1.

A planet of radius R has an acceleration due to gravity of g_(s) on its surface. A deep smooth tunne is dug on this planet, radially inward, to reach a point P located at a distance of (R)/(2) from the centre of the planet. Assume that the planet has uniform density. The kinetic energy required to be given to a small body of mass m, projected radially outward from P, so that it gains a maximum altitude equal to the thrice the radius of the planet from its surface, is equal to (##DSH_NTA_JEE_MN_PHY_C07_E03_017_Q01.png" width="80%">

Answer»

`(63)/(16)mg_(s) R `
`(3)/(8)mg_(s) R `
`(9)/(8)mg_(s) R `
`(21)/(16)mg_(s) R `

Solution :The gravitational potential at a point Q (OQ = x) is given by
`V(x) = {{: (- g_(s) R((3)/(2) - (1)/(2)(x^(2))/(R^(2)) ), " when " x lt R),(-g_(s)R((R)/(x)), " when " x gt R):}}`
The energy required to project the body to a MAXIMUM altitude of 3 R from its surface , is
`m (V_(B) | x = (R)/(2) - V_(p) | x =4 R ) = (9)/(8) mg_(s)R.`


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