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: Centripetal Force and maximum force

  1. Oct 8, 2008 #1


    User Avatar

    Please help with the following problem:

    1. A front-loading clothes washer has a horizontal drum that is thoroughly perforated with small holes. Assuming it to spin dry at 1 rotation per second, have a radius of 42 cm, and contain a 3.9 kg wet Teddy bear, what maximum force is exerted by the wall on the bear?

    2. The first step I took was change 42 cm to .42m

    The main formula I used was Fc= mass (V^2/r)

    T= 1 second
    Mass = 3.9 kg

    I figured V = 2pi(r)/t = 2 (3.14)(.42)/1 = 2.6389

    Fc = 3.9 x (2.6389^2 / .42m = 64.664

    According to the online hwk, this answer is incorrect. Please let me know where I am going wrong. Thanks.
  2. jcsd
  3. Oct 8, 2008 #2
    Here the centripetal force has two components: the normal force from the wall and the weight.
    The way the problem is formulated (with the perforations on the walls) seems to imply a constant speed for the bear during the rotation (the perforations will keep it from sliding). Then the centripetal force should be the same in every position.
    However, you may have Fc=N+mg (top), Fc=N-mg (bottom) and so on.
    Which one gives the maximum N?
  4. Oct 8, 2008 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is a front loading machine, and the problem asks for the max force of the wall on Teddy bear. What are the forces on Teddy when he's at the bottom of the spin?
  5. Oct 8, 2008 #4


    User Avatar

    I am still a little confused, could you please be more specific.

    Thanks for your help.

  6. Oct 8, 2008 #5


    User Avatar

    I figured it out. Thanks.
  7. Oct 8, 2008 #6
    You did calculate the centripetal force (Fc).
    The problem asks for the force exerted by the wall. This is not equal to Fc (excepting the points where bear is at 90 deg from the bottom).

    PhantomJay gave you the solution already: calculate the force when the bear is on the bottom. Here you have N-mg=Fc where N is the force from the wall.
    How much is N?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook