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Homework Help: Physics logic puzzles1

  1. Nov 25, 2007 #1
    Physics "logic" puzzles1

    1. The battery output voltage, the bottle volume, the clock time, and the measure of weight all have something in common. Are they represented by:
    one number (I think this is the answer)
    more than one number

    2. Water is shooting out of the end of a pipe. The end of pipe is bent into a figure 6.
    The water shoots out in a curved arc
    The water shoots out in a straight line (I think this is the answer)

    3. If you drop a boulder and a pebble at the same time, they will fall together with equal accelerations(neglecting air resistance). The principal reason the heavier boulder doesn't accelerate more than the pebble has to do with:
    surface area (I think this is the answer)
    none of these

    4. Suppose that both an elephant and a feather fall from a high tree. Which encounters the greatest force of air resistance in falling to the ground:
    feather (I think this is the answer)
    both the same

    5. A strong man is pulling the spring apart. Is there a force in the spring?
    no (I think this is the answer)
  2. jcsd
  3. Nov 25, 2007 #2


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    I think you have 3 and 5 wrong.
    I must admit though, that I'm not sure what the answer to three should be. It is true that, when you apply the classical laws of physics (in particular, F = m a = m g), the mass drops out of the equation. So I guess the answer would be: "it has to do with weight". The surface area is only important when considering air resistance.

    In 5: as soon as a spring is not in equilibrium, it will exert a force opposite to the displacement from that equilibrium. Of course, if the spring is very light or your man is very strong, the person stretching the spring won't actually feel this force. But if your spring is very stiff, you will! Think about the springs in the suspension of a car: if they are compressed by a hump in the road, they exert an opposite force and you will definitely feel that!

    Can you also give us your reasoning for the other answers? Because for example in 2), one might expect the other answer?
  4. Nov 25, 2007 #3
    If you think about universal gravitation, the force between the boulder and the earth is bigger than the force between the pebble and the earth. Just from this you'd expect that the boulder would accelerate faster, but at the same time, it needs a larger force to be accelerated the same amount as the pebble would. In short, I think their masses help it accelerate faster, but inertia at the same time wants to slow it down so that in the end, everything accelerates at the same speed, g.

    I'm not sure about the difference between weight and inertia, but weight is the final result, not the cause. Unlike the idea of mass, energy, or inertia, weight depends on what the situation is (ex: in space, neither have weight but on earth, the boulder has more weight than the pebble), and I don't think the laws of physics should depend on where you are. It should be universal.
  5. Nov 26, 2007 #4


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    Anadyne, by that reasoning it might as well be "inertia" (i.e. mass), but one might also argue that the answer is "energy" since the final velocity can be calculated from the potential energy before falling, and also there the mass will cancel out.
    That's why I was having doubts about the formulation of 3).
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