PHY 201
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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Seed 18.1
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: The force of gravity of course acts on the ball and brings it back down to the boy. However the car moving at a constant speed is a constant force acting on the ball. The ball is moving the same speed as the car in the horizontal direction but is not accelerating because the car isn't.
Good, but the car doesn't exert a force on the ball as it drops. It did exert a force on the ball, through contact with the child, when it was accelerating, but it is in no way in contact with the ball when it is dropping.
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: The graph of speed vs. clock time would look like a U. The velocity right after release is the greatest velocity which is also the same as when the ball is caught. At the height of the balls trip it will momentarily have a velocity of 0.
Describe the path of the ball as it would be observed by someone standing along the side of the road.
answer/question/discussion: To the boy it appears the ball only moves up then down and does not move horizontally. However, the boy himself is moving the same speed horizontally as is the ball and the car. So to someone standing on the road the ball would not only move up and then back down, but it would be traveling horizontally in the same direction as the car.
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: Moving on a bike, the ball would be traveling along as well, but air resistance would cause it to slow down in its horizontal speed. If the kid didn't catch the ball it would fall to the ground as it's horizontal speed decreases.
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 accelerate by the force of gravity to the floor, so its velocity will increase. Accel would be 9.8 m/s/s
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: It should fall to the ground and come in contact with the ground right beside the child. Its horizontal velocity will stay the same as the cars. If observing from the side of the road the ball will fall down, but also be moving horizontally at the same speed as the car.
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30 minutes
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&#Good responses. See my notes and let me know if you have questions. &#