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  1. J

    Really interesting and tough kinematics problem

    One way might be to place the center of the triangle at the origin and orient the triangle such that the instantaneous velocity of Vc is along the y-axis. Then resolve the velocities into their x-y components. Note that then VBy = VAy, VCx = 0, and VAx = - VBx at that instant in time if the...
  2. J

    Tarzan swinging on a vine -- At what angle does the vine break?

    I get h / L = .0174 for the vertical distance fallen.. If that is the case, then what does 3.2 m have to do with the problem unless that is used to determine the starting angle.
  3. J

    Tarzan swinging on a vine -- At what angle does the vine break?

    I missed the fact that the vine is 18 meters long so you could find the initial angle using 18 meters for the length of the vine and the vertical drop from that point is 3.2 m to the bottom of the swing. (Then, of course, he really wouldn't drop 3.2 m if the vine breaks before he reaches the...
  4. J

    Tarzan swinging on a vine -- At what angle does the vine break?

    Doesn't (r - r cos x) imply that the vine breaks at x = 90 deg.? I think the problem assumes the initial angle from the vertical is 90 deg.
  5. J

    Torque Problem: Finding the weight of a horizontal bar

    I agree with this answer. Your method of balancing torques is interesting - I mean the term (Tsin30⋅4). Usually, one would include the force at the pivot point of the bar and then eliminate this force when balancing the translational forces. You have apparently done this indirectly.
  6. J

    What is the magnitude of the acceleration of cylinder's com?

    The moment of inertia I of a body about any axis is equal to the moment of inertia I CM of the body about a parallel axis through its center of mass plus the mass M of the body times the square of the perpendicular distance L between the axes: I = Icm + M L^2. For a cylinder about an edge I =...
  7. J

    What is the magnitude of the acceleration of cylinder's com?

    Taking torque about the bottom point is a convenient way of eliminating the frictional force. Are you familiar with the parallel axis theorem? You can also apply Newton's equations about the center of mass to eliminate the frictional force.
  8. J

    Ball cyclotron

    Static electricity has a way of dissipating. Try to avoid any sharp points (pointed ends of connecting wires, etc.) Alligator clips should work OK. You might try folding the edges of the foil strips to get rid of the sharp edges. Also, the bowl would have to be clean to prevent leakage between...
  9. J

    Finding the energy density outside of an isolated charged sphere

    You solved for Q using D/2 and V. Why can't you just use E = k Q / r^2 where r is the distance from the center of the sphere to a point external to the sphere?
  10. J

    Finding the energy density outside of an isolated charged sphere

    Your solution looks OK at the surface of the sphere. The problem stated "near" the surface of the sphere.
  11. J

    Should I use time dilation or length contraction?

    Maybe it would help to resolve the issue if you calculated the "contracted" distance that each ship travels and then the time to traverse this distance. Use the 100 min. that you calculated as measured by Liz.
  12. J

    Young's Modulus Experiment With a Glass Screen

    Consider the number of wavelengths of the light as it passes thru the glass as compared to the number of wavelengths of light as it passes thru an equivalent thickness of air.
  13. J

    Maxwell's wheel and the conservation of energy

    In my earlier post I implied that use of a spring scales would show discontinuities in the motion. Kinematics seems to indicate that this is not the case and a scales would be of little or no use. As indicated in post #8 other factors must be at work here such as (work in bending the cords...
  14. J

    Maxwell's wheel and the conservation of energy

    Suppose the strings were attached to spring scales instead of fixed supports. What would you expect to happen to the scale readings during a cycle?
  15. J

    Relative speed of two photons

    What if you replaced photon with "2 protons with speeds of say .999999 c"? It seems that is what the question implies ( reference frames moving at speed c).
  16. J

    Velocity of a hollow cylinder at the bottom of an incline

    The moment of inertia for a solid cylinder of radius R2 is 1/2 m R2^2. If you remove a cylinder of radius R1 from the solid cylinder then you are left with 1/2 m (R2^2 - R1^2) which is what I assumed was meant by a "thick walled cylinder".
  17. J

    Velocity of a hollow cylinder at the bottom of an incline

    You wrote: Then the moment of inertia for a thick walled cylinder = Don't you mean : 1/2m(r2² - r1²)? Also, you could solve the appropriate equations in terms of I = M k^2 where k is known as the radius of gyration. Then you could conveniently compare solutions for various moments of inertia...
  18. J

    Infinite chain of alternating charges (+/-)

    But it seems if you place one end of the chain at the origin then you have to divide by n (infinity) to get the potential energy per ion. But if you place the middle of the chain at the origin then you have the difference of two infinite series that proceed as: 1/2 + 1/4 + 1/6 ........ 1/2n and...
  19. J

    Infinite chain of alternating charges (+/-)

    What if you take one charge out of the infinite chain and then consider the work done in adding the charge back into the chain? For the first two adjacent charges : W1 = -2kq / d and then for the next 2 charges W2 = 2 k q / (2 d) then you get an infinite series of terms like (2 k q / d) *...
  20. J

    Physic (kinematics) — Displacement of an accelerating car

    You used the final speed for 15 sec. You might ask "What was the average speed during those 15 sec.?"
  21. J

    Converting Newtons to Watts

    What seems to be missing is the implication that the train speeds back to 10 m/s in 10 sec. Work (J) = Change in KE = Force * Distance Power (Watts) = Work / time
  22. J

    Ceiling height for a game of Catch

    The range formula is fine as a check, but you do not need to use trig functions to get the answer provided for (R)^1/2.
  23. J

    Physics Help with elastic collision

    One useful fact to keep in mind is that in an (linear) elastic collision of two objects is that the relative speed of approach equals the relative speed of separation after the collision.
  24. J

    Kinetic Energy Equations in MS Excel

    The file you attached is not in Excel format. Were you given the actual spreadsheet., if so you can check the result cells for the formulas used. As an example in cell H1 there must be a formula such as: =B1*D1
  25. J

    Calculate the rate of potential energy loss of water in a pipe

    You wrote U = 36268.82J Shouldn't it be U / t = 36268.82 J / sec?
  26. J

    Finding Total Charge from E-field

    Also, doesn't L'Hopital's give a finite value for E as R goes to infinity? (It does appear to be zero)
  27. J

    Finding Total Charge from E-field

    I tend to agree, but why does R need to go to infinity? Was this implied in the problem?
  28. J

    Finding Total Charge from E-field

    Isn't E = k Q / R^2 (spherical charge distribution)? Then 2 E R dR = k dQ. The integral doesn't diverge unless (1 / R) appears in the result at R = 0.
  29. J

    The length of a pendulum that is equivalent to a rocking hemisphere

    I think you need 3/8 in the denominator instead of 5/8.
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