Page 77 - Physics Form 5 KSSM_Neat

Buoyant force is the force acting upwards on an object immersed in a liquid when there CHAPTER 2
is pressure difference between the lower surface and upper surface of the object. The formula
for buoyant force can be derived as follows: Pressure

Info GALLERY
Fluids consist of liquids and gases.
KEMENTERIAN PENDIDIKAN MALAYSIA
Archimedes' principle is usually
Force, F applied to liquids because liquids
1
h have a higher density than gases.
1
Surface However, Archimedes' principle
h area, A
2 should be applied to gases in
conditions where the magnitude
h
of the buoyant force cannot be
neglected compared to the weight
of the object. An example of this is
the motion of hot air balloons.

Force, F
2

Figure 2.28 Cylinder fully submerged in a liquid

Pressure on the top surface, P = h ρg Info
1 1
Force acting on the top surface, F = P A Height of cylinder, h = (h – h )
2
1
1
1
= h ρgA Volume of cylinder, V = Ah
1 V = A(h – h ),
1
2
Pressure on the bottom surface, P = h ρg The volume of cylinder is the same
2 2
Force acting on the bottom surface, F = P A as the volume of water displaced.
2 2
= h ρgA
2
Resultant force, F (upwards) = F – F 1 Info
2
= h ρgA – h ρgA Since ρ = m , the mass of water
2
1
V
= ρA (h – h ) g displaced is m = ρV.
1
2
= ρAhg
= ρVg Weight of water displaced,
W = mg
This resultant force is the buoyant force, F . W = ρVg
B
Archimedes' principle states that an object which is SCAN ME
SCAN ME
partially or fully immersed in a fluid will experience a EduwebTV:
buoyant force equal to the weight of fluid displaced. Archimedes’
principle
Buoyant force = Weight of fluid displaced http://bit.ly/2t4fBXY
F = ρVg
B

LS 2.5.1 67