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Calculate the velocity with which a body...

Calculate the velocity with which a body must be thrown vertically upward from the surface of the earth so that it may reach a height of `10R`, where `R` is the radius of the earth and is equal to `6.4 xx 10^(6)m.` (Given: Mass of the earth `= 6 xx 10^(24) kg`, gravitational constant `G = 6.7 xx 10^(-11) N m^(2) kg^(-2)`)

Text Solution

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The gravitational potential of a body of mass `m` on earth's surface is
`U(R)=-(GMm)/R`
where `M` is the mass of the earth (supposed to be concentrated at its centre) and `R` is the radius of the earth (distance of the particle from the centre of the earth). The gravitational energy of the same body at a height `10R` from earth's surface, i.e, at a distance `11R` from earth's centre is
`U(11R)=-(GMm)/R`
Therefore, change in potential energy,
`U(11R)-U(R)=-(GMm)/(11R)-(-(GMm)/R)=10/11(GMm)/R`
this difference must come from the initial kinetic energy given to the body in sending it to that height. Now, suppose the body is throuwn up with a vertical speed `v`, so that its initial kinetic energy is `1/2mv^(2)`. Then
`1/2mv^(2)=10/11(GMm)/R` or `v=sqrt(20/11 (Gmm)/R)`
putting the given values:
`v=sqrt((20xx(6.7x10^(-11)Nm^(2)kg^(-2))xx(6xx10^(24)kg))/(11(6.4xx10^(6)m))`
`=1.07xx10^(4)ms^(-1)`
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