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-   -   DV/dS*v = a (http://www.physicsforums.com/showthread.php?t=663069)

carhah Jan8-13 03:45 AM

dV/dS*v = a
 
dV/dS*v=a
now at the higest point when we throw a ball.
v=0
which implies a=0

but that is npot true....any expanations?

tiny-tim Jan8-13 03:58 AM

welcome to pf!
 
hi carhah! welcome to pf! :smile:
Quote:

Quote by carhah (Post 4222159)
v=0
which implies a=0

no, v= 0 does not imply dv/dt = 0 :wink:

(draw a graph of v against t)

carhah Jan8-13 04:04 AM

Re: dV/dS*v=a
 
but if we put v=0 in equation-dV/dS*v=a

we get a=0?

i am in doubt...

carhah Jan8-13 04:06 AM

Re: welcome to pf!
 
Quote:

Quote by tiny-tim (Post 4222165)
hi carhah! welcome to pf! :smile:


no, v= 0 does not imply dv/dt = 0 :wink:

(draw a graph of v against t)


but if we put v=0 in equation-dV/dS*v=a

tiny-tim Jan8-13 04:26 AM

hi carhah! :smile:

(you don't need to send a pm everyone gets automatic email notification of any new post in any thread they've posted in :wink:)
Quote:

Quote by carhah (Post 4222170)
but if we put v=0 in equation-dV/dS*v=a

but dv/ds = ∞ (draw the graph of v against s)

eg, if a is constant, and if so = vo = 0, then s = 1/2 at2 and v = at,

so v = √(2as), so dv/ds|t=0 = √(a/so)= √(a/0) = ∞ :wink:

carhah Jan8-13 04:35 AM

Re: dV/dS*v=a
 
Quote:

Quote by tiny-tim (Post 4222176)
hi carhah! :smile:

(you don't need to send a pm everyone gets automatic email notification of any new post in any thread they've posted in :wink:)


but dv/ds = ∞ (draw the graph of v against s)

eg, if a is constant, and if so = vo = 0, then s = 1/2 at2 and v = at,

so v = √(2as), so dv/ds|t=0 = √(a/so)= √(a/0) = ∞ :wink:




thanks tim :D
BUT DOES that mean that we cannot apply that formula a=dv/ds*v in case of constant acceleration. :)

tiny-tim Jan8-13 04:42 AM

Quote:

Quote by carhah (Post 4222180)
thanks tim :D
BUT DOES that mean that we cannot apply that formula a=dv/ds*v in case of constant acceleration. :)

no, you can always apply that formula
but if it says a = ∞*0, that doesn't help very much! :wink:

carhah Jan8-13 05:02 AM

Re: dV/dS*v=a
 
can u draw graph of dv/ds ? and prove it is infinity?

tiny-tim Jan8-13 05:26 AM

Quote:

Quote by carhah (Post 4222199)
can u draw graph of dv/ds ?

you can draw a graph of v against s (as i've already asked you to)
where it's vertical, dv/ds = ∞


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