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  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.
  16. P

    Circular motion

    Homework Statement A figure is attached. The symmetric system consists of 4 massless rodes with length 'L',the purple thing is a spring(the spring constant is k),when the system is at rest theta=45 deg. the system starts to rotate with angular speed 'w',the rodes are pushing on the...
  17. P

    Potential Energy

    can't believe I didnt see it..really easy thanks :)
  18. P

    Potential Energy

    well this is easy for m1 : l*cos(theta) for m2 : l*sin(theta) I can't see how this is helping me (by the way,this is not homework,I'm doing extra problems from the book)
  19. P

    Potential Energy

    Homework Statement A figure is attached. A string with length L connects two beads with masses M1,M2. what is the equilibrium point?(in terms of the angle theta,as in the figure) The Attempt at a Solution well I know that I need to find the potential energy as a function of theta,and then...
  20. P

    Potential Energy

    I tried to think about this since yesterday. o.k so the energy is conserved,let's assume that the distance between the particles is R.so becuase energy is conserved: E + a/R^3 - b/R^2 = a/r^3 - b/r^2 left side is 'Ei',and the right side is Ef(no kinteic energy just potential energy) is...
  21. P

    Potential Energy

    yea it's clear now thanks :) any suggestions for (b) ?
  22. P

    Potential Energy

    well,the potential energy is zero. as you get them closer(i.e doing work),you increase their potential energy. AlephZero: the intial distance is not given in the problem
  23. P

    Potential Energy

    maybe I'm wrong,but if you integrate the potential you dont get the force!! if you differentiate the potential you get the force.
  24. P

    Potential Energy

    why are you integrating the potential(I dont understand the physical meaning of it)? any hints for (b)? thanks!
  25. P

    Potential Energy

    Homework Statement Particle x is bounded to another particle P by force which is dependent on their distance 'r',the potential derived from this force is: U(r) = a/r^3 - b/r^2 a,b>0 (a)how much energy is required to seperate the particles?. (b)suppose particle x has kinetic energy...
  26. P

    Block on inclined plane

    I got it. thanks alot
  27. P

    Block on inclined plane

    do you have an intuitive explanation for this phenomena?
  28. P

    Block on inclined plane

    are you saying that,if I put a block on a vertical plane,there's no way I could keep this block at rest,by accelerating the plane? thanks for your help
  29. P

    Block on inclined plane

    o.k so now i'm thinking what if alpha=90 deg. tangent doesnt like pi/2 :)
  30. P

    Block on inclined plane

    hello,i'm having some trouble with this problem. a block with mass m,rests on a frictionless plane,inclined at angle alpha relative to the horizon. what is the acceleration 'a' of the plane,if we dont want the block to slide down the plane? here's what i've tried: the x axis will be parallel...
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