BUOYANT
FORCE
INTRODUCTION
Buoyancy is the ability to float. Buoyancy forces are
formed by a principle called Archimedes principle. Archimedes principle said
that buoyancy forces is directly proportional with drowned volume.
Mathematically, Buoyancy force is: F = ρ . g . V, where ρ is the density of fluid (water), g is gravity, and V
is volume which is drowned.
Any object that is completely or partially submerged in a fluid is
buoyed up by a force equal to the weight of the fluid displaced by the body.
Everyone has experienced Archimedes’s principle. As an example of a common
experience, recall that it is relatively easy to lift someone if the person is
in a swimming pool whereas lifting that same individual on dry land is much harder.
Evidently, water provides partial support to any object placed in it. The
upward force that the fluid exerts on an object submerged in it is called the
buoyant force.
According to the Archimedes’s principle, the magnitude of the buoyant
force always equal to the weight of the fluid displaced by the object. The
buoyant force acts vertically upward through what was the centre of gravity of
the displaced fluid.
F = W
Where
F is the buoyant force and W is the weight of the displaced fluid. The units of
the buoyant force and the weight are Newton (N).
The buoyant force acting on the
steel is the same as the buoyant force acting on a cube of fluid of the same
dimensions. This result applies for a submerged object of any shape, size, or
density.
Figure 1: Direction of Buoyant Force
The
boat can float on water based on this principle. Boat has hull to get buoyancy
force and makes the boat float. So, it is very important to keep hull safe. In
the sea, corals are sometimes found in the sea. Hull can causes the boat
drowned since hull is the source of buoyancy force.
ENGAGE
Figure 2 : Example of application in Buoyant Force
- What is buoyant force?
- How buoyant force determine whether an object sinks or floats on water?
- Is there any different if the boat is floating on fresh water and salt water.
- What factors that influence buoyant force?
- What principle related to buoyant force?
EMPOWER
Planning and doing an experiment:
Title :
The effect of mass of different objects on the buoyant force.
Objective :
a)
Use a Force Sensor to measure the
weights of objects in and out of water
b)
Determine the weight of water displaced by each of the objects
c)
Determine the relationship of depth of the immersed object to the
buoyant force.
Hypothesis :
The magnitude of the buoyant force is directly proportional to the
weight of the fluid that the object displaces.
Procedures :
PART 1 : COMPUTER SETUP
1. Connect the Science Workshop
interface to the computer, turn on the interface, and turn on the computer.
2.
Connect the DIN plug of the Force Sensor to Analog Channel A.
3.
Open the document titled as shown:
DataStudio
|
ScienceWorkshop (Mac)
|
ScienceWorkshop (Win)
|
P13 Buoyant Force.DS
|
P18 Buoyant Force
|
P18_BUOY.SWS
|
·
The DataStudio document has a
Workbook display.
·
The ScienceWorkshop document
has a Graph display with Force versus Depth.
·
Data recording is set for 1 Hz. Keyboard Sampling allows the user to
enter the submerged depth in meters.
PART 2 : SENSOR CALIBRATION AND EQUIPMENT
SETUP
1. Mount the Force Sensor on a horizontal rod
with the hook end down.
2. Using the calipers, measure the diameter of
the aluminium cylinder. From the diameter, calculate the radius and the
cross-section area. Record the cross-section area in the Data Table in the Lab
Report section. Recall:
3. Hang the aluminium cylinder from the Force
Sensor hook with a string.
4. Put about 800 mL of water into the beaker and
place the beaker on the lab jack below the hanging cylinder. The bottom of the
cylinder should be touching the water.
5. Position the metric ruler next to the edge of
the lab jack. Note the initial height of the top of the lab jack.
PART 3
: DATA RECORDING
1.
With the cylinder attached to the Force Sensor hook, press the tare
button on the Force Sensor to zero the sensor.
2.
Record Force vs. Depth data as you submerge the cylinder.
In DataStudio,
move the Table display so you can see it clearly.
• Click
on the ‘Start’ button to start recording data. The ‘Start’ button changes to a
‘Keep’.
• Immerse
the cylinder 1 millimeters (1 mm or 0.001 m) by raising the beaker of water 1
mm with the lab jack. Use the metric ruler to measure the distance that you
raise the lab jack.
• Click
the Keep button to record the next Force value at the depth of 0.001 m.
• Increase
the depth of submersion by increments of 1 mm. After each increase in the
submersion, wait for the force reading in the display to stabilize, then click
the Keep button to record a Force value at the appropriate depth.
• Repeat
the data recording procedure until the top of the cylinder is submerged. Stop
data recording by clicking on the ‘Stop’ button. Run #1 will appear in the
Summary window.
In ScienceWorkshop,
click the ‘REC’ button to begin collecting data.
The ‘Keyboard Sampling’ window will open.
Move it so you can also see the Digits display. The default value for ‘Entry
#1’ is 10.000.
- Because the cylinder is not submerged, type in ‘0’ as the depth. Click ‘Enter’ to record the depth and force values. The entered depth value will appear in the Data list.
• Immerse
the cylinder 1 millimeters (1 mm or 0.001 m) by raising the beaker of water 1
mm with the lab jack. Use the metric ruler to measure the distance that you
raise the lab jack.
• For
‘Entry #2’, type in ‘0.001’ (1 millimeters). Click ‘Enter’ to record the depth
and force values.
• Increase
the depth of submersion by increments of 1 mm. After each increase in the
submersion, wait for the force reading in the Digits display to stabilize, then
click the Enter button to record a Force value at the appropriate depth.
• Repeat
the data recording procedure until the top of the cylinder is submerged. Stop
data recording by clicking the ‘Stop Sampling’ button in the ‘Keyboard
Sampling’ window.
• The
‘Keyboard Sampling’ window will disappear. ‘Run #1’ will appear in the Data
List in the Experiment Setup window.
PART 4 : REPEATITION OF THE PROCEDURE USING
DIFFERENT OBJECTS
•
Repeat the procedure in part 2 for step 2 with the brass and copper.
Results
:
Aluminium
|
Brass
|
Copper
|
|
Actual
mass of sample
(g) |
26.16
|
111.39
|
102.65
|
Diameter
of sample (g)
|
1.88
|
1.88
|
1.88
|
Sample
height (cm)
|
3.45
|
4.29
|
4.49
|
Density
(ρ) H20 (g/cm3)
|
1.00
|
1.00
|
1.00
|
Apparent
mass in H20 (g)
|
15.25
|
89.26
|
97.30
|
Calculation
:
Aluminium
|
Brass
|
Copper
|
|
Actual weight of sample
(cm/s2)
|
25654.07
|
100664.75
|
109235.72
|
Density of sample (g/cm3)
|
2.78
|
8.78
|
9.13
|
Area of sample (cm2)
|
2.78
|
2.78
|
2.78
|
Volume of cylinder (cm3)
|
9.58
|
11.91
|
12.46
|
Displaced liquid volume= Fb
|
9.43
|
11.70
|
12.20
|
Discussion :
In this experiment, we study about the
relationship water displaced and buoyancy force. Archimedes principle says that
the buoyant force on a submerged object is equal to the weight of the fluid it
displaces. Thus, in short, buoyancy = weight of displaced fluid. This principle
is useful for determining the volume and therefore the density of an irregularly shaped object by measuring
its mass in air and its effective mass when submerged
in water (density = 1 gram per cubic centimetre). This effective mass under
water will be its actual mass minus the mass of the fluid displaced. The
difference between the real and effective mass therefore gives the mass of
water displaced and allows the calculation of the volume of the irregularly shaped
object. The mass divided by the volume thus determined gives a measure of the
average density of the object. Buoyancy shows that the buoyant force on a volume of
water and a submerged object of the same volume is the same. Since it exactly
supports the volume of water, it follows that the buoyant force on any
submerged object is equal to the weight of the water displaced.
Based on the result of the experiment, we can
see that as the mass of the object increase the volume of the fluid displaced
also increase. This means that the
buoyant force is also increase since the formula for the buoyancy is equal to
the weight of displaced fluid.
For the force against depth graph, we can see
that force is directly proportional to the depth. As the depth of the immersed object increase,
the magnitude of the buoyant force is also increase.
Questions:
1.
Why was the Force
Sensor zeroed after the cylinder was attached to the hook?
The force sensor measures the net force
that is the cylinder’s weight (downward force) minus the buoyant force (upward
force). By taring the force sensor when
the cylinder was attached and out of water, the weight was accounted for during
calibration and the sensor will now report only the buoyant (upward) force.
2.
What is the effects of mass of sample to the buoyant force?
The mass of sample will affect the magnitude
of the buoyant force since formula of the density is the mass over volume. The formula for calculating the buoyant force
is:
3.
In that experiment, what the objects give the lowest and highest buoyant
force?
The object that gives the lowest of buoyant
force is the aluminium whereas the object that gives the highest of buoyant
force is the copper. This results depend
on the density of that objects.
Conclusion:
As conclusion, an object that floats
displaces the amount of water that has the same weight as the object. If it
sinks, it displaces an amount of water that has less weight than the object.
ENHANCE
The
application of buoyant force play an
important roles in our daily life. Discuss the important of buoyancy
control in diving?
Answer:
Controlling buoyancy is a key component of your
diving safety. The physics of floating and sinking are simple concepts, yet
achieving practical control of your buoyancy when outfitted with scuba
equipment and immersed in water is an entirely other matter. Each change in
equipment affects your buoyancy. As your dive equipment grows more complex, the
more attention your buoyancy requires. Given its role as a fundamental element
of dive safety, it's no wonder problems with buoyancy control are often the
underlying cause of a dive injury or fatality.
Divers with proper buoyancy control can maintain their position with very little effort. They can descend or ascend at will. In contrast, divers with poor buoyancy-control skills struggle throughout the dive. In extreme situations, major buoyancy-control issues may cause divers to make grave errors such as descending well beyond their planned depth, negatively affecting gas consumption and no-decompression calculations, or on the flip side, uncontrolled ascents, increasing the risk of decompression illness. There is no doubt buoyancy control affects many aspects of dive safety. Experts in dive training, dive medicine and research all know just how integral it is and are always eager to share thoughts on how to develop and maintain good skills.
Divers with proper buoyancy control can maintain their position with very little effort. They can descend or ascend at will. In contrast, divers with poor buoyancy-control skills struggle throughout the dive. In extreme situations, major buoyancy-control issues may cause divers to make grave errors such as descending well beyond their planned depth, negatively affecting gas consumption and no-decompression calculations, or on the flip side, uncontrolled ascents, increasing the risk of decompression illness. There is no doubt buoyancy control affects many aspects of dive safety. Experts in dive training, dive medicine and research all know just how integral it is and are always eager to share thoughts on how to develop and maintain good skills.
Training
Good buoyancy begins with proper weighting. It is
imperative the amount of weight you use allows you to descend,
not causes you to do so. Weight placement makes a difference, too. A classic
buoyancy-control device (BCD) is generally configured to require a separate
weight belt, whereas newer BCDs often integrate the weights. Each approach
affects a diver's body position in the water, requiring time and attention to
get comfortable. Using rental gear can complicate the process, especially for
new divers, as each change in configuration, responsiveness and other variables
can alter a diver's comfort and buoyancy. Diving with a dry suit, a weight
harness or a rebreather adds to the complexity.
The BCD is the most complex piece
of scuba equipment a diver must master. To truly master buoyancy control, a
diver must understand his BCD inside and out, including knowing how it reacts
to the addition or venting of air. It requires proper maintenance (see
"Gear," Alert Diver, Spring 2011) to prevent sticking
buttons or leaking bladders. Malfunctioning BCDs can lead to uncontrolled
ascents or descents before a diver even realizes what's happening. Like any
piece of equipment, proper function requires proper maintenance. But lack of
maintenance is not the only concern; operator error can also cause loss of
control. Improperly connecting a low pressure inflator can cause negative
buoyancy without a means to correct it. Hitting the inflator button instead of
the vent button can cause a rapid ascent. Every diver needs to be familiar with
his own equipment as well as his buddy's. In a stressful or emergency situation
there may not be time to search for weight releases or inflator/deflator
valves.
Dive Medicine
Many do not equate buoyancy skills with dive
medicine, but there is definitely a connection. The most common dive injury is
consistently middle-ear barotraumas. There are certainly many factors that lead
to this injury, but buoyancy issues are often among them. Every diver is taught
that if discomfort is felt during descent to stop the descent, ascend a few
feet or until the discomfort resolves, and then attempt to equalize again. This
is very difficult to execute without good buoyancy control. When experiencing a
reverse block during ascent, a diver should stop the ascent, descend until the
discomfort resolves and attempt ascent again using appropriate equalization manoeuvres.
The ability to stabilize and adjust position in the water column certainly
takes practice, but as a cornerstone skill, it's worth the effort.
Most marine life injuries are due to incidental contact. Proper buoyancy helps divers avoid contact as it maintains necessary distance from marine life. It also prevents the destruction of the reef and the microscopic critters that live on sub aquatic surfaces, as buoyancy control reduces the need to place hands on those surfaces to steady a diver's position. Buoyancy skills not only protect divers but the environment as well.
Finally, one of the most serious consequences of
inadequate buoyancy control is a rapid ascent. This can place a diver at risk
for a lung overexpansion injury (pulmonary barotraumas), and it also increases
the risk of a potentially fatal arterial gas embolism (AGE). The easiest way to
avoid both these injuries is to learn the best method of prevention good
buoyancy.Most marine life injuries are due to incidental contact. Proper buoyancy helps divers avoid contact as it maintains necessary distance from marine life. It also prevents the destruction of the reef and the microscopic critters that live on sub aquatic surfaces, as buoyancy control reduces the need to place hands on those surfaces to steady a diver's position. Buoyancy skills not only protect divers but the environment as well.
EXTENSION
The
application of buoyancy can be applied
to staying afloat on the water. In the early 1800s, a young Missippi
River flat-boat operator submitted a patent application describing a device for
“buoying vessels over shoals”. The invention proposed to prevent a problem he
had often witnessed on the river-boats ground on sandbars-by equipping the
boats with adjustable buoyant air chambers. The young man even whittles a model
of his invention, but he was not destined for fame as an inventor; instead
Abraham Lincoln (1809-1865) was famous for much else. In fact Lincoln had a sound idea with his proposal to use buoyant
force in protecting boats from running aground.
Buoyancy on the surface of water has
a number of easily noticeable effects in the real world. (Having established
the definition of fluid, from this point onward, the fluids discussed will be
primarily those most commonly experienced: water and air). It is due to
buoyancy that fish, human swimmers, icebergs, and ships stay afloat. Fish offer
an interesting application of volume change as a means of altering buoyancy: a
fish has an interesting swim bladder, which is filled with gas. When it needs
to rise or descend, it changes the volume in its swim bladder, which then changes
its density. The examples of swimmers and icebergs directly illustrate the
principle of density- on the part of the water in the first instance, and on
the part of the object itself in the second.
To a swimmer, the difference between
swimming in fresh water and salt water shows that buoyant force depends as much
on the density of the fluids as on the volume displaced. Fresh water has a
density of 62.4lb/ft3 (9,925N/m3), whereas that of salt
water is 64lb/ft3 (10,167N/m3). For this reason, salt
water provides more buoyant force than fresh water; in Israel’s Dead Sea, the
saltiest body of water on Earth, bathers experience an enormous amount of
buoyant force.
Water is an unusual substance in a number of regards, not
least its behaviour as it freezes. Close to the freezing point, water thicken
up, but once it turns to ice, it becomes less dense. This is why ice cubes and
icebergs float. However, their low density in comparison to the water around
them means that only part of an icebergs stay atop the surface. The submerged
percentage of an iceberg is the same as the ratio of the density of ice to that
of water: 89%.
UNIQUE
FEATURE OF THIS EXPERIMENT
·
Buoyancy is defined as
the tendency of a fluid to exert a supporting upward force on a body placed in
the fluid.
·
Buoyant force must
equal to the weight of the displaced fluid.
·
A solid object would
float if the density of the solid object were less than the density of the
fluid and vice versa.
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