Physics Forums

Definition of parallel

nateja — Wed, 19 Jun 2013 14:32:41 GMT

1. The problem statement, all variables and given/known data
The question states True or False: Two lines parallel to a third line are parallel

2. Relevant equations

You need to know the difference between skew, parallel and perpendicular

3. The attempt at a solution

I thought of three parallel planes (which have an infinite amount of lines) and those lines never cross as long as they stay on their separate planes. But do parallel lines have to be in the same direction?

Does mass exist when matter is completely inert?

_233\/3 — Wed, 19 Jun 2013 14:17:10 GMT

I've been thinking about mass and the question occurred to me: If a particle is completely inert i.e. absolutely no motion whatsoever then does it still have a mass? I ask because, as far as I understand it, everything we've ever measured cannot strictly be said to be inert. That is: Every particle we've measured is always under some kind of motion.

Thanks, :)

Angular Momentum and Kepler's Second Law

Bashyboy — Wed, 19 Jun 2013 13:54:21 GMT

1. The problem statement, all variables and given/known data
A particle of mass m moves along a straight line with constant velocity v in the x direction, a distance b from the x axis. (a) Does the particle possess any angular momentum about the origin? (b) Explain why the amount of its angular momentum should change or should stay constant. (c) Show that Kepler's second law is satisfied by showing that the two shaded triangles in the figure have the same area when t_D - t_C = t_B - t_A

2. Relevant equations

3. The attempt at a solution

Here are my answers to part (a) and (b):

(a) Examining one of the forms of angular momentum, [itex]\vec{L} = \vec{r} \times m \vec{v}[/itex], we can easily see that neither the position vector function, nor the velocity, is the zero vector at any time, meaning that angular momentum is some nonzero value. For [itex]\vec{r}[/itex], we see that the y-distance (projection of the [itex]\vec{r}[/itex] on the [itex]\hat{j}[/itex] will remain constant; however, because the object with a velocity in the x-direction, the x-component of the position vector function is varying as some function of time. So, the position vector function will have the form [itex]\vec{r}(t) = f(t) \hat{i} + b \hat{j} [/itex] Now, the derivative of [itex]\vec{r}(t)[/itex] should equal [itex]\vec{v}_0[/itex].

[itex]\vec{r}'(t) = f'(t) \hat{i}= \vec{v}_0[/itex] Since the velocity does not vary with time (it is constant) [itex]f'(t) = a[/itex]

That the three quantities in the expression for angular momentum are never zero implies that angular momentum will never be zero.

(b) Since [itex]\vec{r}[/itex] varies with its x-component, which varies with time, and since [itex]\vec{L}[/itex] depends upon [itex]\vec{r}[/itex], then [itex]\vec{L}[/itex] must vary.

Does this appear correct? Even if it is correct, I would be interested in knowing other ways of answering this particular question.

For part (c), I am not sure of how to employ kepler's law. I feel as the object travels farther and farther, it will take longer to sweep out certain angles.

Good physics problem

jingu — Wed, 19 Jun 2013 13:49:25 GMT

1. The problem statement, all variables and given/known data

There are two roads of length x.There separation is also x.They have mass k and p.Than what is the force exerted by one on the other.

2. Relevant equations

I think f=gmM/R^2 IS THE EQUATION TO BE USED.
3. The attempt at a solution
When I solved (dx)^2 was coming during integration which is not solvable for me.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Magnetization and polarization.

peripatein — Wed, 19 Jun 2013 13:47:21 GMT

Hi,
Suppose I have an infinite cylinder with radius R, axis along the z axis and constant magnetization M[itex]\hat{z}[/itex]. I wish to find the magnetic field everywhere. (This is NOT a HW question! I don't have to solve it nor submit it. I wish merely to check and see whether I understand this topic correctly.)
Now, as M is constant and [itex]\vec{j}[/itex] = cM[itex]\hat{θ}[/itex], i.e. tangential current, may I consider this to be an infinite solenoid, whose magnetic field is zero everywhere, except for within the solenoid where it is equal to 4πnI/c (using c.g.s), where n is the density of wraps?

Quantum tunneling - Decay

omiros — Wed, 19 Jun 2013 13:26:39 GMT

Hello everybody, I am a first year physics student and I have a question about the probability for 'heavy' nucleus to decay.

I was thinking the other day, how can we know the probability for that nucleus to decay. Let's suppose that we have an alpha decay. Is the probability of the nucleus decay, just equals the He nucleus probability to tunnel? If not, what else should we consider?

complex number ??

Asla — Wed, 19 Jun 2013 12:18:24 GMT

1. The problem statement, all variables and given/known data
Let ω be the solution to the equation x²+x+1=0
Get the value of ω¹⁰+ω⁵+3=

2. Relevant equations
complex numbers???

3. The attempt at a solution
When I try solving the first equation I hit a complex number which is making me think I am wrong.
(x+1/2)²=-3/4
Again if the method is right, what is the relationship between the complex number and the later expression?

Walt Mossberg: New laptops last all day

Naty1 — Wed, 19 Jun 2013 11:55:54 GMT

From the Wall Street Journal: Interesting article

http://online.wsj.com/article/SB1000...od=djemptech_t

Power Testing: Can Two New Laptops Really Last All Day?

.......Intel 4th generation core processsors

yes they can!

Anybody have insights on some of the design tricks used to reduce power consumption??

what happens after Compton scattering event?

mitch_1211 — Wed, 19 Jun 2013 11:42:15 GMT

Hi All,

So the photoelectric effect is the phenomenon where an orbital electron fully absorbs an incoming photon (assuming the energy of the photon is greater than the binding energy of the electron) and is ejected from its shell. The electron can then undergo its own interactions in the material. Meanwhile there is a 'hole' left in the atoms shell where the electron was so if no free electron fills is the shell energy levels are reshuffled so that the hole is filled resulting in a release of another photon (characteristic x-ray) to compensate for the energy difference of the shells involved in the reshuffle.

Now my question is, when a photon is Compton scattered from an electron and it deposits enough energy to release the electron from the shell does a reshuffle occur here also resulting in characteristic x-ray emission? Surely the 'hole' cant just remain there?

Thanks in advance

Mitch

Why insertion sort works better than quick-sort for small data?

Avichal — Wed, 19 Jun 2013 11:20:39 GMT

I have seen in books that when number of elements is small ~ 30-40 insertion sort is recommended. Why is that? The worst case of insertion sort is n² whereas for quick-sort it is nlogn.

nlogn beats n² for all values of n isn't it? Then why?

electric potential

tsgkl — Wed, 19 Jun 2013 11:10:14 GMT

1. The problem statement, all variables and given/known data
Twenty seven charged water droplets each with a diameter of 2 mm and a charge of 10^-12 C coalesce to form a single drop .Calculate the potential of the bigger drop.

2. Relevant equations
V(potential)=[itex]\frac{q}{4∏εr}[/itex]

3. The attempt at a solution
i don't really have an idea from where to start.....please help...
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Work against gravity confusion

Kinhew93 — Wed, 19 Jun 2013 11:04:32 GMT

Hi I've just started learning work, energy etc and the first question in the book is in the form:

An object of mass m is lifted by a tension through a displacement of h. Find the work done on the object.

I know that the answer is simply mgh J, there is something that I don't quite get.

Why can't it be that the tension could be greater than mg but still lifting the object the same distance (in less time) so therefore does more work? I hope that makes sense.

Thanks :)

differentiability & graph...

ARG17 — Wed, 19 Jun 2013 10:51:36 GMT

is the function (x-a)cos(1/(x-a)) differentiable at x=a???????draw the graph of function????????////

Slope at point X and the next closest value to X

Atran — Wed, 19 Jun 2013 10:48:57 GMT

Hi,

Say we have [itex]f(x) = sin(x)[/itex], then [itex]f'(x) = cos(x)[/itex] and [itex]f''(x) = -sin(x)[/itex]

Let [itex]x = π/2[/itex], the slope at that point is zero: [itex]cos(x) = 0[/itex]. The second derivative gives, [itex]-sin(x) = -1[/itex]: that means the slope of the first derivative at point x is negative. Thus for the next closest value to x, the slope of the original function is negative.

I want to know why the next closest value, say t, to any x has f(t)>f(x) if the slope at point x is positive, and f(t)
Thanks for help.

quadratic equation

Asla — Wed, 19 Jun 2013 10:42:42 GMT

1. The problem statement, all variables and given/known data
The total number of integers that satisfy the equation x²-4xy+5y²+2y-4=0