height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?  TOPIC_SOLVED

Limits, differentiation, related rates, integration, trig integrals, etc.

height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?

Postby stellar on Mon Apr 06, 2009 11:28 pm

If the height of an elevator above the ground is given as h(t)=4t^3-21t^2+18t+30 (h in meters, t>0 sec.) when is acceleration zero?
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Re: height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?

Postby Martingale on Tue Apr 07, 2009 12:58 am

stellar wrote:If the height of an elevator above the ground is given as h(t)=4t^3-21t^2+18t+30 (h in meters, t>0 sec.) when is acceleration zero?


find h''(t) and set it equal to zero and solve for t
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Postby stapel_eliz on Tue Apr 07, 2009 2:03 pm

stellar wrote:If the height of an elevator above the ground is given as h(t)=4t^3-21t^2+18t+30 (h in meters, t>0 sec.) when is acceleration zero?

To expand upon the earlier reply (which is completely correct, by the way), you need to keep in mind that "velocity" is the derivative of "position", and "acceleration" is the derivative of "velocity". So "acceleration" is the second derivative of "position".

That's why you need to find the second derivative, and then set it equal to zero and solve. :wink:
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Re: height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?

Postby stellar on Sat Apr 11, 2009 1:42 am

stellar wrote:If the height of an elevator above the ground is given as h(t)=4t^3-21t^2+18t+30 (h in meters, t>0 sec.) when is acceleration zero?

i did 12t^2-42t+18= 24t-42=0 t=7/4.is that right?
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Re: height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?

Postby Martingale on Sat Apr 11, 2009 2:39 am

stellar wrote:
stellar wrote:If the height of an elevator above the ground is given as h(t)=4t^3-21t^2+18t+30 (h in meters, t>0 sec.) when is acceleration zero?

i did 12t^2-42t+18= 24t-42=0 t=7/4.is that right?


since when does 12t^2-42t+18= 24t-42 :wink:

yes...

and


and when
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Re: height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?

Postby stellar on Sat Apr 11, 2009 1:11 pm

should i use 42/24 insted of 7/4? :confused:
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Re: height h(t)=4t^3-21t^2+18t+30: when is acceleration zero?  TOPIC_SOLVED

Postby Martingale on Sat Apr 11, 2009 2:23 pm

stellar wrote:should i use 42/24 insted of 7/4? :confused:



does it matter...they are the same number
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