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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Seed question 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:
It has horizontal motion, and vertical motion (up). But the only force acting on it is gravity. I don’t think the horizontal motion is considered a force.
You are correct. To elaborate a bit:
Motion is not a force in any case.
An object with mass has zero force in any direction in which it its acceleration is zero. If its acceleration is not zero in a direction, then it has a nonzero force in that direction.
Force is associated with acceleration, not motion.
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 slows down (due to gravity) as it goes up. It reaches a point where it stops and then starts the free fall back down. As it falls it increases speed. The graph would look like a V, with the top left point of the V being when it leaves the child’s hands at it’s maximum velocity. It will slow down as time passes and the bottom of the V will be touch the x axis at y=0 and then as it starts to fall the velocity increases. When it reaches the child’s hands it should be back to the same velocity it was at when it left the child’s hands. I am assuming the horizontal velocity will be constant and have no effect on the vertical velocity.
Very good.
Describe the path of the ball as it would be observed by someone standing along the side of the road.
answer/question/discussion:
The ball would arc up and back down as it goes by. If you took the car out of the visual picture the ball would travel from one side of the scene to the other in an arch.
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 think it would still appear the same if the child was on a bicycle. If the kid didn’t bother to catch it the ball would continue on it’s trajectory until it hit the ground, but it’s vertical velocity would speed up some due to a longer distance vertically.
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 vertical velocity would change just as if it had been dropped from a stationary point. The acceleration would be that of gravity. It also would have a horizontal velocity that would remain constant. Looking at it from outside it would appear to travel at an angle down (as if coming off a ramp).
It would in fact arc downward in a manner similar to that you described earlier.
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:
??? I think it would appear to the child as if it was going backwards, but I’m not sure about how it would look to an innocent bystander. That bystander better get a good look at the license of the car before he gets hit on the head by a steel ball though. It’s hard to visualize without the air resistance.
Without the air resistance the ball will stay next to the child as it falls, and the child was see it fall straight down just as it would if dropped in the car.
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30 mins
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&#Good responses. See my notes and let me know if you have questions. &#