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ICSE-IX-Physics

04: Pressure in Fluids and Atmospheric Pressure

with Solutions -

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    Section : A
  • Qstn #1
    Define the term thrust. State its S.I. unit.
    Ans : Thrust is the force acting normally on a surface.
    Its S.I. unit is ‘newton’.
  • Qstn #2
    What is meant by pressure? State its S.I. unit.
    Ans : Pressure is the thrust per unit area of the surface.
    Its S.I. unit is ‘newton per meter2‘ or ‘pascal’.
  • Qstn #3
    (a) What physical quantity is measured in bar?
    (b) How is the unit bar related to the S.I. unit pascal?
    Ans : (a) Pressure is measured in ‘bar’.
    (b) 1 bar = 105 pascal.
  • Qstn #4
    Define one pascal (Pa), the S.I. unit of pressure.
    Ans : One pascal is the pressure exerted on a surface of area 1 m2 by a force of 1N acting normally on it.
  • Qstn #5
    State whether thrust is a scalar or vector?
    Ans : Thrust is a vector quantity.
  • Qstn #6
    State whether pressure is a scalar or vector?
    Ans : Pressure is a scalar quantity.
  • Qstn #7
    Differentiate between thrust and pressure.
    Ans : Thrust is the force applied on a surface in a perpendicular direction and it is a vector quantity. The effect of thrust per unit area is pressure, and it is a scalar quantity.
  • Qstn #8
    How does the pressure exerted by thrust depend on the area of surface on which it acts? Explain with a suitable example.
    Ans : Pressure exerted by thrust is inversely proportional to area of surface on which it acts. Thus, larger the area on which the thrust acts, lesser is the pressure exerted by it.
    Example: If we stand on loose sand, our feet sink into the sand, but if we lie on that sand, our body does not sink into the sand. In both the cases, the thrust exerted on the sand is equal (equal to the weight of the body). However, when we lie on sand, the thrust acts on a large area and when we stand, the same thrust acts on a small area.
  • Qstn #9
    Why is the tip of an allpin made sharp?
    Ans : The tip of an allpin is made sharp so that large pressure is exerted at the sharp end and it can be driven into with less effort.
  • Qstn #10
    Explain the following statements:
    (a) It is easier to cut with a sharp knife than with a blunt one.
    (b) Sleepers are laid below the rails.
    Ans : (a) It is easier to cut with a sharp knife because even a small thrust causes great pressure at the edges and cutting can be done with less effort.
    (b) Wide wooden sleepers are placed below the railway tracks so that the pressure exerted by the rails on the ground becomes less.
  • Qstn #11
    What is a fluid?
    Ans : A substance which can flow is called a fluid.
  • Qstn #12
    What do you mean by the term fluid pressure?
    Ans : Due to its weight, a fluid exerts pressure in all directions; the pressure exerted by the fluid is called fluid pressure.
  • Qstn #13
    How does the pressure exerted by a solid and fluid differ?
    Ans : A solid exerts pressure only on the surface on which it is placed, i.e. at its bottom, but a fluid exerts pressure at all points in all directions.
  • Qstn #14
    Describe a simple experiment to demonstrate that a liquid enclosed in a vessel exerts pressure in all directions.
    Ans : Take a can or large plastic bottle filled with water. Place it on a horizontal surface. Make a series of holes in the wall of the vessel anywhere below the free surface of the liquid. The water spurts out through each hole. This shows that the liquid exerts pressure at each point on the wall of the bottle.

    Liquid exerts pressure at all points in all directions
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