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PHCC 141: Physics for Scientists and Engineers I  Fall 2007
10a. Angular Dynamics
Due at 11:59pm on Thursday, October 25, 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.
Torque and Finding It
Torques on a Seesaw: A Tutorial
Learning Goal:
To make the connection between your intuitive understanding of a seesaw and the standard formalism for
torque.
This problem deals with the concept of torque, the "twist" that an offcenter force applies to a body that tends to make it rotate.
Try to use your intuition to answer the following question. If your intuition fails, work the rest of the problem and return here
when you feel that you are more comfortable with torques.
Part A
A mother is helping her children, of unequal weight, to balance on a seesaw so that they will be able to make it tilt back and
forth without the heavier child simply sinking to the ground. Given that her heavier child of weight
is sitting a distance
to
the left of the pivot, at what distance
must she place her second child of weight
on the right side of the pivot to balance the
seesaw?
Hint A.1
How to approach the problem
Hint not displayed
Express your answer in terms of
,
, and
.
ANSWER:
=
The figure shows the seesaw slightly tilted, as will be the case when in use. This does not change the torque balance because
the horizontal distances from the pivot to each child (which are called the moment arms for the vertically directed weight
and must be used to calculate the torque instead of the distance along the seesaw) are reduced equally, so the sum of the
torques is zero at any angle. Given that the torque is zero at all times (except when one or both children push on the ground),
there will generally be no angular acceleration of the seesaw, and the seesaw will rotate at a constant velocity between
pushes from the feet of the children on the ground.
Now consider this problem as a more formal introduction to torque. The torque of each child about the pivot point is the
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This note was uploaded on 10/28/2008 for the course PH 141 taught by Professor Toki during the Fall '08 term at Colorado State.
 Fall '08
 TOKI
 Physics

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