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: Centripetal Force Theory Questions

  1. Oct 3, 2015 #1
    1. The problem statement, all variables and given/known data


    1. Answer True, False, or Cannot tell to each of the five statements below.
      1. A small projectile is launched horizontally 1 m above the surface of a smooth, airless planet, with sufficient speed for orbit. A bug riding in a small hole in the projectile has apparent weight equal to zero.
      2. A ball on a string moves in a vertical circle. When the ball is at its highest point, the tension in the string is less than the ball's weight.
      3. The figure shows the radial acceleration ar at four sequential points on the trajectory of a particle moving in a circle. The tangential acceleration at point 3 points up.
        circle2.gif
      4. All points on a rotating wheel have the same radial acceleration.
      5. A marble rolls around the inside of a cone. For the marble at the right side of the cone, the free body diagram is correct.
        marblecone.jpg fbd2.gif
    2. Relevant equations

    Not many equations for this, just theory

    3. The attempt at a solution
    1) True! Apparent weight = 0 N in free fall.
    2) Unsure, but am leaning towards false.
    3) Cannot tell! Depends on which way the particle is moving.
    4) False, depends on the way the wheel is rotating.
    5) False, Fc should be pointing to the left, not the right.

    I have only one try left, and I'm just very unsure about b on the most part. Any guidance concerning #2? Thanks in advance!
     
  2. jcsd
  3. Oct 3, 2015 #2

    RaulTheUCSCSlug

    User Avatar
    Gold Member

    For number two think what is actually happening when the ball is at the highest point. Draw a force diagram for the ball at the highest point and note what force is holding the ball down and what is holding it up (tension).
     
  4. Oct 4, 2015 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    For 3 and 4 you have the right answer but the reason you offer is wrong.
    For 5, is it clear what the c in Fc stands for?
     
  5. Oct 4, 2015 #4
    I've drawn a diagram and I know that at the top both the weight and tension are pointing down, but I am unsure of how to know which is smaller or larger.
     
  6. Oct 4, 2015 #5
    Hmmmm, okay, I'll give it some thought.

    I am basically positive it stands for centripetal force.
     
  7. Oct 4, 2015 #6

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Correct!

    The sum of those two forces equals ...
     
  8. Oct 4, 2015 #7
    The centripetal force! So Fc = Ft + Fg, which can be rearranged to give Ft = Fc - Fg. Does that mean the tension in the string is greater than the ball's weight?
     
  9. Oct 4, 2015 #8

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Correct!

    I don't see how that rearrangement helps determine if Ft is less than Fg.

    Think about the ball's speed if Ft = 0. This is the minimum speed needed to maintain circular motion, a question sometimes asked on tests!
     
    Last edited: Oct 4, 2015
  10. Oct 4, 2015 #9
    So the minimum speed needed to keep a ball in circular motion occurs when Ft = 0. Does that mean the weight is what's keeping the ball going in a circular motion? And if that's the case, then the tension in the string is definitely less than the ball's weight.
     
  11. Oct 4, 2015 #10

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    When the ball is at that position, yes.

    And if the speed is larger than that minimum value Ft will be larger. Anything happen if Ft is larger yet and equal to Fg? Larger than that? What can you say about the ball's speed in each of these cases?

    Thinking about these things now will definitely help you later when you're taking a test.
     
    Last edited: Oct 4, 2015
  12. Oct 4, 2015 #11
     
  13. Oct 4, 2015 #12

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    You got it!
     
  14. Oct 4, 2015 #13

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Suppose the particle is moving in the clockwise direction. If the tangential component of the acceleration points upward the particle is speeding up. If downward the particle is slowing down.

    If the particle is moving in the counter-clockwise direction we would have the opposite situation.

    In general, if the directions of the velocity and the tangential component of the acceleration are the same, the particle is speeding up.

    If the directions of the velocity and the tangential component of the acceleration are opposite, the particle is slowing down.
     
  15. Oct 4, 2015 #14

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Points on the wheel that are different distances from the center will have different radial accelerations.

    a
    c = v2/r.
     
  16. Oct 4, 2015 #15
    Thank you!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted