A strip of wood of mass M and length l is placed on a smooth horizontal surface. An insect of mass m starts at one end of the strip and walks to the other end in time t, moving with constant speed. The speed of the insect as seen from the ground is
A
lt[MM+m]
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B
lt[mM+m]
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C
lt[Mm]
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D
lt[mM]
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Solution
The correct option is Alt[MM+m]
Co-ordinates of COM will not change as no external force is present. ⇒m(l−x)=Mx⇒x=mlm+M
Distance moved by insect w.r.t. ground is l−x=l−mlm+M=Mm+Ml
Speed of the insect as seen from the ground : =distancetime=Mlt(M+m)