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  1. J

    Help Solving Harmonic Motion Problem

    Can you offer some help on the following question? I've been trying different things for a while but can't get the right answer. Thanks! At time t = 0, consider a 1/2 wavelength long section of the rope which is carrying the wave y = 0.06 cos(2.1 t - 2.4 x) between two points which have zero...
  2. J

    HELP Angular Acceleration

    Okay... treating them as point masses: alpha = torque / I = (m1)gr - (m2)gr / m1(r^2) + m2(r^2) = 12(9.8)(0.25) - 6(9.8)(0.25) / ((.25^2) (m1 + m2)) = 13.1 Is that right? So the position of the center of mass doesn't really matter in this case? Thanks!
  3. J

    HELP Angular Acceleration

    A dumbbell consists of a slender rod of negligible mass and small spheres attached to each end with mass 12-kg and 6-kg respectively. It is pivoted at its center about a fixed horizontal frictionless axle and initially held in place horizontally. The length of the rod is 50 centimeters and...
  4. J

    Net Torque & Net Force: Exploring the Relationship

    if there is a net torque on a system does that mean there must be a net force on the system
  5. J


    Suppose now that I want to calculate the tangential force. I set torque = I(alpha) = F(R) F = I(alpha) / R = (1/3)ML^2*(a/R) / R F = (1/3)ML^2*((g sin_theta)/(L/2)) / (L/2) I got that F_tangential = 4/3Mgsin_theta, but they got that the answer is 1/4 Mg sin_theta. What did I do...
  6. J


    thanks Everything did come together. Thank you for your help!
  7. J


    Mechanics...PLEASE HELP VERY SOON! Can someone please help me with this problem? I've been working on it for hours - anything would help thanks! A system consists of a uniform stick of length L and mass M hinged at one end. The hinge is released from rest at an angle theta_0 with respect to...
  8. J

    Rotational Motion: Who Reaches Bottom First?

    help! can someone please answer/
  9. J

    Rotational Motion: Who Reaches Bottom First?

    would the smaller inertia hoop be going faster or does it not matter? I think we did this experiment where we found acceleration to be g sin theta / (1 + c), where c was the constant of inertia. (i.e., 1/2 for a cylinder and 1 for a hoop).
  10. J

    Rotational Motion: Who Reaches Bottom First?

    I'm a lil confused: so by conservation of energy I have: mgh = 1/2mv^2 + 1/2Iw^2 mgh = 1/2mv^2 + 1/2I(v^2/r^2) V^2(1/2m + 1/2(I/R^2) = mgh v^2 = mgh / (1/2m + 1/2(I/R^2)) For spoke one, the mass is greater, so speed decreases... But wouldn't I be smaller? Wouldn't that increase...
  11. J

    Rotational Motion: Who Reaches Bottom First?

    re; The part of I'm confused on is this: The wheel with the spokes has more mass, which increases inertia and decreases speed. But it also has more mass concentrated around the radius, which increases speed...
  12. J

    Rotational Motion: Who Reaches Bottom First?

    Two hoops of identical radii roll without slipping down identical inclined planes under the influence of gravity. Hoop 1 is identical to Hoop 2 except for some massive spokes which are fixed to the rim. If the two hoops are released at the same time from the same height, which hoop makes it...
  13. J

    Simple momentum problems

    Hey, just want to double check on these: Rain falls vertically into an open boxcar coasting freely on a horizontal frictionless track. As a result the momentum of the system consisting of the boxcar plus accumulated water increases. - I said true - since there is no friction, velocity...
  14. J

    Elastic Collision: Who Experiences Larger Accel?

    A cart of mass M and a second cart of mass 2M collide head on elastically and bounce apart. Which cart experiences a larger magnitude of acceleration during the collision?
  15. J

    Magnitude of Total Momentum

    Say there was 2 collisions. There are 2 carts of identical mass. In the first case cart A starts at height h above the ground and crashes into cart B which is stationary on the flat ground. They stick together. In the second case, cart A starts at height h/2 above the ground, cart B starts at...
  16. J

    HELP Block up incline

    There's a block of mass M that's pulled up an incline by a force T. The block starts from rest and is pulled a distance L by the force T. The incline, which is frictionless, makes an angle (a) with respect to the horizontal. 1) Write down the work done by force T. That would just be W =...
  17. J

    HELP mechanics problems

    Thanks for your help! I just need one more clarification. So I understand that external forces are not the same as conservative forces, so then it's FALSE to state that if the work is done by conservative forces, then momentum is conserved?
  18. J

    HELP mechanics problems

    Hey, can any of you clarify a few things for me? I'd greatly appreciate it, thanks! If the work done on an object is by conservative forces, does that mean the momentum is conserved? I know if there aren't any external forces, then momentum is conserved. Does conservative forces mean no...
  19. J

    Really simple problem - can't get right answer

    Suppose a 50kg block slides along a horizontal surface where the coefficient of kinetic friction between the block and the surface is uk = 0.60. A force F = 400 N is now applied, where the angle of the force above horizontal is 20°. What is the magnitude of the acceleration of the...
  20. J

    Solve Circular Motion: Blocks on Turntable, ASAP!

    Circular Motion - please help ASAP! Two identical blocks are tied together with a string and placed along the same radius of a turntable that is spinning about its center. The inner block is 4 cm from the center and the outer block is 6 cm from the center. The coefficient of static friction...
  21. J

    Physics 1D problems: Please help

    and... thanks doc, but I still don't see how I can eliminate Vob...
  22. J

    Physics 1D problems: Please help

    Problem 1 Ball A is dropped from the top of a building of height H at the same instant ball B is thrown vertically upward from the ground. First consider the situation where the balls are moving in opposite directions when the collide. If the speed of ball A is m times the speed of ball B...
  23. J

    Find the distance d that places the final image at infinity.

    Anyone know how to solve the following problems? Thanks! 1.) An object is placed 12 cm to the left of a diverging lens of focal length -6cm. A converging lens of focal length 12 cm is placed a distance of d to the right of the diverging lens. Find the distance d that places the final image...