Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Need Help

  1. May 6, 2005 #1
    Here are a couple questions I could use help on, please help.

    1. What force must be applied to a small cylinder piston with a diameter of 0.32 m to lift a car weighting 12,000 N that is supported by the piston in a large cylinder that has a diameter of 1.4 m? Assume that there is an enclosed chamber in common between the two pistons and that the chamber is filled with hydraulic oil.m?

    2. What is the height in cm of the column of mercury for a mercury barometer when the atmospheric pressure at a certain location is 1 atm? The denisty of mercury is 13.55g/cm^3.

    3. A sperical-shaped hollow steel shell, with a radius of 2.5m, containing some instruments, is lowered by a cable into a lake. The shell with itss contents weighs 7.2 X 10^5 N on the dect of the ship from which it is being lowered. What is the tension in the cable when the sphere is completely immersed in the water?

    4.During level airplaine flight at a constant velocity, the pressure created by air flowing over the curved upper surface of the wing compared to the pressure created by air flowing along the flat underside of the wing is?

    6. Which phase (State) of matter has a fixed volume and takes the shape of its container?

    a. plasma
    b. gas
    c. liquid
    d. solid

    If someone could please explain and answer these for me, i would be forever in their debt. :zzz: :zzz:

    -------------------------------------------------------------------- |
    Why be you, when you could be me! |
    ____________________________________________________________ |
  2. jcsd
  3. May 6, 2005 #2


    User Avatar
    Gold Member

    This is the only one that I can address, and I don't know if this is the answer you want. The pressure differential between the top and bottom of an airfoil in level flight is equal to the weight of the aircraft.
  4. May 6, 2005 #3
    Should these questions belong to homework forums?

    1. Check the pressure exerted by the car on the 1.4m dia piston. This is the same pressure acting upon the 0.32m piston also. So force to be applied can easily be calculated.(627N)

    2. You must be joking.

    3. Check the volume of water displaced. Check its weight. Deduct it from sphere weight. This is the net force acting on the cable.

    4. Check Bernoulli's theorem?

    6. You must be joking again.

    100. Google is a great resource. Try your hands on Physics by Resnick and Haliday
  5. May 6, 2005 #4

    This is for a buddy of mine, he is having a bit of trouble in his physics class.. he is the one doing all the work, he would like some answers to those, so that he can figure out how they were reached. If anyone else knows any more answers please don't hesitate to answer :)

    :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye: :bugeye:
  6. May 7, 2005 #5

    Did I say Physics by Resnick and Haliday? You should read it as Fluid Mechanics by Steiter.

    1. This solution is based on Pascal's law which states that pressure exerted by a fluid in a closed container is same in all directions. Thus a 12000N force creates a pressure of 12000/(3.142x1.4x1.4)/4 N/sq.m pressure on a 1.4 m radius piston. This same pressure acts upon the 0.32m piston also. So force to be applied on 0.32m piston is [(3.142x0.32x0.32)/4]x[12000/(3.142*1.4*1.4/4). Or simply the forces are inversely proportional to the square of the piston diameter.

    2. Rise in mercury column at atmospheric pressure is 76cm. Just use the forumla P = rhoxgxh, where rho is the density of the material(more precisely specific weight), g is acceleration due to gravity and h is the liquid column height.

    3. You should refer Archimedes Principle to solve this problem. When a body submerges in water, it looses its weight equal to that of the water displaced. So, the spherical steel shell displaces water to a volume of 4x3.142x2.5^3/3. Density of water is 1kg/cu.m. So total weight of water displaced is (4x3.142x2.5^3/3)x9.81 which gives you a value in N. Deduct this value from weight of shell on the deck and you will get the tension in the rope.

    4. If the pressure on the lower side of the wing is lower than that of the upperside, the plane can't float in the air. Danger had it right.

    6. I still feel you are joking.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook