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Homework Help: Really Fustrated Help

  1. Dec 5, 2005 #1
    Really Fustrated...Help :(

    I have been studying newtons 3 laws in class and at the moment we are working with friction, particularly friction working on a "box" moving down a ramp of some sort or a force being held in equilibrium. I cannot for the life of me wrap my head around the methods for solving these problems that are presented to me. So here are my questions.

    How exactly should i go about looking at and solving a problem such as the one that follows or one similar to it.

    E.G. 1 (box on incline w/o acceleration):

    Two 10 kg boxes are connected by a massless string that passes over a massless frictionless pully. The boxes remain at rest the one on the right hanging vertically and the one on the left 2 meters from the bottom of an inclined plane that makes an angle of 60 degrees with the horiz. The coefficients of kinetic friction and static friction between the left hand box and the plane are .15 and .3 respectively. you may use g=10m/s^2, sin 60 deg = .87 and cos 60 deg=.50. what is the tension T in the string?

    My questions on this one: how do you go about solving it first. How exactly do you look at the problem and determine what to do? How do kinetic and static friction play in this problem? What exactly do you do with them and why? How do you add all the forces of both the boxes together so you can get your answer?

    E.G 2 (Acceleration.):

    Two blocks are connected by a weighless string over a frictionless pully. The box on the left weighing 7 kg is on an incline of 37 deg with the horiz.
    The box on the right weighing 12 kg is hanging vertically. The coefficient of friction is .25. What is the acceleration of the 7 kg block?

    My questions on this one: How is this problem solved? How exactly do you find the forces acting on the box to the right and how does that have an effect on the box to the right?

    Thank you for your help and im sorry i couldn't formally introduce myself in a previous post. Im in a rather tight crunch at the moment and so all the help i can in the least amount of time is essential. Real briefly, my name is jrew, im a full time dj. Im a senior in high school and honestly, science has always been a difficult deal (although math hasn't been). I find physics to be difficult because i seem to have to warp my thoughts in ways i never have before. Anywho, thank you for your help, i truly appreciate it.
  2. jcsd
  3. Dec 5, 2005 #2
    i just read the faq, apparently and i didn't prefrence this post correctly. This is not a homework problem, this is mearly an example to work with for conceptual purposes. If the admins need to move the post please feel free. i just need to know how to work these sorts of problems out. Thank you.
  4. Dec 5, 2005 #3


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    Staff Emeritus
    Science Advisor

    Well one has to determine all the forces involved, and in this case means weights and friction.

    The string simply transfers force between masses, and tension depends on the direction of the forces involved.

    Friction is a force which opposes movement, and is proportional to the 'normal' force on the surface of contact. If the system is static, one uses static coefficient of friction, and if the system is in motion, one uses dynamic or kinetic coefficient of friction, which is usually less than static.

    There is no motion if the friction equals or exceeds the pull of the string. In the case cited, the suspended mass applies a force = mg to one end of the string, and the friction of the block on the incline provides an opposing force. The block on the incline also has a force component pointing down the incline.

    Tension in the string comes from opposite forces. If the oppsing forces are equal, then the magnitude of tension = the magnitude of the force.
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