As an open elevator rises at a constant speed of 8 m/s, a boy throws an apple with a speed of 10 m/s at a 45o angle (in the reference frame of the elevator) when the elevator is 20 m above the ground

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the boy's acceleration toward the girl is 2.08 m/s^2.

explanation:

according the newton's third law, the force causing the girl accelerate towards the boy will be me with equal counter-force causing the boy to accelerate towards the girl. these forces will be of equal magnitude and allows us to determine the boy's acceleration:

the boy's acceleration toward the girl is 2.08 m/s^2.

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