Recent content by Parallel

  1. P

    Electro-static energy

    of course I meant (3/5)kQ^2/R :) and you were right,I had a stupid math mistake(I really hate those) thanks alot.
  2. P

    Electro-static energy

    Homework Statement hello I have a question,it's not homework related..I know the electro-static energy of a sphere is:(3/5)kQ/R I tried to calculate it today using the expression: U = 1/2 * integral (phi * rho dV) where: phi is the potential, rho is charge density(uniform), dV is the...
  3. P

    Center of mass

    thank you so much P3X-018...I finally got it. thanks again :)
  4. P

    Center of mass

    is the relation between the velocities is just: 0 = -m1u + m2v,that is total momentum has to be zero(at least for a short time dt,as you suggested) but if I do it this way,then i get Vcm=0,which is not true.
  5. P

    Center of mass

    I wrote the euqation of motion for the CM,in my first post,it looks o.k to me. [-u*m1 + v*m2] / (m1+m2) where u and v are found by using the equation for constant accel': u=u0-at but from this equation I cant deduce anything about the direction. I dont know how to get rid of the friction...
  6. P

    Center of mass

    there is no oscillation here...and yes the spring is useless here,the problem is to find in which direction the beam will start rotating. cutting the spring only gives the masses initial speed,and from now on,you just need to focus on the masses,and friction,to find the direction the beam...
  7. P

    Center of mass

    hmm o.k i'll try to explain it again. 2 masses are connected by a SPRING,the spring is contracted,you can think of it as someone is holding those masses with his hands,bringing them closer,so the system will be in equilibrium..and another person comes and cuts the spring.. and at the same...
  8. P

    Center of mass

    can someone please try to help..(sorry for bumping but my post,got to the second page). thanks
  9. P

    Center of mass

    Homework Statement figure attached: Uniform beam with mass M,is placed horizontally,pivoted about it's center on the beam there are two small masses m1 and m2,connected by a spring,the system is in equilibrium,and at t=0 the spring is cut,and m1 and m2 slide on the beam.(their speed is not...
  10. P

    Rigid Body: bullet and a rod

    thank you very much..(I finally got it..spent like 3 hours on this) thanks again :)
  11. P

    Rigid Body: bullet and a rod

    I tried to work in the center of mass frame,(although it's messy),but how can I find V_cm?..should I just compute angular momentum before the hit and after the hit,in the CM frame,and equate them? thanks for your help
  12. P

    Rigid Body: bullet and a rod

    I really don't see,how the equation for P should give me a hint,can you be more specific? can you tell me what is the velocity I calculated?
  13. P

    Rigid Body: bullet and a rod

    Homework Statement a rod with mass 'M' and length 'L' is pivoted about a frictionless axle through it's end .a bullet with mass 'm' and speed 'v' is shot and sticks to the rod a distance 'a' from the axle. I need to find the LINEAR momentum of the system just after the hit. Homework...
  14. P

    Circular motion

    I think you can call the central axis a pivot,and imagine that it is nailed to floor,and the system rotates about this central axis. I guess the rods are pushing on the string because the system rotates(it makes sense)..there's a remark that says that this "device" can explain why Earth is...
  15. P

    Circular motion

    the rods are connected to the balls on one side and to the spring on the other side. because the system is rotating(imagine it's spining like a top) the balls are getting farther from each other and the rods are pushing on the spring.
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