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Homework Help: 9th Homework doubts!

  1. Jan 30, 2007 #1
    My doubts:
    1. Why doesnt the time period of a simple pendulum depend either on mass or on the magnitude of amplitude?

    2.Why is pressure regardeed as a scalar quantity?

    I cant say anything about the first question. But about the second I'll share my views. Pressure = Force/ Area : Force is vector and 1/Area is scalar. So their product should be a vector. There was one more question regarding this in my book, A very thin foil of a metal is placed at a certan depth in water. My book states that it will remain in that position (it will niether rise up nor will sink down) because pressure acts on both sides is equal in magnitude but opp. in direction. If pressure has a direction, then why is it scalar? This is really confusing.

    3. For fluids the magnitude of pressure is given by hdg. Why isnt it applicable for solids?

    For solids as well: A*h*d*g/A =Pressure.
  2. jcsd
  3. Jan 30, 2007 #2


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    For the first question, do you know how period is defined? and what is the angular frequency of a simple pendulum?

    For the 2nd question, in short pressure is a scalar because of having no direction unlike its twin by units stress (a tensor). This has to deal mostly with how we define pressure. If you have a body and you have a normal vector to its surface, which magnitude is the area of finitessimal segment of the body, and you apply a force on the segment, pressure can be defined as:

    [tex] P = \frac{\vec{F} \cdot \vec{A}}{A^2} [/tex]
  4. Jan 31, 2007 #3
    Time period(T) is the time taken by a simple pendulum to complete one oscillation.
    Well, whats the definition of pressure according to you?
    I cant understand. Please use some simple words so that I can grasp easily.
    Last edited: Jan 31, 2007
  5. Jan 31, 2007 #4
    More questions...
    1. Which is more accurate stop watch or a stop clock?
    My view: Stop watch, becoz it has a lower L.C.
    2. Quote from my textbook:
    My correction: Then only the y coordinate of the moving particle will vary with time!
    3. Example of my book:
    My correction: uniform acceleration! am i right?
  6. Jan 31, 2007 #5


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    Wrong. If the motion is parallel to the x axis then the y coordinate is constant, by definition.

    Wrong. (Well OK, if you are being pedantic, the accleration is zero, which is "uniform"). You forgot about the drag force caused by the viscosity of the air.
  7. Jan 31, 2007 #6


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    It doesn't depend on the mass, because the force of gravity on the pendulum is proportional to the mass, so the accleration of the pendulum = force/mass is independent of the mass.

    The period does depend on the amplitude for large amplitudes. for small amplitudes it is approximately constant.

    No, in the context area is also a vector. The force caused by a pressure is always normal to the area you are considering. (People who know about tensors are free to nit-pick about that statement if they want, but from the questions I guess the OP doesn't know about tensors).

    Force = Pressure * Area, Force and Area are vectors, therefore Pressure is a scalar.

    See above - but the statement that the foil will neither rise up nor sink down is complete nonsense, and if what the book says implies this only works at one particular depth, it's very misleading nonsense. Maybe you should burn the book.

    It is applicable to solids. Why do you think it doesn't apply to solids?

    However, often the pressure in a solid caused by its own weight is small compared with the other forces acting on it, so the pressure effects can be ignored.
  8. Jan 31, 2007 #7
    That formula says that the force that counts for the pressure is only the normal (perpendicular) part of the vector, and that only its modulus if of interest.

    Side commentary, IMO, the given definition of pressure at a point in most textbooks lacks an explanation of why the orientation of the surface whose area if infinitesimally small doesn't affects the defined pressure.
  9. Feb 1, 2007 #8
    Thanks a lot....
  10. Feb 1, 2007 #9
    I have studied that the slope of a velocity time graphy gives the acceleration directly. Now when we make a graph for S vs. t^2 from a graph of velocity-time for a freely falling body. So why does the slope of the graph obtained gives g/2?

    Action is hammering but whats the reaction????

    These were some other questions which came to my mind while reading my text book. Please help!!!
    Last edited: Feb 1, 2007
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