Question is from Barron's SAT subject test Physics

"A flat railroad car is moving to the right at 5m/s. A person standing on the car throws a ball straight upward at 20m/s. If air resistance is negligible, where will the ball be in relation to the person's new position at the time when the ball returns to its original starting height?"

The answer is C) the ball will land in the person hand. The book explains that before throwing the ball vertically, the ball and the person have a horizontal velocity of 5m/s. I don't get this bit, if we intentionally throw the ball vertically, how can it get horizontal velocity? I initially answered E)"The ball will land 20 m behind the person"

  • $\begingroup$ This concerns the very basics of vectors and can easily be understood by googling. $\endgroup$ – N.S.JOHN Jan 21 '16 at 14:09

It is because the ball was traveling with you when you threw it. Imagine the following question: I am in a car traveling at 5m/s holding a ball. Where will the ball be relative to me in 10 seconds? Answer in my hand. To be obtuse: ball has velocity 5m/s. After 10s it will have moved forward 10 x 5 meters = 50. I have velocity 5m/s. After 10s it will have moved forward 10 x 5 meters = 50. The only difference between my question and yours is that the ball travels up and down over the time peroid in question. The important thing is that the ball has the same horizontal velocity as you when it was thrown vertically and that the term "vertically" is defined in the referance frame of the person in the car. To a stationary observer the ball would have been thrown up and forward.


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