Download FREQUENTLY ASKED QUESTIONS Content Questions

FREQUENTLY ASKED QUESTIONS
January 13, 2015
Content Questions
What did Dr. Goshaw’s demo with the cup and water falling out
of holes in the sides fail?
I’m afraid I didn’t see it so I don’t know! I’ll ask him and update later if I
find out.
What extra math will we need to know?
Dr. Roy’s handout handles the main “more advanced” topics. However you
are expected to know basic algebra, geometry and trigonometry. I’ll note
things you need to know as we come across them. Most people should have
seen these things in high school, but if you find you are rusty on anything,
or didn’t happen to learn something previously, let me know.
Can you explain the uncertainty problem?
Problem 1-16 asked for the uncertainty on the time for the pulsar to rotate 1
million times. The uncertainty on the time T to rotate once (i.e., the period)
is δ = 0.00000000000003 ms (the ±3 refers to the last decimal place of T ).
We found the fractional uncertainty, which is δ over T . The time to rotate
106 times is 106 times the period. The fractional uncertainty on that time
will be the same as the fractional uncertainty on T . If we know the fractional
uncertainty on a quantity, we can calculate the absolute uncertainty on the
quantity by multiplying the fractional uncertainty by the quantity.
What did you mean when you did the unit conversion with the T
on the bottom?
This was a calculation of the quantity “how many rotations in 7.00 days”.
(It was not only a unit conversion, although it included a unit conversion.)
The number of rotations in 7.00 days is 7.00 days, divided by T , the time for
one rotation. But if T does not have units of days, then you need to convert
units so that numerator and denominator have the same units. I converted
the numerator to seconds, and the denominator T (which we were given in
ms) to seconds as well.
What causes a star to collapse into a pulsar?
Well, this is a bit beyond the scope of this class, but extremely interesting! (and actually somewhat related to my research). Essentially, gravity
is the culprit... here’s a quick description: stars normally hold themselves
up against gravity by thermal pressure, where heat comes from nuclear reactions. However, really massive stars eventually reach a point at which
they’ve burned all their nuclear fuel, and they can’t hold themselves up
anymore, so they collapse gravitationally on themselves. The pressure is
so great at the center that protons and electrons in the star’s atoms get
squeezed together to form neutrons (and in the process they emit neutrinos, a kind of particle I study). This creates incredibly high-density neutron matter and extreme conditions. By conservation of angular momentum (which we’ll be getting to later in this course), the super-dense neutron
star ends up spinning very rapidly, in the same way that a figure skater
spins up when she pulls in her arms. Here’s more if you are interested:
http://en.wikipedia.org/wiki/Pulsar.
What was the “head to tail” method of adding vectors?
~ and B
~ geometrically, do the following:
To add vectors A
~ in parallel with itself so that its tail (starting point)
• Move vector A
~
touches the tip (arrow head) of vector B.
~ to the tip of A.
~ This is your resultant
• Draw a line from the tail of B
(summed) vector.
This is also sometimes called “tip to tail”.
~ or B
~ that you move, since vector addiIt doesn’t matter whether it’s A
tion is commutative. Also, to subtract vectors: first invert the direction of
the vector with the − sign in front of it, then follow the same head-to-tail
procedure.
You can also add vectors algebraically, by summing the components.
I’d like to see the solution to problem 3-17.
Note that I won’t put problem solutions in these FAQs. However I may give
hints and clarify solutions to problems done in class.
Problem 3-17 can be done by component addition. My suggestion for
approaching it: draw the vectors, write down their components, add or subtract components as indicated, and then use the formulae for magnitude and
angle.
What is an AU?
An “AU” is an “astronomical unit”— it’s defined as the average distance
between the Earth and the Sun (the Earth moves in a slightly elliptical orbit
around the Sun, so the distance varies a bit, which is why it’s an average
distance).
Can you clarify what a “parsec” is?
A “parsec” is a unit of distance, not time, and it’s defined as the distance
at which one astronomical unit (the distance from the Earth to the Sun)
subtends and angle of one arcsecond. We figured out how long a parsec is
in AU by using the expression for distance subtended by an angle, s = rθ
(where θ must be in radians), and converting seconds of arc to radians. A
parsec is a long distance! The closest star to Earth is about 1.3 parsecs away
(about 4 light years).
What is an arcsecond?
An “arcminute” is 1/60 of a degree. An “arcsecond” (or “second of arc” or
just “second” for short) is 1/60 of an arcminute, or 1/3600 of a degree.
Can you explain problem 2 on homework 2?
This is a relatively hard one. Hints: draw x vs t for both red car cases and
for the green car. Write down expressions for the times at which the cars
meet. Your goal is to get enough equations to solve for the unknowns.
Can you explain problem 3 on homework 2?
This one can be handled by constant acceleration equations for intervals
when the car is moving.
Jokes of the Week
(Chemistry but nonetheless...)
There once was a boy named Johnny,
But Johnny is no more.
For what he thought was H2 0
Was H2 SO4 .
What’s the best part of Switzerland? I don’t know but their flag is a big plus.
May the mass times acceleration be with you.