1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: By what factor help?

  1. Apr 10, 2017 #1
    1. The problem statement, all variables and given/known data
    pipe factor question.PNG cake volume.PNG

    2. Relevant equations
    Cake one...
    Area of circle times height
    Pipe one... Av = Av

    3. The attempt at a solution
    Hello, i would like some help in the right direction/
    I had a go at these problems and got B) for the pipe one and C for the cake one.
    However the correct answers for the pipe one is A) whereas the one for the cake is E.).

    I dont understand what im doing wrong. If you try substituting values and work backwards the constants i get dont fit.
    EG: for the pipe one i made r = 4...
    pi * 4^2 * 0.5 = pi* x * 4^2 * 2
    x = 0.25 so i got B, but the correct answer is A...

    For the cake problem i made h=5 and diametre = 8cm (radius = 4)...
    pi * 4^2 * 5 = 251 m-3
    i then doubled volume, and worked backwards to find factor to increase radius and therefore diamtre by...

    502 = pi * r^2 * 5
    r = 5.6418
    5.6418/4 = 1.44... so i got C, but correct answer is E

    Please help me
  2. jcsd
  3. Apr 10, 2017 #2


    User Avatar
    Science Advisor

    Pipe: if x is the scaling of the radius, you should have pi*x^2*4^2*2
    Cake: it is a scale model, so all dimensions increase by the same factor: pi*x^2*4^2*x*5 = 502
  4. Apr 10, 2017 #3
    Hello, thank you so much!! I get the one with the pipe, but i dont quite get the second one, where did the other x come from? Do you have a site i can read up on to explain this i'm too stupid.
  5. Apr 10, 2017 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It says it is a scale model, so all dimensions increase in the same ratio. If the cake starts with radius r and height h, and all dimensions are scaled up by a factor x, what is the new radius? What is the new height? What is the new volume?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted