A world-class tennis player can serve a tennis ball at 110 mi/h (about 49 m/s ). The length of a tennis court is 78 ft . Approximately how long does an opponent have to react from the instant the ball is served until the ball reaches the opponent?

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Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other

There was no intial y-momentum so the net final y-momentum must also be 0. they can have final velocities in the y-direction