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A body weighs 5.6 kg wt on the surface of the Earth. How much will be its weight on a planet whose mass is 7 times the mass of the Earth and radius twice that of the Earth’s radius?

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Question

A body weighs 5.6 kg wt on the surface of the Earth. How much will be its weight on a planet whose mass is 7 times the mass of the Earth and radius twice that of the Earth’s radius?

Sum
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Solution

Given: 

  • Force/weight of the body on earth = 5.6 kg wt
  • Mass of the planet = 7 × Mass of Earth
  • Radius of the planet = 2 times Radius of the Earth

To find: Weight of the body on the surface of the planet

Calculation: 

Let the mass of Earth be 'm' kg.

Let the distance between the two bodies be 'r' m.

F = G`("m" " m"_2)/"r"^2`   ...(i)

Force of Gravitation between the two bodies when mass of Earth is 7 times and the distance is doubled.

F' = G`(7 "m m"_2)/(2"r")^2`

F' = G`(7 "m m"_2)/(4"r"^2)`   ...(ii)

Now, dividing Equation (i) from Equation (ii), we get:

`=> ("F'")/"F" = (cancel"G"(7 "m m"_2)/(4"r"^2))/(cancel"G"("m m"_2)/"r"^2`

`=> "F'"/"F" = (7 "m m"_2)/(4"r"^2) xx "r"^2/"m m"_2`

`=> "F'"/"F" = (7 cancel("m m"_2))/(4cancel("r"^2)) xx cancel("r"^2)/cancel("m m"_2)`

`=> "F'"/"F" = 7/4`

Now, by substituting the value of F in the Equation, we get:

`=> "F'"/5.6 = 7/4`

`=> "F'" = 7/4 xx 5.6`

`=> "F'" = 7 xx 1.4`

⇒ F' = 9.8 kg wt

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Chapter 5: Gravitation - Exercises [Page 99]

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Balbharati Physics [English] Standard 11 Maharashtra State Board
Chapter 5 Gravitation
Exercises | Q 4. (xi) | Page 99

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