WEIGHT W IS THE FORCE ACTING VERTICALLY DOWNWARDS, THROUGH THE CENTRE OF GRAVITY G. W HAS A TENDENCY...
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Transcript of WEIGHT W IS THE FORCE ACTING VERTICALLY DOWNWARDS, THROUGH THE CENTRE OF GRAVITY G. W HAS A TENDENCY...
WE I GHT W I S T HE FO RC E AC T I N G VE RT I C AL LY DO W NWARDS, T HRO UG H T HE C E N T RE O F
G RAVI T Y G. W HAS A T E N DE NC Y T O S I NK T HE B O DY.
WHI L E UPT HRUST F B O F L I Q UI D AC T I N G VE RT I C ALLY UPWAR DS, T HRO UG H T HE
C E N T RE O F B O U YANC Y B I .E . T HE C E N T RE O F G RAVI T Y O F T HE DI S PL AC E D L I Q U I D.
NO T E : B C O I NC I DE S W I T H G FO R A B O DY WI T H UN I FO R M C O M PO SI T I O N, I F T HE B O DY
I S C O M PLE T E LY SUB ME RG E D.B L I E S VE RT I C AL LY B E L O W T HE C E NT RE O F
G RAVI T Y, I F T HE B O DY I S FL O AT I NG WI T H I T S PART SU B M E RG E D.
FLOATATION AND RELATIVE DENSITY –
GRADE 9
W = volume of the body X density of the body X g
FB = volume of submerged part of the body X density of liquid X g
•THE BODY WILL SINK•W-F B WILL BE THE APPARENT WEIGHT
ACTING VERTICALLY DOWNWARDS
Case 1 W > FB (this is the case when the density of
solid > density of the liquid.
•THE BODY WILL JUST FLOAT BELOW THE SURFACE OF THE LIQUID
• APPARENT WEIGHT OF THE BODY WILL BE ZERO.
Case 2 W = FB (this is the case when the density of
body = density of the liquid.
•THE BODY WILL FLOAT PARTIALLY ABOVE AND PARTIALLY BELOW THE
SURFACE OF THE LIQUID•ONLY THAT MUCH PORTION GETS
SUBMERGED BY WHICH THE WEIGHT OF DISPLACED LIQUID
BECOMES EQUAL TO THE WEIGHT OF THE BODY.
•W ACTS AT THE CENTRE OF GRAVITY OF THE BODY, WHILE FB
ACTS AT THE CENTRE OF BUOYANCY B WHICH IS VERTICALLY BELOW G
•W= FB ONLY DUE TO THE SUBMERGED PART OF THE BODY.
Case 3 W < FB (this is the case when the density of
body < density of the liquid.
PRINCIPLE OF FLOATATION
The weight of a floating body is equal to the weight of the liquid displaced by its submerged part.
Thus the weight of the body is balanced by upthrust (weight of the liquid displaced by its submerged part) on it .
APPARENT WEIGHT = TRUE WEIGHT – UPTHRUST = 0
Thus a floating body appears to have no weight (or its apparent weight is zero)
W = V ρs gFB = v ρL gFor floatation W = FB
V ρs g = v ρL g
APPLICATIONS OF THE PRINCIPLE OF FLOATATION
Floatation of iron ship An iron nail sinks in water while a ship floatsA loaded ship is more submerged while an
unloaded ship is less submergedA ship begins to submerge more as it sails
from sea water to river waterPlimsoll line indicates the safe limit for
loading the ship in waterAn unloaded ship is filled with sand at its
bottom
Floatation of human body – It is easier for a man to swim in sea water
than in fresh water e.g. a man can easily swim in dead sea with very small portion of his body inside water and shoulders all the time above water.
Floatation of Submarines –Submarine is a fish shaped water-tight boat
provided with several ballast tanks. It can be made to dive into water or rise up to
the surface of water as and when desired.
Floatation of ice-bergThe density of ice (0.917 g/< density of water
1 g/)
Floatation of fishWhen the fish has to rise up in water, it
diffuses gas from its fluid into the bladder so its volume increases and its average density decreases.
When it has to come down, it empties its bladder to the required extent so its volume decreases and density increases
Rising of balloonsLighter gases like hydrogen or helium is filled
in a balloon.The weight of he air displaced by he inflated
balloon (upthrust) becomes more than the weight of the gas filled balloon and it rises up.
The balloon does not rise indefinitely.