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Homework Help: Pressure Problem

  1. Nov 2, 2005 #1
    Consider a helium-filled balloon floating around in a car. The driver suddenly stops the car. With respect to the car,
    *the driver is pushed backward, but the balloon moves forward
    *none of these
    *only the driver moves-and that is forward
    *the driver is pushed forward as is the air in the car, so the balloon moves backward
    *the driver, the air in the car, and the balloon are pushed backward

    For this one is ayd that the only thing that moves is the driver, and he moves forward. It didn't make sense for anything in the car to be pushed backward, since they all had a forward momentum, stopping quickly wouldnt cause anything to go backward right?

    I'm not as sure with what the balloon does, but it would make sense if it didn't move at all since it is lighter than air correct?

    A 154.9 kg polar bear standing 6 ft tall walks onto a floating sheet of ice 0.350 m thick. How big is the ice sheet if it sinks just below the surface while supporting the bear?

    For this problem i used the equation Fbuoy=density*g*Vdisplaced. I know what Fbuoy must be, 154.9kg*9.81m/s=1519.57N, because the ice just barely goes beneath the surface.

    Then i divided both sides by 9.81, giving me 154.9=density*Vdisplaced. (redundant i know)

    I'm just not sure what i value i should use for the density of ice. I looked it up on the internet and got that the density of ice = 917kg/m^3. Is that the value i should use?

    I'm kinda stuck at this point. If i use the density of ice = 917kg/m^3, i get a really small number for the volume displaced, which cant be right. i just need a prod in the right direction.

    Determine the blood pressure at the brain, taking its height above the heart to be 40 cm.

    This is a hydrostatic pressure problem right? so P=density*g*h. i looked up the density of blood, and it is 1060kg/m^3, and plugged in all the numbers giving me P=1060kg/m^3*9.81m/s^2*.4m= 4159.44Pa correct?
  2. jcsd
  3. Nov 2, 2005 #2

    James R

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    For the first question, the balloon moves backwards. The air moves to the front of the car, increasing the pressure at the front compared to the rear. The bouyant force on the balloon then pushes the balloon towards the back of the car, for the same reason that the difference in air pressure between the ground and a height makes a helium balloon rise.
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