phy 121
Your 'cq_1_22.2' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
--------------------------------------------------------------------------------
A 70 gram ball rolls off the edge of a table and falls freely to the floor 122 cm below. While in free fall it moves 40 cm in the horizontal direction. At the instant it
leaves the edge it is moving only in the horizontal direction. In the vertical direction, at this instant it is moving neither up nor down so its vertical velocity is zero.
For the interval of free fall:
What are its final velocity in the vertical direction and its average velocity in the horizontal direction?
answer/question/discussion: ->->->->->->->->->->->-> :
'ds= v0'dt+ .5a'dt^2
1.22m=0 + .5(9.8m/s^2)'dt^2
.249= 'dt^2
.5 = 'dt
vf= v0 + a*'dt
vf= 9.8m/s^2 (.5)
vf=4.9m/s
Average velocity= 122 cm/.5=244 cm/s= 2.44 m/s
#$&*
Assuming zero acceleration in the horizontal direction, what are the vertical and horizontal components of its velocity the instant before striking the floor?
answer/question/discussion: ->->->->->->->->->->->-> :
vertical = 1.22
This is half of your average vertical velocity. The final vertical velocity will be double the average; and you figured out the correct final vertical velocity previously.
horizontal= .4
.40 m is the displacement in the horizontal direction during the .5 second fall, not the horizontal velocity.
vector=1.22^2 + .4^2= h^2
1.4884+.16= h^2
h=1.28
angle = arctan (vy/vx) = arctan(1.22/.4)=arctan 3.05=71.85 degrees
#$&*
What are its speed and direction of motion at this instant?
answer/question/discussion: ->->->->->->->->->->->-> :
vf= 4.9 m/s The direction of motion is a vector (hypotenuse) from the table to the floor.
#$&*
What is its kinetic energy at this instant?
answer/question/discussion: ->->->->->->->->->->->-> :
Kef= 1/2 mvf^2
Kef= 1/2(.7g)(4.9 m/s)^2= 8.4 J
#$&*
What was its kinetic energy as it left the tabletop?
answer/question/discussion: ->->->->->->->->->->->-> :
Ke0= 1/2mv0^2
Ke0= 1/2(.7g)(0)^2=0
The ball was moving when it left the tabletop; otherwise it wouldn't have traveled 40 cm in the horizontal direction.
#$&*
What is the change in its gravitational potential energy from the tabletop to the floor?
answer/question/discussion: ->->->->->->->->->->->-> :
Gravitational potential energy will be greatest when the object is first dropped and then change to kinetic as dropped. Potential energy would probably be equal to 8.4 J
because it is transferred to kinetic.
#$&*
How are the the initial KE, the final KE and the change in PE related?
answer/question/discussion: ->->->->->->->->->->->-> :
The initial Ke would be zero while the potential energy is 8.4, and then as the object falls, the PE changes into KE as the object falls.
#$&*
How much of the final KE is in the horizontal direction and how much in the vertical?
answer/question/discussion: ->->->->->->->->->->->-> :
I think the KE vertically is probably about 3 times the amount that is horizontal due to the distance that is moved in these two directions.
#$&*
--------------------------------------------------------------------------------
Copy and paste your work into the box below and submit as indicated:
Your instructor is trying to gauge the typical time spent by students on these questions. Please answer the following question as accurately as you can, understanding that your
answer will be used only for the stated purpose and has no bearing on your grades:
Approximately how much time did you spend on this question?
30 min. or so
You may add optional comments and/or questions in the box below.
--------------------------------------------------------------------------------
Author information goes here.
Copyright © 1999 [OrganizationName]. All rights reserved.t
Revised: 06 Feb 2010 17:15:36 -0500
** **
30 or so min.
** **
Most of your work is very good, but there are some errors.
&#Please compare your 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. &#