Wikipedia:Reference desk/Archives/Mathematics/2008 September 9

= September 9 =

Acceleration dependent on position
I've been trying to solve this problem: Given a particle with mass m, initial position (0,0,0), and initial velocity $$\dot{x}_0\bar{i} + \dot{y}_0\bar{j} + \dot{z}_0\bar{k}$$, find equations for the particle's position and velocity if the force acting on it is $$(4-y)\bar{j}$$. I've figured out that since there is no x or z acceleration, the velocity in those directions will be constant, so all I need to find is velocity in the y direction, $$\dot{y}$$. From F=ma I know the acceleration in y: $$\ddot{y} = (4 - y)/m$$ My first thought was just to integrate both sides with respect to t, but I can't figure out how to integrate the y term on the right side. Any suggestions? — jwillbur 16:24, 9 September 2008 (UTC)
 * See ordinary differential equation. --Trovatore (talk) 16:45, 9 September 2008 (UTC)


 * Bah, I knew I was missing something obvious. Thank you, Trovatore. — jwillbur 19:35, 9 September 2008 (UTC)

About statistics
Q. What is the target population? What property a representative sample is expected to hold? —Preceding unsigned comment added by 89.108.14.159 (talk) 17:51, 9 September 2008 (UTC)


 * This sounds a lot like a couple of questions out of a homework assignment to me, but in the interest of good faith I will provide some hints and motivating examples. Firstly, a "target population" is something that assumes that you are interested in some piece of information. If you know what information you want to know, then you should know what your target population is. Suppose you are going to run a survey to find out about the study habits of students at a university. Who is the target population? As for what property a representative sample should hold, consider an unrepresentative sample - in this case, let's say I just ask everyone in my statistics class. Why is this sample not representative of my target population? Confusing Manifestation (Say hi!) 00:55, 10 September 2008 (UTC)

Index numbers
What are index number and their uses in business? —Preceding unsigned comment added by 89.108.14.159 (talk) 17:53, 9 September 2008 (UTC)


 * This doesn't really sound like a maths question. An index number is a way of referring to an item or concept or ... when you have several of them and want a short way of consistently referring to each thing (or group of things). A car registration number is an "index" supplied by the government to uniquely identify a particular vehicle (eg XY08ABC may refer to "Ford Ka chassis number 123456789 Engine number 987654321". It can be used for almost anything.


 * Alternatively an index number may be a statistic. Eg the cost of a typical week's shopping. Usually these are set to 100 when they start, so an index of 110 means that the cost has gone up 10% with respect to the start date. These are good for observing trends and may be of help in future planing. -- SGBailey (talk) 10:02, 10 September 2008 (UTC)


 * From my undergrad in finance, index numbers are often used in discussions about inflation. i.e. what does an apple cost today in 1945 dollars.  You can learn more at Price index and List of price index formulas Sentriclecub (talk) 15:00, 10 September 2008 (UTC)

statistics
Argue that descriptve statistics and inferential statistics are must for each other.

Explain the difference between population and statistical population? —Preceding unsigned comment added by 89.108.14.159 (talk) 17:57, 9 September 2008 (UTC)


 * Sounds like homework to me... Try searching Wikipedia for those phrases and see if you can't do it yourself. (If that fails, try google). --Tango (talk) 18:17, 9 September 2008 (UTC)


 * Three homework questions in a row? Come on, you're not learning anything by just asking others, and there's no point in studying a course if you're not learning anything. -- Aeluwas (talk) 11:42, 10 September 2008 (UTC)