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Rate of change question (pretty confusing)

  1. Jul 29, 2010 #1
    3. The volume of a tank used to provide water for animals at an animal enclosure at the local show is given by V(h) = (0.2h^3+3h)m^3 where h metres is the depth of the water in the tank at any time t seconds. Water is being added to the tank at a constant rate of 0.4 m3 per second. Find the rate at which the water level is rising when the depth of water in the tank is 2.1 metres.

    So i differentiated the equation: v(h) = (0.2h^3+3h) m^3, which is v'(h) = (0.6h^2+3) m^3. But because the limit is 2.1m, and the amount of liquid going into the object is going in at 0.4m^3 a second. so if i use v'(2.1)= (0.6h^2+3) m^3 and solve for h.. i can divide h by 0.4 and find out the time it took to reach 2.1m?

    Thank you.
  2. jcsd
  3. Jul 29, 2010 #2
    U need to find dH/dt, i.e the rate at which water level is rising,,
    lets say, dV/dH=(dV/dt).(dt/dH).
    write dV/dH expression, from the relation given at h=2.1 m.
    since know what dV/dt is i.e, o.4 m^3/sec, u can find out dH/dt!!!!!!!
  4. Jul 29, 2010 #3
    What was the answer you got? I got 0.071 m/s.
  5. Jul 29, 2010 #4
    i got 0.053
  6. Jul 29, 2010 #5


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    Then I'll confirm that Anakin_k is correct.
  7. Jul 29, 2010 #6
    I think it should be dV/dT = dH/dT * dV/dH rather than what you had, Mandeep.
  8. Jul 29, 2010 #7
    its all the same!!! u cn write it whichever way u wish too
    u have dV/dt, dV/dh... its not a big deal to get dH/dt
  9. Jul 29, 2010 #8
    dude m really sorry..... i messd up the calc..
    its 0.071 only!!!!
  10. Jul 29, 2010 #9
    A piece of writing advice. You don't really need to put exclamation marks in everything you write. At some point it starts to lose its meaning. In fact, in a math homework forum I dare say an exclamation point would probably never be necessary.
  11. Jul 29, 2010 #10


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    But if I couldn't use exclamation marks then what good does does it this equality? 3+3=3!

  12. Jul 29, 2010 #11
    I think you must be really excited about 3+3=3. I, on the other hand, know that 3+3!=3. :p
  13. Jul 30, 2010 #12


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    I never claimed this. Please, quote specifically where I ever claimed such an invalid response in any of my posts here.

    You don't know very much then do you :tongue:
  14. Jul 30, 2010 #13


    Staff: Mentor

    A few spaces would make it more clear that you meant 3 + 3 != 3, or better yet, 3 + 3 [itex]\neq[/itex] 3.
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