A man of mass m starts moving w.r.t. a platform of mass 2m with a velocity u =913m/s as shown in the figure. The platform is fitted with a concave mirror of focal length f. The velocity of image (in m/s) at the initial moment is
A
3
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B
2.5
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C
2
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D
3.5
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Solution
The correct option is A 3 Using the relation m=ff−u we get
Magnification m = −f−f+3f2⟹ m= -2
Taking the direction in the right hand side Vimage/mirror=−m2Vobject/mirror
Vimage/mirror is velocity of image w.r.t. mirror
Vobject/mirror is velocity of object w.r.t mirror
⟹Vimage/mirror=−4u
assume velocity of platform is v towards left
as the man is moving with velocity u towards right w.r.t. platform