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Homework Help: Can anyone help provide an explanation?

  1. Jun 19, 2005 #1
    Qn1: When paddling a canoe, one can attain critical speed with relatively little effort, but to go faster beyond this critical speed requires more effort. Why?

    Qn2: Why do jet planes usually fly at altitudes above 10000m although it takes a lot of fuel to climb to that height?

  2. jcsd
  3. Jun 19, 2005 #2
    What are your thoughts?

    Hint: Friction
  4. Jun 19, 2005 #3
    yup.. around there. Friction and the drag force
  5. Jun 19, 2005 #4
    but how..? when u fly higher, u get lesser friction..?
  6. Jun 19, 2005 #5
    Right. I may have the number wrong but when flying at the altitudes planes do on long flights (around 30,000 feet) the air is about 20% as thick as it is at sea level.
  7. Jun 19, 2005 #6
    u mean air is less dense at high altitudes, therefore, by using F=pAv^2, we get a lesser drag force?
  8. Jun 19, 2005 #7
    Yeah, the air density is much less up there. This also affects the plane's lift though.

    Can you think of anything for the canoe?
  9. Jun 19, 2005 #8
    no.. that's really what i don't know
  10. Jun 19, 2005 #9
    Look at the force equation you gave above for drag resistance. Although that is the application for air, it is a simliar principle for fluids. What do you notice as velocity increases?
  11. Jun 19, 2005 #10
    oOO.. I see. so as the speed of the canoe increases, the larger the drag force is on the canoe.

    Whao, thx guys!
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