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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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Asst 18 question 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: ->->->->->->->->->->->-> :
Gravity (gravitational force)
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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: ->->->->->->->->->->->-> :
As the ball is thrown up, it rises, and the speed decreases. It then falls and speeds up since gravitational force is exerted on the ball at 9.8m/s^2. The speed vs clock time graph shows the acceleration, it increases as the velocity decreases and decreases as the velocity increases. At velocity=0, the ball is at rest.
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The position vs. clock time graph will reveal acceleration as the curvature of the graph (more precisely as the degree to which the rate indicated by the slope is changing). The slope of the position vs. clock time graph indicates the velocity.
The acceleration is the rate at which the velocity changes.
A graph which is increasing at an increasing rate has positive velocity and positive acceleration.
A graph which is increasing at a decreasing rate has positive velocity and negative acceleration.
etc.
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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: ->->->->->->->->->->->-> :
The ball would speed up going in the vertical direction, then it would curve down back to the child's hand to catch.
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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: ->->->->->->->->->->->-> :
The faster the bike is going the faster the ball will come back to the child. ???? I am not sure what would happen if the child does not catch the ball.
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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: ->->->->->->->->->->->-> :
Acceleration will increase until the ball hits the ground (9.8m/s^2-force of gravity)
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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: ->->->->->->->->->->->-> :
It will go horizontally alongside the car, then as it looses air resistance it will fall straight down.
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• Approximately how much time did you spend on this question?
15 minutes
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Author information goes here.Copyright © 1999 [OrganizationName]. All rights reserved. Revised: 26 Jul 2012 19:10:21 -0400
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15 min
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Asst 18 Question 2
A child in a car tosses a ball upward so that after release it requires 1/2 second to rise and fall back into the child's hand at the same height from which it was released. The car is traveling at a constant speed of 10 meters / second in the horizontal direction.
• Between release and catch, how far did the ball travel in the horizontal direction?
answer/question/discussion: ->->->->->->->->->->->-> :
vAve=10m/s horizontal direction
'dt=1/2 s
'ds=vAve*'dt
'ds=10m/s*0.5 s
'ds=5 meters
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• As observed by a passenger in the car, what was the path of the ball from its release until the instant it was caught?
answer/question/discussion: ->->->->->->->->->->->-> :
The ball would go up and back down in the same path.
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• Sketch the path of the ball as observed by a line of people standing along the side of the road. Describe your sketch. What was shape of the path of the ball?
answer/question/discussion: ->->->->->->->->->->->-> :
The people on the side of the road would see the ball go straight up and back down. But in reality the car has moved 5 meters so the ball did not just go straight up and down in the same pathway.
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Those people would see the ball travel 5 meters in the horizontal direction as it rose and fell. They would not see it rise straight up and fall straight down. That would be the perception from inside the car.
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• How fast was the ball moving in the vertical direction at the instant of release? At that instant, what is its velocity as observed by a line of people standing along the side of the road?
answer/question/discussion: ->->->->->->->->->->->-> :
vf=v0+a*'dt
0m/s=V0+(-9.8m/s^2)*0.25 s
???I think it would be 0.25 s since it is half of the time going.
v0=2.45 m/s
It would look faster.
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• How high did the ball rise above its point of release before it began to fall back down?
answer/question/discussion: ->->->->->->->->->->->-> :
vAve=(2.45m/s+0m/s)/2
=1.23 m/s
'dt='dv/a
=1.23/-9.8m/s^2
=0.25 s
'ds=vAve*'dt
=1.23m/s*0.25 s
=0.31 m
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You've got most of the details correct, but you're not seeing the paths as perceived from the road.
Check out the discussions and let me know if you have questions:
&#See any notes I might have inserted into your document, and before looking at the link below see if you can modify your solutions. If there are no notes, this does not mean that your solution is completely correct.
Then please compare your old and new solutions with the expanded discussion at the link
Solution
Self-critique your solutions, if this is necessary, according to the usual criteria. Insert any revisions, questions, etc. into a copy of this posted document. Mark any insertions with &&&& so they can be easily identified.If your solution is completely consistent with the given solution, you need do nothing further with this problem. &#
&#See any notes I might have inserted into your document, and before looking at the link below see if you can modify your solutions. If there are no notes, this does not mean that your solution is completely correct.
Then please compare your old and new solutions with the expanded discussion at the link
Solution
Self-critique your solutions, if this is necessary, according to the usual criteria. Insert any revisions, questions, etc. into a copy of this posted document. Mark any insertions with &&&& so they can be easily identified.If your solution is completely consistent with the given solution, you need do nothing further with this problem. &#
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