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course Phy 121
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 ball has momentum from the velocity it received by being inside the car and moving at the same velocity. Relative to the car, the only forces acting on the ball are the child giving it an initial velocity and then gravity acting on it.
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The ball's momentum is not a force. It is the result of a force previously applied, but that force is no longer acting.
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The force exerted by the child is no longer acting either, during the specified interval.
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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 ball would loose some horizontal velocity because there is no force propelling it forward while it is in the air.
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It will only lose horizontal velocity if there is a force opposing its horizontal motion.
Is there any such force? If so what is the source of that force?
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Describe the path of the ball as it would be observed by someone standing along the side of the road.
answer/question/discussion:
Some one on the side of the road would see the ball go up in the air and slow down horizontally and then speed back up once the ball was caught.
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The child neither speeds up nor slows down. If the ball slows, then it will fall behind its original position and the child won't be likely to catch it.
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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 don't see any reason why a bicycle would be any different unless air resistance was factored in, in which case there would be more of it when the kid was on the bicycle. If the child didn't catch the ball, it would continue to slow down.
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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 acceleration of the ball in the vertical direction will be the 9.8m/s^2 and the horizontal acceleration will be zero, assuming no air resistance, so the velocity will be constant.
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The ball won't encounter any air resistance in the horizontal direction, since the air and the ball are both moving along with the car. When you're riding in a car with the widows closed and the fan off, you don't feel any air hitting you in the face, and the ball won't feel any either.
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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:
The ball would fall strait down as observed by the child, but it would appear to move forward and fall from the view of someone on the side of the road.
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As it moves forward while falling, what will be the shape of its path, as observed from the side of the road?
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Good descriptions, but there might be some contradictions in your thinking.
You'll benefit from thinking a little more about these questions.
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