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Question

A body is projected up with a speed V
along the line of greatest slope of an inclined plane of angle of inclination β. If the body collides elastically perpendicular to the inclined plane, find the time after which it crosses through its point of projection after collision ?

A
2Vg(sinβ)
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B
2Vg(tanβ)
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C
2Vg(1+3sin2β)
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D
2Vg(1+3tan2β)
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Solution

The correct option is C 2Vg(1+3sin2β)

Taking x axis along the inclined plane and y axis perpendicular to plane and resolving gravity in that direction
On striking the plane perpendicularly, the velocity along x axis should be zero. So using vx=ux + axt we get
0 = Vcosα gsinβt t=Vcosαgsinβ. Also sy = 0 from A to B.
So using sy = uyt+ ayt22 we get,
0 = 2V(sinα)t g(cosβ)t22 t = 2Vsinαgcosβ
Equating t from above two equation , we get
Vcosαgsinβ = 2Vsinαgcosβ tanα = cotβ2
cosα = 24 + cot2β = 2sinβ1 + 3 sin2β
Putting value of cosα in t we get t = 2Vg1 + 3sin2β . Since collision is elastic at perpendicular to inclined plane , so it will trace back same path and will take same time t as mentioned above to get back to point of projection .

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