🚀 go-pugleaf

 
  ... large number of unknown newsgroups snipped, two of your other 
      newsgroups kept but omitted from followups ... 

In article <38ca66c1.0@news2.tm.net.my> 
"koktc" <koktc@maxis.com.my> writes:
>
>This is a multi-part message in MIME format.

 Please do not post both plain text and HTML to a newsgroup that is 
 an ascii-only medium.  It wastes bandwidth to duplicate the text.

>Anyhow, there's a simplified version of my question is:
>" Why does an object being thrown up has a negative acceleration and 
>fall freely with also a negative accelerations ?"

 Simplified answer: because that is how you defined your coordinate 
 system when you drew it as your first step in solving the problem.

>My Point of view is:
>" Any object being thrown up will definately have a negative 
>acceleration because the direction of the object is the opposite of the 
>gravitational force. But when the object starts to fall freely will 
>surely have a positive acceleration because the direction of the object 
>is the same as the gravitational force. "

 You almost said that the acceleration is positive because it is 
 in the same direction as the negative velocity, which would have 
 made your misconception explicit.  

 That is, it looks like your misconception is that a "positive 
 acceleration" results in an increase in speed, a view that is 
 consistent with casual usage but which ignores the vector nature 
 of the quantities used in physics.  

 If you use the casual terminology, you have to change signs manually 
 to make the equations give the right answer, whereas if you draw a 
 coordinate system and use it for all of your vectors, the equations 
 (F=ma and the others that follow from this) apply without any fiddling.

 Since one of the main things you need to learn in a first physics class 
 is how to use precise mathematical expressions in place of casual language, 
 this is something you need to understand and that your teacher should be 
 able to explain further by having you work out DeltaV/DeltaT for specific 
 cases and see that the acceleration can be negative if a negative velocity 
 is becoming more negative as an object falls.

-- 
 James A. Carr   <jac@scri.fsu.edu>     | Commercial e-mail is _NOT_ 
    http://www.scri.fsu.edu/~jac/       | desired to this or any address 
 Supercomputer Computations Res. Inst.  | that resolves to my account 
 Florida State, Tallahassee FL 32306    | for any reason at any time.