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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 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: ->->->->->->->->->->->-> :
The forces acting on the ball between the instant of release to instant it is caught are as followed:
Acceleration of gravity acts on the ball = -9.8 m/s^2
The velocity is positive (upward). The highest point the velocity final = 0.
The displacement is unknown.
@& None of your answers specifies a force.
The force is the force exerted by gravity, and it does accelerate the ball downward at 9.8 m/s^2.*@
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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 speed at any height when going up is equal and opposite coming down.
• Describe the path of the ball as it would be observed by someone standing along the side of the road.
answer/question/discussion: ->->->->->->->->->->->-> :
As the ball rises, its speed decreases until it reaches the highest point where its speed is zero for an instant, then it descends, with increasing speed. Displacement is unknown.
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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 path would be the same for the ball as it rises, the speed decreases until it reaches the highest point, then it descend with increasing speed until reach the grown.
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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 speed of the ball will be greater and cover greater distance with each second it takes to reach the floor The acceleration would be +9.8 m/s^2, which is chosen as positive. The initial velocity will equal 0.
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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: ->->->->->->->->->->->-> :
@& This question is important and should be answered.*@
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20 mins
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SEED Question 18.1 submitted 9 Mar 2011 around 2:56 PM.
SEED Question 18.2
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: ->->->->->->->->->->->-> :
Given:
dt = .5 s
a = 0, horizontal motion is characterized by zero acceleration.
vAve = 10 m/s
vAve = ds / dt
ds = vAve * dt
ds = 10 m/s * 0.5 s
ds = 5 m
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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: ->->->->->->->->->->->-> :
As the ball rises, its speed decreases until it reaches the highest point where its speed is zero for an instant, then it descends, with increasing speed. Displacement is 20 meters.
@& As observed from within the car the ball doesn't travel forward or backward. It would travel forward as observed from the side of the road.*@
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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 shape of the path for the ball is line graph.
@& Does the forward speed of the ball change?
Does its up-and-down speed change?
If both are unchanging then the motion is a straight line. If not, the motion might not be straight.
What are your thoughts?*@
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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: ->->->->->->->->->->->-> :
Given:
dt = .5 s
a = 9.8 m/s^2
vAve = 10 m/s
vf = v0 + a * dt
vf = 0 + (9.8 m/s^2 * 0.5 s)
vf = 19.6 m/s
@& Good, but 9.8 * .5 = 4.9, not 19.6.*@
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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: ->->->->->->->->->->->-> :
ds = v0 * dt + 0.5 * a * dt^2
ds = 0.5 * 9.8 m/s^2 * 0.5 s^2
ds = 0.5 * 9.8 m/s^2 * 0.25s^2
ds = 1.23 m
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25 mins
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SEED Question 18.2 submitted 9 Mar 11 around 4:04 PM.
@& Good answers on many questions.
Some, however, do require revision.
&#Please see my notes and, unless my notes indicate that revision is optional, submit a copy of this document with revisions and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes.
If my notes indicate that revision is optional, use your own judgement as to whether a revision will benefit you. &#
*@