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: Inclined plane (A ball rolling down a slope)

  1. Apr 26, 2013 #1
    1. The problem statement, all variables and given/known data
    The mass of the ball: 136 G
    Length of the slope that its sliding down: 132 Cm
    Angle of the slope that its sliding down: 11,5228 °

    All I have to do, is to find the velocity, and the acceleration of the ball. Though my teacher has given us no time at all for this project, and due to many projects overlapping my 3 current physics projects, which are still in progress of being made. I would be grateful if you guys could provide me with the formulars. I can find the moment of inertia, and the impuls force myself.

    3. The attempt at a solution


    v^2=2*V[m/s^]*D => v=V[m/s]
  2. jcsd
  3. Apr 26, 2013 #2
    You might start by finding the velocity at the bottom of the ramp.
    PE is convertd into KE.
  4. Apr 26, 2013 #3
    so mgh = 1/2mv^2, and then isolate V?
  5. Apr 26, 2013 #4
    Consider the angular KE also
  6. Apr 26, 2013 #5
    So after isolating the equation, then add *cos^-1(ω) ?
  7. Apr 26, 2013 #6
    Think of it this way.
    mgh = (1/2)mv^2 + (1/2)I omega^2
  8. Apr 26, 2013 #7
    mgh = (1/2)mv^2 + (1/2)I omega^2

    Could you explain the last part of that equation? Omega, what does that stand for?
  9. Apr 26, 2013 #8
    kinetic energy (rotational) = (1/2) (moment of inertia) (angular velocity)^2
    Note: the problem didn't state whether this is a solid ball or a shell, like a basketball. This is important in knowing the moment of inertia
  10. Apr 26, 2013 #9
    Its a sort of marble. Its made out of hard rock materials.
  11. Apr 26, 2013 #10
    There are actually two ways you can approach this problem. One is the way I have proposed, and another is to draw a freebody diagram of the ball and figure out all the forces and torques. Remember, once you have the velocity at the bottom and the distance covered, you can figure out the average acceleration.
  12. Apr 26, 2013 #11
    How can I know the angular velocity, if I don't know the velocity nor the Acceleration?
  13. Apr 26, 2013 #12
    Recall that tyhe angular velocity is related to the linear velocity..

    (omega) = velocity X radius
  14. Apr 26, 2013 #13


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In dealing with the energy, the acceleration doesn't matter. You don't need to know either velocity individually, you just need to know the relationship between them. That means you can write your energy equation with only one unknown.
  15. Apr 26, 2013 #14
    Let me explain without giving you the answer.
    If the ramp was frictionless, then the ball would slide and not rotate. However if there is friction and the ball doesn't slide, then it will start to rotate. So, if there is rotation, then you must consider the linear kinetic energy at the bottom of the ramp as well as the rotational kinetic energy. The linear KE is (1/2)mv^2 the rotational KE is (1/2)Iw^2. w is the angular or rotational velocity and I the moment of inertia. The reason I asked about whether the ball was solid or a shell is because they have different moments of inertia. You can determine w from the radius of the ball and the linear velocity of the ball. When you put all of this together properly, you can solve for v at the bottom of the ramp.
  16. Apr 27, 2013 #15
    One thing that I still wonder is, how that I can find the angular velocity? Doesn't that require that I know the velocity at first? (Sorry that I ask this much)
  17. Apr 27, 2013 #16

    Doc Al

    User Avatar

    Staff: Mentor

    Since the ball rolls without slipping, the angular and linear velocities are connected. (As barryj already stated.) You can express one in terms of the other and solve for both.
  18. Apr 27, 2013 #17
    So the angular velocity just turns into regular velocity? I am just still curios as to what the angular velocity might be, just cant imagine it.
    Last edited: Apr 27, 2013
  19. Apr 27, 2013 #18

    Doc Al

    User Avatar

    Staff: Mentor

    No, but they are related. If you know one, you know the other. (Or you would if you knew the radius.)
    Last edited: Apr 27, 2013
  20. Apr 27, 2013 #19
    I don't want to give you the answer but... remember that the angular velocity is the linear velocity divided by the radius of the ball.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted