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PHCC 141: Physics for Scientists and Engineers I  Fall 2007
7a. Potential Energy and Total Mechanical Energy
Due at 11:59pm on Thursday, October 4, 2007
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Number of answer attempts per question is:
5
You gain credit for:
correctly answering a question in a Part, or correctly answering a question in a Hint.
You lose credit for:
exhausting all attempts or requesting the answer to a question in a Part or Hint, or incorrectly answering a question in a Part.
Late submissions:
reduce your score by 100% over each day late.
Hints
are helpful clues or simpler questions that guide you to the answer. Hints are not available for all questions. There is
no penalty
for leaving questions in Hints unanswered.
Grading of
Incorrect Answers
For
MultipleChoice
or
True/False
questions, you lose
100%
/
(
# of options  1
)
credit per incorrect answer.
For
any other
question, you lose 3% credit per incorrect answer.
Work on a Sliding Box
A box of mass
is sliding along a horizontal surface.
Part A
The box leaves position
with speed
. The box is slowed by a constant frictional force until it comes to rest at position
.
Find
, the magnitude of the average frictional force that acts on the box. (Since you don't know the coefficient of friction,
don't include it in your answer.)
Hint A.1
How to approach the problem
Use the workenergy theorem. As applied to this part, the theorem states that the work done by friction is equal to the change
in kinetic energy of the box:
.
Find
,
, and
(which will depend on
), then solve for
.
Part A.2
Find the initial kinetic energy
What is
, the kinetic energy of the box at position
?
ANSWER:
=
Part A.3
Find the final kinetic energy
What is
, the kinetic energy of the box when it reaches position
?
ANSWER:
=
0
Part A.4
Find the work done by friction
Find
, the work done by friction on the box. Note that the work done by friction is always negative (i.e., friction always
dissipates energy).
Express your answer in terms of
and other given variables.
ANSWER:
=
Express the frictional force in terms of
,
, and
.
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ANSWER:
=
Part B
After the box comes to rest at position
, a person starts pushing the box, giving it a speed
.
When the box reaches position
(where
), how much work
has the person done on the box?
Assume that the box reaches
after the person has accelerated it from rest to speed
.
Hint B.1
How to approach the problem
Again, use the workenergy theorem. In this part of the problem, both the person and friction are doing work on the box:
.
Part B.2
Find the work done by friction
What is
, the total work done by friction on the box as the person pushes it from position
to position
?
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 Fall '08
 TOKI
 Physics, Energy, Force, Potential Energy

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