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Cambridge Physics Problems Help

  1. Jan 18, 2012 #1
    I am a CIE A Level student and the following question is of 1st year undergraduate level.

    A bag containing a mass M of sand is suspended from a hook on the arm of a balance at a height h above the balance pan. At a time t=0, the sand starts to pour from a hole at the bottom of the bagm falling onto the pan beneath, and continues at a constant rate r (mass/unit time) until the bag is empty.

    i. Find the mass required on the other pan to maintain balance under steady conditions when a continuous stream of sand is falling from the bag (a simple balance with equal arms is envisaged).

    ii. Show graphically the variation with time of the mass required to maintain balance throughout the experiment, indicating by suitable labelling the quantities involved. Assume ideal conditions, under which air resistance, balance inertian and damping effects may be ignored.

    My working:

    When we consider the pouring of sand from a height h above the pan, there exists 3 phases: the falling sand yet to reach the pan; the steady stream of sand whose momentum is destroyed upon reaching the pan and the final stage when the steady stream of sand reduces to zero when all of the sand in the bag is depleted.

    So for the "mass required on the other pan to maintain balance under steady conditions when a continuous stream of sand is falling from the bag", I tried starting from the conservation of energy gh = 0.5 v^2. Since the height is yet to be found, I couldn't use the impulsive force on destroying of momentum too.

    I apologise for not having complex mathematical workings to present. please help. Thank you.
  2. jcsd
  3. Jan 18, 2012 #2


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    Staff: Mentor

    Welcome to PF johnconnor.

    Here's a diagram to consider:

    Note that the mass of the sand is distributed in three places at any given time so long as the sand continues to stream from the sac to the pan. When the falling sand stops at the pan it will deliver up its momentum by way of exerting a force Fp on the pan. You should be able to determine what that force will be based upon the mass-delivery rate of the sand and the velocity it gains as it falls.

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  4. Jan 19, 2012 #3
    Thank you, gneill. Pardon me if my question sounds elementary, but may I know what questions did you ask yourself when you were attempting the question?

    Could you please demonstrate my query with the question below?

    An aerator nozzle is fitted 0.40m below the surface of the water in a fish tank. It is noted that air bubbles from the nozzle accelerate upwards for the first few millimetres of their motion but then rise to the surface with a constant terminal speed. For bubbles of diameter 3.0mm, this speed is found to be 0.31m/s. The passage of the bubbles through the water is turbulent.

    i. Deduce the value of the constant K, in the expression for the retarding force due to turbulence in this system, expressed as KAv2ρ, where ρ is the liquid density, v is the uniform velocity under turbulent conditions and A is the cross-sectional area of the bubble at right angles to the direction of motion.
    (The density of water is taken as 1.0x103 kg m-3 and the density of air in the bubbles is 1.3 kg m-3.)

    ii. As the bubbles rise, the pressure due to the water above them decreases. Estimate the resulting fractional change in the raduis of a bubble as it rises from the nozzle to the surface.


    i. Condition for resultant force = 0.
    i.e., upthrust = weight + retardation force
    ρVg = 1.8E-7 + KAv2ρ
    Taking 1.8E-7 ≈ 0,
    K = rg/v2
    Substituting values, K = 0.15

    In the answer, the constant K is given the expression 4rg/3v2 whereas its value is 0.2. May I ask why is there a 4/3 factor? What did I miss out? Since the bubbles are so small, why is the factor of 4/3 - which I suppose relates to the dimension of a sphere - play a role? What other questions am I supposed to ask myself?

    ii. I have no idea how to do. pV = nRT didn't help much; perchance I applied the wrong methods involving the formula.

    Your help is greatly appreciated. Thank you.
  5. Jan 19, 2012 #4


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    Staff: Mentor

    I simply asked myself what forces are present (weights), what sources of energy exist (potential energy), what conditions have to hold to satisfy the problem (system remains balanced). Then I drew a Free Body Diagram for left hand pan.
    Looks like a whole new problem. In general you should start a new thread for a new problem.
    What forces are acting on the bubble? There's buoyancy force, weight, and retardation. What expressions are associated with each? Can you write them out?

    When the conditions stated in the problem are in effect (constant speed), how are the forces related to each other? (Free Body Diagram again).
    Assume constant temperature for the rising bubble. The pressure inside the bubble must equal the pressure in the surrounding water. So what determines the pressure in the water surrounding the bubble?
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