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(B) Power is defined as the dot product of force and velocity: $P = \vec{F} \cdot \vec{v} = Fv \cos 	heta$.At the highest point,the velocity $v = 0$,

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at the instant just before the body hits the earth.

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(B) Power is defined as the dot product of force and velocity: $P = \vec{F} \cdot \vec{v} = Fv \cos 	heta$.At the highest point,the velocity $v = 0$,so power $P = 0$.During the motion,the gravitational force $\vec{F}$ is always directed towards the center of the earth.As the body falls back,the velocity $\vec{v}$ is directed downwards,which is in the same direction as the gravitational force $\vec{F}$ (i.e.,$	heta = 0^\circ$).Since $P = Fv \cos(0^\circ) = Fv$,the power is proportional to the speed $v$.The speed $v$ of the body is maximum at the instant just before it hits the earth's surface.Therefore,the power exerted by the gravitational force is greatest at the instant just before the body hits the earth.

$A$ body projected vertically from the earth reaches a height equal to earth's radius before returning to the earth. The power exerted by the gravitational force is greatest

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