Styrofoam, balloons, and
ships are some examples of objects that float while rocks, heavy steels, and
bricks are some examples of objects that sink. The question is: How come a big
object like a ship float while a small object like a brick sinks inside the
water? First of all let’s see and understand the meaning of float and sink. Float
could be defined “to remain suspended within or on the surface of a fluid
without sinking”, and according to the dictionary sink could be defined “to go
down below the surface of something especially of a liquid; become submerged.”
Buoyancy is the upward
force of an object submerged into fluid. As an object is pushed under water, it
displaces a volume of liquid equivalent to it’s own volume. The weight of fluid
displaced in this case is called Buoyant Force.
The formula of Buoyant Force is as the following:
Fa = ρ
x V x g
In which here,
·
Fa = Buoyant force of the object, in
Newton
·
ρ = Density, in kg m-3
·
V = Volume of the object, in m3
·
g = Acceleration
due to gravity, in m/s2
Here is an example of a problem related with
buoyant force:
Question:
A floating object displaces 0.6 m3
of water. Calculate the buoyant force on the object and the weight of the
object.
Answer:
According to Archimedes’ principle the
buoyant force is equal to the weight of the displaced fluid. We know the volume
of fluid displaced, so we can calculate the mass of the displaced fluid.
m = d V
m = (1000 kg / m3) (0.6 m3)
m = 600 kg
The weight of is calculated as the product of
mass times the acceleration due to gravity as
wt = m × g
wt
= (600 kg) (9.8 m/s2)
wt
= 5880 N
The buoyant force is equal to the weight of the displaced fluid,
so the buoyant force is 5880 N. Because the problem stated that the object was
floating, the buoyant force must be equal to the weight of the object.
Therefore the weight of the object is also 5880 N.
In physics,
density is defined as the degree of consistency measured by the quantity of mass per
unit volume. The formula of density is as the following:
Here are some examples of problems related
with density:
Question:
1) A piece
of glass has a mass 10 kg and has a volume 2 m³. Calculate density of the glass!
2) The density of air is 1.3 kg/m³.
What is the mass of 5 m³?
Answer:
1) p = m/V
p = 10/2
p = 5 kg/m3
m = 5 × 1.3
m = 6.5 kg
When there is a floating
object, or what they say is a
positively-buoyant object, it means that the buoyant
force is greater than the density of the object because it is able to push the
object upwards until it remains on top and float.
While when there is a negatively-buoyant object, or in
other words an object that sinks, it means that the buoyang force is less than
the dinsity of the object because it is not able to push the object upwards,
thus letting he object sink.
In conclusion, buoyant
force plays a great role on whether an object will float or sink. When the object’s density
is more than the buoyant force, then the object will float, while when the
object’s density is less than the buoyant force, the object will sink. For instance,
we have two objects with the same mass but different density, then the object
that has a greater density will float and the object with less density will
sink.
Resources:
