Physics Forums

can string theory/gravitons/broken SUSY reproduce GR time dilation?

ensabah6 — Sat, 21 Nov 2009 06:11:38 GMT

looking at this thread,
Has einsteins experient with time been proven, why hasn't it been applied or used ( 1 2 3)

I've wondered if string theory and or QFT w/gravitons can reproduce time dilations, or changing time background.

i understand how gravitons exchange can create an attractive force but I'm less clear as to whether this can also change time

does broken SUSY also allow for time dilation

Pascal's pyramid

davedave — Sat, 21 Nov 2009 06:09:26 GMT

I have heard that Pascal's pyramid is a 3-dimensional analogue of Pascal's triangle .

The edges of Pascal's pyramid come from a particular row in Pascal's triangle.

For example, let's take the row of Pascal's triangle having entries 1 5 10 10 5 1.

So, the corresponding Pascal's Pyramid is given below.

1
5 5
10 20 10
10 30 30 10
5 20 30 20 5
1 5 10 10 5 1

Can someone explain how to find the internal entries, such as 20 and 30?

Thanks.

nuclear model

joebo — Sat, 21 Nov 2009 06:03:42 GMT

recently,i am focusing on studying the nuclear model, can you recommend some articles ,good review and Classic books about nuclear model,especally the cluster model to me?
thanks

delta(t) potential

najima — Sat, 21 Nov 2009 06:01:46 GMT

Consider the potential 0 for X<0 and X>L
what is α If we have just one allowed energy that is zero.
first of all I must write the eigen function of well potential? what can I do with ?

how to prove that the symmetry group of a regular polygon has only 1 and 2 dim irredu

wdlang — Sat, 21 Nov 2009 05:39:41 GMT

how to prove that the symmetry group of a regular polygon has only 1 and 2 dim irreducible representations?

Vertical simple harmonic motion concept

caljuice — Sat, 21 Nov 2009 05:37:17 GMT

1. The problem statement, all variables and given/known data
A 0.12-kg block is suspended from a spring. When a small stone of mass 30 g is placed on the block, the spring stretches an additional 5 cm. With the stone on the block, the spring oscillates with an amplitude of 12 cm. What is the net force of the stone when it is at a point of maximum upward displacement

3. The attempt at a solution

The answer is Fnet= mass of pebble x gravity. The solution says this is the case because the spring isnt moving.

But shouldn't Fnet = force of gravity + force of spring? Since at maximum compression, the spring will exert a down acceleration. Just because it's not moving doesn't have to mean it isn't causing acceleration right? And when I look at the position graph and the acceleration graph of horizontal simple harmonics, the greatest magnitude of acceleration of a spring is when the spring is at maximum compression or stretch- the amplitude. If there's acceleration then there must be force? Or is this different because this is vertical? Thanks.

Write a working matlab function

tuablink — Sat, 21 Nov 2009 05:30:59 GMT

1. The problem statement, all variables and given/known data
Write a working Matlab function that takes as input a positive integer n and outputs a
2n by 2n matrix whose top left n by n block and bottom right n by n block are zero,
whose top right n by n block is the n by n identity matrix and whose bottom left n by n
block is filled with uniform [0,1] random numbers.

2. Relevant equations

3. The attempt at a solution
function MatrixMul
n = input('Please enter a positive integer: ');
A(2*n,2*n)=0;
rand(n,n);
eye(n)
display(A)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Voltage drop in diode

sudar_dhoni — Sat, 21 Nov 2009 05:09:15 GMT

the depletion region is resposible for voltage drop
all that is fine
but in forward bias there is no depletion region hence there can be no voltage drop
but it is mentioned in my book that the voltage will drop by .7 volt
how come this can happen

also i have many doubts in transistor amplifiers
waiting for reply's

Gravities effect on the boundaries of the Universe

TheDoorsOfMe — Sat, 21 Nov 2009 05:05:57 GMT

Some theories of the expanding universe say that eventually the universe's expansion could be slowed and eventually pulled back in on itself. My question is why can we assume the edge of the universe is effected by gravity? Does this barrier necessarily have to be made up of a physical substance?

Also this might be a weird question but what do yo think would happen if I flew up next to this expanding barrier and tried to put my hand through it? Would I lose my hand or possible my hand wouldn't penetrate it.

Curve Sketching help!!

Slimsta — Sat, 21 Nov 2009 05:00:45 GMT

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

2. Relevant equations

3. The attempt at a solution
For part E, ive been sitting on it for the last 2 hours trying to figure out why this is wrong..
i came up with a solution but it doesnt seem to be right..
if solving for f'(x) i get x=6.279
so f'(x) > 0 ==> x<6.279
f'(x) < 0 ==> x>6.279
decreasing (-inf, 6.279)U(6.279, inf) and increasing on (6.279, 0) ? im not sure about the increasing part..
but when graphing it on my calc, it look like 1.521 would work better..

Simple Torque on Rigid Object

IAmSparticus — Sat, 21 Nov 2009 04:44:49 GMT

1. You are installing a new spark plug in your car, and the manual specifies that it be tightened to a torque that has a magnitude of 26 N�m. Using the data in the drawing, determine the magnitude F of the force that you must exert on the wrench.

2. Torque= Force * Lever arm length

3. The force is said to be 26 N*m, and the lever arm length is shown to be 0.28 cm in the picture.

26 N*m = F * 0.28 m
F= 26 N*m/0.28m
F= 92.86 N

Which is wrong. What did I do wrong?

Attached Images

Torque 1.gif (6.9 KB)

When A Mass is Falling from an Inclined Plane

Procrastinate — Sat, 21 Nov 2009 04:28:37 GMT

A mass M is released from rest on a smooth plane inclined to the horizontal at 14 degrees. Find the time taken for the mass to travel 1m down the incline and the velocit attained at the end of that time.

I had no trouble with the first part - measuring the velocity and time for the part where it slides down the inclined plane.

However, the part where I am having trouble with is attempting to find the angle in which the mass makes with the ground and its line of motion when it completes the second part of its journey.

Anyone have any hints to do this? I drew a free body force diagram but I was really confused with the lines of motion.

why oxygen, cannot act as a pi-donor while NO can?

queries — Sat, 21 Nov 2009 04:26:14 GMT

Hi,

I have learnt that pi-donor are those ligands which donate their pi electrons to metal centre while pi-acceptor accepts electron from filled d(pi) orbital to their pi* orbital.
Now, I'm confused, why oxygen, having two pi* electrons cannot act as a pi-donor while NO, having one pi* electron only can?

Thank you!

LHC Restarts

Vanadium 50 — Sat, 21 Nov 2009 04:20:30 GMT

Last night the LHC re-established circulating beams.

CERN has issued a press release.

[Probability] Seating arrangement & Selection

CountNumberla — Sat, 21 Nov 2009 04:17:19 GMT

PROBLEM 1
7 people sit in a row, and 2 of them want to sit together, in how many ways can they be arranged?

ATTEMPTED SOLUTION:
6! x 2 = 1440
-------------------------------------------------------------------------------------------------------
PROBLEM 2
7 people sit in a row, and 2 of them do not want to sit together, in how many ways can they be arranged?

ATTEMPTED SOLUTION:
No ideas
-------------------------------------------------------------------------------------------------------
PROBLEM 3
7 people sit in a circular formation, but 2 of them do NOT want to sit together, in how many ways can they be arranged?

ATTEMPTED SOLUTION
No ideas
-------------------------------------------------------------------------------------------------------
PROBLEM 4
There are 10 red & 6 blue marbles in a bag. You pick 2 of them at the same time.
In how many ways can you pick 2 marbles of the same color? Different colors?

ATTEMPTED SOLUTION:
Same colors: 10/16 x 9/15 + 6/16 x 5/15
Different colors: 10/16 x 6/16 + 9/15 x 5/15