The Official Math/Physics/Whatever Homework questions thread - Page 64

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04-21-2011 , 02:14 PM

#1576

Cueballmania

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Oh, I guess that does work. Too easy IMO.

Alternatively, I found a way to do it via contour integration. Thanks for the speedy responses.

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04-24-2011 , 07:34 PM

#1577

Ryanb9

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Sister is doing math homework and i suck at maths. Answer is 1.11111E14 on the calc, teacher says "dont write andswers as "E..." so how should it be written?

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04-24-2011 , 10:21 PM

#1578

Cueballmania

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E14 is the same as times 10^14

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04-29-2011 , 04:38 PM

#1579

keysopendoors

stranger

Join Date: Apr 2011 Posts: 2

Can anyone explain how centrifugal force works?

Quote:

John Harrison, a Yorkshire carpenter, then submitted a project in 1730, and completed in 1735 a clock based on a pair of counter-oscillating weighted beams connected by springs whose motion was not influenced by gravity or the motion of a ship.

*His first two sea timepieces H1 and H2 (completed in 1741) used this system, but he realised that they had a fundamental sensitivity to centrifugal force, which meant that they could never be accurate enough at sea.

How could centrifugal force affect this?

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05-01-2011 , 05:20 AM

#1580

astj

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Join Date: Feb 2009 Posts: 66

If any Chemistry majors would be willing to help me out with a fairly simple real life chemistry problem please PM me.

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05-02-2011 , 10:57 AM

#1581

Csaba

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Join Date: Jan 2009 Posts: 3,131

Briefly describe what three conditions need to be satisfied for a self sustaning nuclear fission reaction to uccur.

I'm assuming the following conditions must be satisfied:

Adequate supply of fuel
Nuetrons must be slowed down
The binding energy of the product nuclei must be smaller than the binding energy of the reactant's nucleus.

I'm a bit unsure about the final factor. Does this look OK?

Thanks

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05-03-2011 , 12:00 AM

#1582

Krumb Snatcha

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Join Date: Mar 2010 Posts: 3,051

Can someone help me differentiate e^2x/x^4. So far, I think you need to use the quotient rule to solve this so: (x4* 2e^2x)- (e^2x*4x^3)/ (x^4)^2. Now if this is correct, how do you factor/simplify this to its simplest form?

Thanks.

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05-03-2011 , 12:14 AM

#1583

Wyman

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Quote:

Originally Posted by Krumb Snatcha

(x4* 2e^2x)- (e^2x*4x^3)/ (x^4)^2.

= [2 * e^(2x) * x^3 * (x - 2) ]/ x^8

= 2(x-2)e^(2x) / x^5

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05-03-2011 , 12:22 AM

#1584

Krumb Snatcha

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Join Date: Mar 2010 Posts: 3,051

Quote:

Originally Posted by Wyman

= [2 * e^(2x) * x^3 * (x - 2) ]/ x^8

= 2(x-2)e^(2x) / x^5

Can you explain how you arrived at x^3 and (x-2)? thanks.

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05-03-2011 , 12:29 AM

#1585

Wyman

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pull out the e^(2x)
look at what's left and pull out the x^3
look at what's left and pull out the 2

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05-03-2011 , 02:47 PM

#1586

Comstizzle

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Join Date: Nov 2009 Posts: 2,298

as a general rule it's easier to differentiate sometime like e^(2x)*x^(-4) than e^(2x)/(x^4).

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05-03-2011 , 06:01 PM

#1587

Krumb Snatcha

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Join Date: Mar 2010 Posts: 3,051

Quote:

Originally Posted by Wyman

pull out the e^(2x)
look at what's left and pull out the x^3
look at what's left and pull out the 2

thanks.

Quote:

Originally Posted by Comstizzle

as a general rule it's easier to differentiate sometime like e^(2x)*x^(-4) than e^(2x)/(x^4).

Yea, I never thought about that. Thanks.

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05-03-2011 , 08:05 PM

#1588

Krumb Snatcha

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Join Date: Mar 2010 Posts: 3,051

I know f'(x) is the derivative, but what is f"(x)? Is it the derivative of the derivative? If so, what does this mean or what does this tell us and how is knowing f"(x) useful? thanks.

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05-03-2011 , 08:12 PM

#1589

Comstizzle

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Join Date: Nov 2009 Posts: 2,298

Think about a car moving. As time goes by, it's position is f(t). The change in position, f'(t), is obviously speed (velocity w/e). Can you think of what (f'(t))' is?

acceleration

It tells us how quickly the derivative changes with time.

While the derivative tells us the slope of the graph, the second derivative tells us the convexity, or it's curvature.

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05-03-2011 , 08:20 PM

#1590

Wyman

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The derivative F ' is the rate of change of F. When it's positive, F is increasing. When it's negative, F is decreasing.

F '' is the derivative, hence the rate of change, of F '. When it's positive, F ' is increasing, When it's negative, F ' is decreasing.

So now you can say things (knowing the sign of F ' and F '') like: F is increasing at an increasing rate, i.e., accelerating (common usage, physics nits). This corresponds to F ' > 0, F '' > 0.

See if you can draw the graph of a function that has that property.

Then see if you can draw & think about the other possibilities -- F ' > 0, F '' < 0; and the 2 where F ' < 0.

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05-03-2011 , 08:40 PM

#1591

Cueballmania

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Quote:

Originally Posted by Comstizzle

as a general rule it's easier to differentiate sometime like e^(2x)*x^(-4) than e^(2x)/(x^4).

I'm a fan of using logarithmic derivatives.

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05-06-2011 , 06:58 PM

#1592

Krumb Snatcha

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Join Date: Mar 2010 Posts: 3,051

A car loan is 2100, the interest rate is 8.75%, and the loan period is for 3 years. what is the monthly payment?

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05-07-2011 , 02:39 AM

#1593

Comstizzle

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Join Date: Nov 2009 Posts: 2,298

Make a spreadsheet

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05-07-2011 , 03:11 AM

#1594

Pawntificator

33/6

Join Date: Dec 2008 Posts: 13,094

Get a better job and pay cash up front. Damn, I suck at homework now that I don't have to do it

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05-08-2011 , 08:30 PM

#1595

stueycal

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Join Date: Jan 2009 Posts: 2,903

What program do I use for weird characters again? Anyways...This is from my logic/reasoning 101 take home test on logic/circuits.

Draw the circuit corresponding to the negation of the bi-conditional statement P<-->Q.

Thanks in advance.

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05-08-2011 , 11:41 PM

#1596

Cueballmania

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Are you talking about greek letters? You can use LaTex. But if you want to insert it somewhere, look for the ASCII value and enter it in with "Alt+3digits".

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05-09-2011 , 12:24 PM

#1597

dmkai3

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Join Date: Feb 2008 Posts: 2,619

Real simple question.

At a rate f/3 feet per m minutes, how many feet can a bicycle travel in s seconds?

How do I solve this algebraically in terms of f, m, and s?

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05-09-2011 , 01:32 PM

#1598

Wyman

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multiply your ratio by 1 minute / 60 seconds to cancel out your 'minutes' units and you'll be left with feet/second

then mult by s seconds to kill your 'seconds' units, and you're left with just feet.

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05-10-2011 , 02:24 AM

#1599

tiltowhirl

grinder

Join Date: Nov 2006 Posts: 607

Solve the diferential equation:
y"(t) + 4y(t) = sin(t) - u(t - 2pi)sin(t - 2pi), y(0)=y'(0)=0 using Laplace transforms.

Brain is mush after 9 problems like this, I'm just bad with what I think is a unit step and a tricky sin(t-2pi) which is just sin(t). On the last term e^(something)/(s^2+1) unless I'm missing some snazzy substitution method using sin(t-2pi).

I guess I just basically need the laplace of the last term unless the inverse is difficult in the end.

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05-10-2011 , 08:34 AM

#1600

Wyman

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Laplace transform of u(t-a)f(t-a) should be in your book.

L{u(t-a)f(t-a)} = e^(-as) F(s), where
F(s) = L{f(t)}

You should post your work itt; it's very easy to screw these up until you get the hang of handling the shifts, etc.

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