Phy 121
Your 'cq_1_18.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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A child in a slowly moving car tosses a ball upward. It rises to a point below the roof of the car and falls back down, at which point the child catches it. During this time the car neither speeds up nor slows down, and does not change direction.
• What force(s) act on the ball between the instant of its release and the instant at which it is caught? You can ignore air resistance.
answer/question/discussion: ->->->->->->->->->->->-> : Gravity initially acts against the force the child applied to the ball as it leaves the hand of the child. Right when it leaves the hand, it should also have the momentum of the car which would move the ball in the direction of the car. Once the ball reaches its maximum height, it will fall gaining velocity from gravity.
• What happens to the speed of the ball between release and catch? Describe in some detail; a graph of speed vs. clock time would also be appropriate.
answer/question/discussion: ->->->->->->->->->->->-> : It will decrease at 9.8m/s from the second it takes off from the hand. The graph will increase at a steady rate, peak out, then decrease at a steady rate.
• Describe the path of the ball as it would be observed by someone standing along the side of the road.
answer/question/discussion: ->->->->->->->->->->->-> : I believe it would look to almost “slide” in the direction of the car as it moved up and forward at the same time.
• How would the path differ if the child was coasting along on a bicycle? What if the kid didn't bother to catch the ball? (You know nothing about what happens after the ball makes contact with the ground, so there's no point in addressing anything that might happen after that point).
answer/question/discussion: ->->->->->->->->->->->-> : I would assume that if he did not attempt to catch the ball it would fall substantially behind the bike. I don’t see a reason why the motion of a ball tossed perfectly up in the air on a bike versus a car would change if we can assume similar speeds and air resistance.
• What if the child drops the ball from the (inside) roof of the car to the floor? For the interval between roof and floor, how will the speed of the ball change? What will be the acceleration of the ball? (You know nothing about what happens after the ball makes contact with the floor, so there's no point in addressing anything that might happen after that point).
answer/question/discussion: ->->->->->->->->->->->-> : The ball will speed up at close to 9.8m/s^2. The ball will end up directly under the boy if we don’t have wind resistance in the equation since objects don’t lose horizontal velocity.
• What if the child holds the ball out of an open window and drops it. If the ball is dense (e.g., a steel ball) and the car isn't moving very fast, air resistance will have little effect. Describe the motion of the ball as seen by the child. Describe the motion of the ball as seen by an observer by the side of the road. (You know nothing about what happens after the ball makes contact with the ground, so there's no point in addressing anything that might happen after that point).
answer/question/discussion: ->->->->->->->->->->->-> : To the child, it probably seems to drop straight down…to an on looker it probably seems to fall behind the car.
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15minutes
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Good thinking throughout. Review the discussion at the link below (no revision necessary):
&#Please compare your solutions with the expanded discussion at the link
Solution
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