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Physics Problem

  1. Oct 6, 2004 #1
    Hi,
    I think I posted this in the wrong place, so im posting it here. I'm taking Physics w/ cal 1, and Im having problems solving a couple of question, I was hoping some one could help, I would appreciated it..

    1) block of mass m1=8kg is connected over an idead (masless and frictionless) pulley to block 2, of a mass m2=4kg. The magnitude of blocks acceleration is 3 m/s^2 and the angle of incline Theta=30 deg.
    a) find the coefficient of the kinetic friction between block 2 and the plane of the incline.
    b) what is the magnitude of the tension in the rope.

    and also this problem which is projectile motion -
    2) Projectile is fired from the top of a 40m tower at an angle of 60 deg above the horizontal hits the ground at the point 100m from the base of the tower.
    a) find the speed at which the stone was thrown.
    b) find the speed of the stone just before it hits the ground.
    c)find the time at which the projectile hits the ground.
    d) find the maximum height.

    for a) i got 35 m/s

    Thank You.
     
  2. jcsd
  3. Oct 6, 2004 #2

    Pyrrhus

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    Do a freebody diagram for the first problem, If the pulley is ideal then it has the same tension throughout it.

    For the second problem consider.

    Vy = 0 at max height
    Y = 0 when it hits the ground.
     
  4. Oct 6, 2004 #3
    do you know how to get Part a in problem 1. Im having problems setting up the equation.
     
  5. Oct 6, 2004 #4

    Pyrrhus

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    Yes, i know.

    Here's a hint:

    Put your coordinate system with the same inclination as theta, so the normal force will have one non-zero component, while the gravity will have two non-zero components. Also consider the other two forces Tension and friction on the coordinate system.
     
  6. Oct 6, 2004 #5
    Thanks for responding to my question.

    In problem two part "b" Is it F= w*sin(30)= 4N..this is what I get, Im probably doing it wrong. and In part one I still dont undrestand how to solve the problem.
     
  7. Oct 6, 2004 #6

    Pyrrhus

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    Draw a freebody diagram, you can use the hint i said, to solve it.

    Do you know your geometry (specially trigonometry) well?

    If you still don't understand, let me know.
     
  8. Oct 6, 2004 #7

    Gokul43201

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    I can't seem to say this enough (as, I imagine does cyclo) ...yet it seems, no one listens : Draw the free-body diagram. That's half the work in the problem.
     
  9. Oct 6, 2004 #8
    Was my answer for "b" right?
    Im still working on part a.
     
  10. Oct 6, 2004 #9

    Gokul43201

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    Did you draw the free body diagram ?
     
  11. Oct 6, 2004 #10
    Yes I did.

    I used the book as guide to draw it.
    I also worked on part a, and I get 0.38 N.
    is this right?
     
    Last edited: Oct 6, 2004
  12. Oct 6, 2004 #11

    Pyrrhus

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    no, that's not correct.

    show me your equations.
     
  13. Oct 6, 2004 #12
    I also get 0.64 when I use cos(theta).

    Im using the formula F=Mu_k*mg
     
  14. Oct 6, 2004 #13

    Pyrrhus

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    Try to make the equations again remembering this

    [tex] \sum_{i=1}^{n} \vec{F}_{i} = m \vec{a} [/tex]
     
  15. Oct 6, 2004 #14
    i used N=mg*cos(theta), F=mg*sin(theta)

    and then mu_k = F/N and I get 0.57 is this one right?
     
  16. Oct 6, 2004 #15

    Pyrrhus

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    Ok let's try this with a different approach, you're beginning to understand, but still not there.

    What are the forces acting on the x-axis on the first and second block?

    What are the forces acting on the y-axis on the first and second block?
     
  17. Oct 6, 2004 #16
    i also did it using F_k = ma, N= mg, and F/N, and for this one i get 0.30
     
  18. Oct 6, 2004 #17

    Pyrrhus

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    Please, answer the questions of my reply above.
     
  19. Oct 6, 2004 #18
    for the second block, the forces acting on it are Gravity and Normal force.
    for the first block g, N, T
     
  20. Oct 6, 2004 #19

    Pyrrhus

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    Try answering this questions

    Aren't both blocks tied together to a string?

    Do you know what's an ideal pulley?

    What is the normal force?

    What is friction and how does it affect movement?
     
  21. Oct 6, 2004 #20
    Yes both blocks are tied to a string. I do not know what the ideal pulley is. The normal force is m*a, is the the upward motion of the block. The friction I believe slows down the movement of the block.
     
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