Physics Help Forum Physics Homework Question??

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 1st 2009, 10:42 AM #1 Junior Member   Join Date: Aug 2009 Posts: 18 Physics Homework Question?? 2. Two students stand on top of a building of height h. One throws a ball straight upward at the same speed as as the other student who simultaneously throws a ball straight downward toward the ground. Which of the following must be true?(Select all that apply.) A) The balls hit the ground at the same time. B) The ball thrown downward hits the ground first. C) The balls hit the ground with the same speed. D) After both balls leave the hand, the ball thrown downward has a larger acceleration than that thrown upward. E) The ball thrown downward hits the ground before the ball thrown upward reaches its peak. F) None of the above. Answer: this i what i got. Is this right?? A) False B) True C) False, the ball thrown upward falls for a longer period of time, so it's going faster when it hits the ground. D) False, the only acceleration acting on the balls is gravity, and its the same for both. E) False, (or not necessarily true), it depends on how high the building is and how hard they throw the balls.
 Sep 3rd 2009, 12:43 AM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 You got everything right except C. C) False, the ball thrown upward falls for a longer period of time, so it's going faster when it hits the ground. We neglect air resistance for this problem. If the initial speed upwards is v, when the ball reaches its peak say h feet above its K.E. = 0 and P.E. = mgh. This gets converted back to K.E. and from conservation of energy, when it passes the point from where it was thrown up, it is now travelling downward with the same speed v as it was thrown up. The situation henceforth as you can see is the same as that of the second ball which was thrown downwards with speed v. Since both have the same initial speed, both will have the same final speed also The ball falls for a longer period of time, as you say, but when it starts falling its initial speed is zero unlike the one thrown down.

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