Unformatted text preview: Sorry I wasn’t clear on what I meant by my comments on the chapter 2 homework. To try again to be clear about what you need
to do, I’ll give some examples and more in depth discussion.
‘numbers’ and ‘arithmetic’
All your math except the final numerical answers explicitly asked for in the problem should be symbolic.
For example
do this : d = Hp rL2 + H2 rL2 not this : d = 141.32 + 902 (1) If you’re specifically asked to find d, you can always put down the actual number,
do this : d = Hp rL2 + H2 rL2 = 167.6 m not this : d = 141.32 + 902 = 167.6 m (2) but you don’t have to as long as your final equation for d is expressed in terms of things you’re given.
If you start with
d= dx 2 + dy 2 (3) and you don’t know dx , then when finding it to be p r, you can say dx = p r but don’t say dx = p r = 141.3 m. The fact that it’s
141.3 is not relevent to the general problem, it’s only relevent to the final number that comes out. When you’re calculating the
number at the end, you may well have to first calculate dx to be 141.3, but that’s not relevent to the general reasoning that leads
to the final numerical answer.
If you look at my answer to the wheel problem, if the wheel had been of a different radius, then the final number wouldn’t have
been 168 cm, but everything else in the line of reasoning would have been the same.
What you should write down is an argument that is as independant of as many numbers as possible. In most cases, it will be
completely independant of the actual numbers, as the wheel problem is.
However, sometimes the numbers matter, as in the ball problem of chapter four. In order to determine whether the ball has
peaked by the time it hits the wall, you have to either calculate a number for the time of flight, a number for the maximum range,
or a number for the ycomponent of the velocity. In this case, you can’t answer the problem if you don’t do that, so you should
have one of those numbers somewhere in the problem.
stating the obvious/why not just that
Your goal is to give me a reasoned argument that the answer is whatever it is. Since the problems we’re working on in intro
physics are neither fiendishly complicated, like determining the vibrationally induced stress on a bolt somewhere inside the space
shuttle during touch down nor profoundly subtle, like the connections between quantum mechanics and general relativity, your
arguments are bound to include statements like “it is raining outside and I am outside and I don’t have a raincoat on, so I’m
getting wet”.
Think of this as an advantage, not a detriment; we’re not concerned with truely hard problems, just with problems that may be
hard for you only because you’re not used to working with these concepts. So the fact that you can produce a solid and complete
argument by saying lots of obvious things is good. It makes it far easier to work on understanding what a good argument is in
general because you don’t have to spend so much time on complicated details.
An example:
Say this: The wheel rotated half a revolution, so P rotated half a revolution and P started out
exactly at the bottom, so it must now be exactly at the top. Since the radius is r and the
distance across a circle is twice the radius then the vertical distance is 2 r. 2 OnHomework.nb Your goal is to give me a reasoned argument that the answer is whatever it is. Since the problems we’re working on in intro
physics are neither fiendishly complicated, like determining the vibrationally induced stress on a bolt somewhere inside the space
shuttle during touch down nor profoundly subtle, like the connections between quantum mechanics and general relativity, your
arguments are bound to include statements like “it is raining outside and I am outside and I don’t have a raincoat on, so I’m
getting wet”.
Think of this as an advantage, not a detriment; we’re not concerned with truely hard problems, just with problems that may be
hard for you only because you’re not used to working with these concepts. So the fact that you can produce a solid and complete
argument by saying lots of obvious things is good. It makes it far easier to work on understanding what a good argument is in
general because you don’t have to spend so much time on complicated details.
An example:
Say this: The wheel rotated half a revolution, so P rotated half a revolution and P started out
exactly at the bottom, so it must now be exactly at the top. Since the radius is r and the
distance across a circle is twice the radius then the vertical distance is 2 r. Not this: The wheel rotated half a revolution so the vertical distance is 2 r. I know you know it’s 2 r just by looking at it; this is a consequence of the knowledge and skill you’ve alredy developed before
you enrolled in this class. But this is beside the point. The point is to show me the reasoning, not just the conclusions of the
reasoning.
The reason I want to you to do that is because your reasoning will be wrong often enough to matter. That’s why there’s a class
for learning physics. If it were obvious and within the average person’s skill set to answer questions like this, you wouldn’t have
to take a class on it.
If your reasoning is wrong and you don’t show me in detail what that reasoning is, I can’t help you fix it.
If anyone still thinks they might lose 15% for ‘numbers’ or 10% for ‘why’ on their chapter 5 solution because they’re not sure
they know what I mean, see me before class on Monday. I’m free pretty much all day today and I’m here on the weekends for
some period of time, just email me to find out when. ...
View
Full
Document
This note was uploaded on 12/13/2011 for the course PHYS 521 taught by Professor Staff during the Fall '10 term at South Carolina.
 Fall '10
 Staff
 Physics, Work

Click to edit the document details