PRESSURE IN FLUIDS : PASCAL'S LAW
A substance which can flow is called a fluid. All liquids and gases are thus fluids. We know that a solid exerts pressure on a surface due to its weight. Similarly, a fluid exerts pressure on the container in which it is contained due to its weight. However, unlike a solid, a fluid exerts pressure in all directions. A fluid contained in vessel and in all
directions.
a vessel exerts
pressure at all points of the This fact can easily be demonstrated in a very
simple way by taking a hollow rubber ball and by making a number of fine pin
holes at different positions on it as shown. The ball is filled with water by
making one slightly wider hole and pouring water in it. The ball is then held
in hand with a finger closing the wider hole and the ball is squeezed. Water is
seen to rush out of the fine pin holes in the form of fine streams. If you
place your finger of the other hand on any one of the holes, you will feel a
thrust due to liquid. This demonstrates that the liquid contained in the vessel
exerts a thrust at all points below its free surface. The thrust on a unit area
at a point gives the pressure due to the liquid. It is further observed that
all the streams of water reach almost the same distance in the air. This shows that
the increase of pressure produced on squeezing the ball has been transmitted to
every part of water contained in the ball and is responsible for pushing out
the fine water jets. This observation is summed up by Pascal's law which states
that:
|
In an enclosed
fluid, if pressure is changed in any part of the fluid, then this change of
pressure is transmitted undiminished to all the other parts of the fluid. 4.7. BUOYANCY In order to
understand buoyancy, let us perform the following simple experiments. 1. (a) Take an
empty sealed can and place it in a tub of water. We find that the can floats. (b) Push the can
into water. When you do so, you feel an upward force acting on the can. This
upward force on the can goes on increasing as you push the can deeper into
water till the whole can is immersed in water. Once completely inside the
water, a fixed force is needed to keep the can in position. (c) Release the can
when it is completely in water. You will notice that it bounces that the
force of gravity pulls it downwards. Obviously, water exerts an upward force
it upwards and this upward force is greater than the force of gravity acting
on the can. back in spite of
the fact on the can which pushes can, i.e., the weight of the 2. (a) Take a cork
and place it on water surface. We find that it floats on water with nearly
(2/3)rd of its volume inside water. (b) If you push the
cork into water, it returns to water surface and floats again. In case, the
cork is to be kept immersed in water with our fingers, we feel an upward
force acting on the fingers. From the above
experiments, we conclude that an upward force acts on a body which is
immersed in a liquid. This force is called the buoyant force or upthrust and
the property due to which a liquid exerts upthrust is called buoyancy. Thus, When a body is
partially or wholly immersed in a liquid, an upward force acts on it which is
called upthrust or buoyant force. The property of the liquids responsible for
this force is called buoyancy. NOTE All objects,
including ourselves, are subject to a buoyant force due to surrounding air.
But we do not feel this force as it is negligibly small as compared to our
weight. This is due to the reason that the density of air is very small. But
on the other hand, a balloon filled with hydrogen gas (or any other gas which
is lighter than air), rises up as the upthrust on it due to surrounding air
is more than the weight of the balloon and hydrogen gas filled in it. Let Us See Some Examples of balanced and unbalanced
forces 1. Let us consider a
wooden block placed on a horizontal table. A string X is attached to the
right face of the block. When we apply a small force P to the string X,
the block does not move. This is because two pairs of balanced forces are
acting on the block : (i) weight mg of the block acting vertically
downwards on the table. (ii) force of reaction R of the horizontal
table acting on the block vertically upwards. As Ra=mg, therefore, this is one pair of
balanced forces, which fails to move the block in the vertical direction. The
other pair of forces is : (i)
applied force P tending to move the
block to the right, (ii)
force of friction F between the rough table
top and rough bottom surface of the block, acting to the left and opposing
the motion. As F =
P, therefore, this is the second pair of balanced forces which fails to move
the block in the horizontal direction. If we go on increasing the applied force to
the string X, a stage reaches when the block begins to move on the table. At
this stage, the applied force P becomes greater than the maximum value of
opposing force of friction F. Thus the second pair of forces becomes
unbalanced. That is why motion is produced in the block. 2. In a tug of war, i.e., rope-pulling by two
teams, the rope does not move in any direction, because the forces applied by
the two teams are equal and opposite, i.e., the forces applied are balanced
forces. However, when one team releases the rope, an unbalanced force acts on
the other team due to which the other team falls backwards. So, That’s It For today Guys I we
Upload Next Blog As Soon As Possible. Thank You!! |
0 Comments