses are doubled, then force is$F'=\dfrac{4GMm}{R^2}=4F$ the force gets quadrupled.

Question 2

Two particles of equal mass m go around a circle of radius R under the action of their mutual gravitational attraction. The speed of each particle is

Accepted Answer

Given,Masses of the two particles are $m_1=m_2=m$And radius of the two particles are $r_1=r_2=R$So ,The Gravitational force of attraction between the particles $F=\dfrac{G\times{m}\times{m}}{R^2}--------(1)$Ans, centripital force $F=\dfrac{m\times{v^2}}{R}------(2)$From given condition (1) and (2)$\dfrac{Gmm}{(2R)^2}=\dfrac{mv^2}{R}$, $v=\dfrac{Gm}{4R}=\dfrac{1}{2}\sqrt{\dfrac{Gm}{R}}$So the speed of the each particle is $v=\dfrac{1}{2}\sqrt{\dfrac{Gm}{R}}$

Question 3

A stone is released from the top of a tower of height $19.6$m. Calculate its final velocity just before touching the ground.

Accepted Answer

Initial Velocity $ u = 0$Fianl velocity $ v = ?$Height, $s = 19.6 m$By third equation of motion$v^{2} = u^{2} + 2 gs$$v^{2} = 0 + 2 \times 9.8\times19.6$$v^{2}  = 384.16$$\Rightarrow v = 19.6 m/s$

Question 4

Discuss the variation of g with(a) altitude (b) depth

Accepted Answer

( a )  The acceleration due to gravity on the earth is given by&#10;&#10;$g = \dfrac {GM}{R^2} i.e. GM = R^2g$  . . . . . . .( 1 )&#10;&#10;The acceleration due to gravity at height 'h' from the surface of the&#10;earth is given by&#10;&#10;$g_h = \dfrac{GM}{(R+h)^2} i.e. GM = (R+h)^2 g_h$ . . . . . . ( 2 )&#10;&#10;From ( 1 ) and ( 2 ) we have,&#10;&#10;$\dfrac{g_h}{g} = \dfrac {R^2}{(R+h)^2}$&#10;&#10;$\dfrac{g_h}{g} = (1 - \dfrac{2h}{R})$  &#10;&#10;( b )  The acceleration due to gravity on the surface of the earth&#10;is given by.&#10;&#10;$g = \dfrac{GM}{R^2}$ . . . .( 1 )&#10;&#10;let 'Q' be the density of the material of the earth.&#10;&#10; Now, mass = volume $\times$ density&#10;&#10; $M = \dfrac{4}{3} \pi R^3 \times \rho$&#10;&#10; Substituting in equation ( 1 ) we get &#10;&#10; $g = \dfrac{G}{R^2} \times \dfrac{4}{3} \pi R^3 \times \rho = \dfrac{4}{3}&#10;\pi GR \rho$&#10;&#10; $g = \dfrac{4}{3} \pi GR \rho$ . . .. .. ( 2 )&#10;&#10; $g_d = \dfrac{4}{3} \pi G(R-d) \rho$  . . . .. .( 3 )&#10;&#10; Dividing equation( 3 ) by ( 2 ) we get &#10;&#10; $\dfrac{g_d}{g} = \dfrac{R-d}{R} = (1 - \dfrac{d}{R})$&#10;&#10; $g_d = g ( 1 - \dfrac{d}{R})$

Question 5

What happens to the gravitational force between two objects, if $(i)$ The mass of one object is doubled?$(ii)$ The distance between the objects is doubled and tripled?$(iii)$ The masses of both objects are doubled?Give reasons in each case

Accepted Answer

Gravitational force             $F = \dfrac{Gm_1m_2}{r^2}$Case $(i)$ :      Mass of one object is doubled   i.e    $m'_1 = 2m_1$$\therefore   $  $F' = \dfrac{G (2m_1) m_2}{r^2}$    $\implies  F' = 2F$    Thus gravitational force gets also doubledCase $(ii)$ :      Distance between the objects is doubled    i.e    $r' = 2r$ $\therefore   $  $F' = \dfrac{G m_1 m_2}{(2r)^2}$   $\implies  F' = \dfrac{F}{4}$      Thus gravitational force gets reduced by $4$ times.   Distance between the objects is tripled    i.e    $r' = 3r$ $\therefore   $  $F' = \dfrac{Gm_1 m_2}{(3r)^2}$   $\implies  F' = \dfrac{F}{9}$      Thus gravitational force gets reduced by $9$ times.Case $(iii)$ :      Mass of both the objects are doubled   i.e    $m'_1 = 2m_1$     and      $m_2' = 2m_2$ $\therefore   $  $F' = \dfrac{G (2m_1) (2m_2)}{r^2}$   $\implies  F' = 4F$      Thus gravitational force becomes $4$ times.

Question 6

Derive an expression for acceleration due to gravity at a depth 'd' below the earth's earth surface.

Accepted Answer

Let $M$ be mass of the earth$,$ $R$ be the radius of the earth$g_d$ be gravitational acceleration at depth $'d'$ from the earth surface$g$ be gravitational acceleration on the earth surfaces$.$$p$ be the density of the earth$.$$'p'$ be the point inside the earth at depth $'d'$ from earth surfaces$.$$\therefore CS - CP = d,\ \, \, \,\therefore CP = R - d$-----------$(1)$ $($since CS=R$)$$g = \dfrac{{GM}}{{{R^2}}}$$\therefore g = \dfrac{{G\frac{4}{3}\pi {R^3}p}}{{{R^2}}}$$\therefore g = \dfrac{{4G\pi Rp}}{3}$--------------$(2)$$gd=$ acceleration due to gravity at depth $'d'$$g_d=\dfrac{{G \times Mass\,of\,the\,sphere\,with\,radius\,CP}}{{C{p^2}}}$$\therefore g_d = \dfrac{{G\frac{4}{3}\pi C{P^3}\rho }}{{C{P^2}}}$$\therefore g_d = \dfrac{{4G\pi CP\rho }}{3}$-----------$(3)$Dividing eq$.$ $(3)$ by eq$(2)$$\dfrac{{g_d}}{g} = \dfrac{{CP}}{R} = \dfrac{{R - d}}{R}$$\therefore g_d = g\left( {1 - \dfrac{d}{R}} \right)$

Question 7

The time period of oscillation of a simple&#10;pendulum is $T = 2\pi\sqrt{(l/g)}$ .measured value of length is 20cm known to have 1mm&#10;accuracy and time for 100 oscillation of the pendulum is found to be 90s using&#10;wrist watch of 1s resolution.what is the accuracy in determining the value of&#10;acceleration due to gravity?&#10;&#10;

Accepted Answer

$T=$ Time period of one oscillation$t=$ Time period of 100 oscillationFor 100 oscillation, $t=100\,T$Take ln and differentiate.$\dfrac{\Delta t}{t}=\dfrac{\Delta T}{T}=\dfrac{2}{90}\,\,\,.....\,\,(1)\,$Time period of one oscillation$T=2\pi \sqrt{\dfrac{L}{g}}$$g=4{{\pi }^{2}}\dfrac{L}{{{T}^{2}}}$Take ln on both side and differentiate$ \dfrac{\Delta g}{g}=\pm \left( \dfrac{\Delta L}{L}+2\dfrac{\Delta T}{T} \right) $$ \dfrac{\Delta g}{g}=\pm \left( \dfrac{0.1}{20}+\dfrac{2}{90} \right)==\pm 2.7% $Hence, error in gravity is $\pm 2.7\,$%

Question 8

A body weighs $63 $N on the surface of the earth. What is the gravitational force (in N) on it due to the earth at a height equal to half the radius of the earth ?

Accepted Answer

Weight of the body,$W=63\ N$Acceleration due to gravity at height $h$ from the Earths surface is given by the relation:$g'=\dfrac{g}{(1+\dfrac{h}{R_e})^2}$Substituting $h=R_e/2$,$g'=4g/9$Weight of body of mass m at height h is given as:$W'=mg'$       $=4/9\ mg$       $=4/9\times 63=28\ N $

Question 9

What is escape velocity? Obtain an expression for it.

Accepted Answer

Escape velocity: -The minimum speed with which a body must be projected in order that it will escape from the earth gravitational filed is called as escape velocity.Expression of it:-consider a body of mass $m$ on the surface of earthwhose mass and radius are respectively represented by $M$ and $R$.and earth is $F = \dfrac{{GMm}}{{{R^2}}}$gravity potential energy $\left( {GPE} \right) = \dfrac{{GMm}}{{{R^2}}}$$w = F.S = \dfrac{{GMm}}{{{R^2}}}$In order to projector body into the space overcoming the earth's gravitational field it must be give a kinetic energy which is equal topotential energy.therefore;kinetic energy is equal to potential energy.

Question 10

If $v_e$ and $v_0$ represent the escape velocity and orbital velocity of a satellite corresponding to circular orbit of radius $R$, then

Accepted Answer

escape velocity $v_e=\sqrt{GM/R}$Orbital velocity $v_o=\sqrt{GM/2R}$so, $v_e=\sqrt 2v_o$