{"id":68,"date":"2024-09-03T16:52:04","date_gmt":"2024-09-03T16:52:04","guid":{"rendered":"https:\/\/courses.asiascubainstructors.com\/?post_type=lessons&p=68"},"modified":"2024-10-18T06:06:17","modified_gmt":"2024-10-18T06:06:17","slug":"physics-3-buoyancy","status":"publish","type":"lessons","link":"https:\/\/courses.asiascubainstructors.com\/de\/lessons\/physics-3-buoyancy\/","title":{"rendered":"Restricted content"},"content":{"rendered":"

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Physics<\/h1>\n

 <\/p>\n

3. Buoyancy<\/h2>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row use_custom_gutter=”on” gutter_width=”1″ make_equal=”on” _builder_version=”4.22.0″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”0px||||false|false” locked=”off” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.22.0″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” background_color=”#FFFFFF” custom_padding=”5%|5%|5%|5%|true|false” hover_enabled=”0″ locked=”off” global_colors_info=”{}” theme_builder_area=”post_content” sticky_enabled=”0″]<\/p>\n

| <\/span>
<\/strong><\/span><\/p>\n

Background story: Archimedes’ Principle<\/strong><\/span><\/p>\n

In ancient Syracuse, King Hiero II thought a goldsmith had cheated him by mixing silver into a crown that was supposed to be pure gold. He asked the clever mathematician Archimedes to find out if the crown was really pure gold without damaging it.<\/p>\n

Archimedes was confused. He knew he could weigh the crown but could not tell the volume without damaging it.\u00a0 One day, while stepping into a bath that was filled up to the rim, he noticed the water spilling over the edge. Suddenly, he realized that the water spilling over the edge must have the same volume as his foot.<\/p>\n

Excited, Archimedes jumped out of the bath and ran through the streets shouting, \u201cEureka! Eureka!\u201d\u2014\u201cI\u2019ve found it!\u201d\u2014forgetting he was completely naked! Unfortunately for the goldsmith, Archimedes discovered that the crown wasn\u2019t pure gold. The king was furious and punished the goldsmith for cheating him.<\/p>\n

Archimedes conducted more experiments with floating objects and noticed the weight of the water spilling over the edge was always equal to the weight of the floating object. And so he came up with Archimedes’ Principle which states that an object submerged in a fluid is buoyed up by a force equal to the weight of the fluid it displaces.<\/p>\n

 <\/p>\n

\"archimedes\"<\/p>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row make_equal=”on” _builder_version=”4.22.0″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”||||false|false” use_custom_width=”on” width_unit=”off” custom_width_percent=”100%” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.22.0″ _module_preset=”default” custom_padding=”||||false|false” custom_padding_phone=”|5%||5%” custom_padding_last_edited=”off|desktop” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” custom_margin=”10%|||||” custom_padding=”|||” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

Sinking or floating<\/h3>\n

An object that floats is positively<\/em> buoyant.
An object that sinks is negatively<\/em> buoyant.
An object that neither sinks nor floats is neutrally<\/em> buoyant.<\/p>\n

If an object is neutrally<\/em> buoyant in salt water it will sink in fresh water.
If an object is neutrally<\/em> buoyant in fresh water it will float in salt water.
If an object is positively or negatively<\/em> buoyant in either fresh or salt water, you cannot determine exactly what will happen when placed from fresh to salt or vice versa unless you know exactly how much positive or negative buoyancy the object has.<\/p>\n

[\/et_pb_text][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” custom_margin=”5%||” custom_padding=”|||” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

Calculating buoyancy<\/h3>\n

When you push an object under water, the water pushes back with a force that is equal to the weight of the water that is displaced. 1 Liter of fresh water weighs 1 kilo. So, an object with a volume of 5 liters is pushed up with 5 kilos. The calculation is very simple because kilos and liters is the same number.<\/em><\/p>\n

In seawater, the calculations are also easy but kilos and liters are not the same number. <\/em>1 Liter of seawater weighs 1.03 kilo.
5 liters in a liftbag gives it 5 x 1.03 = 5.15 kilos of lift.<\/p>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row column_structure=”1_3,2_3″ make_equal=”on” _builder_version=”4.22.0″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”||||false|false” use_custom_width=”on” width_unit=”off” custom_width_percent=”100%” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”1_3″ _builder_version=”4.16″ _module_preset=”default” custom_padding=”|||0px” custom_padding_tablet=”” custom_padding_phone=”|||0px” custom_padding_last_edited=”on|desktop” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_image src=”https:\/\/courses.asiascubainstructors.com\/wp-content\/uploads\/2024\/09\/PADI-IDC-theory-buoyancy-schema.jpg” force_fullwidth=”on” align_tablet=”center” align_phone=”” align_last_edited=”on|desktop” _builder_version=”4.16″ _module_preset=”default” animation_style=”fade” animation_delay=”500ms” global_colors_info=”{}” theme_builder_area=”post_content”]   [\/et_pb_image][\/et_pb_column][et_pb_column type=”2_3″ _builder_version=”4.22.0″ _module_preset=”default” custom_padding=”||||false|false” custom_padding_phone=”10%|||” custom_padding_last_edited=”off|desktop” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” custom_padding=”||5%|” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

When an object is neutrally buoyant, the force pushing the object down (the weight of the object) is equal to the force pushing it up (the force of the water).
It can help to draw a little scheme to visualise this.<\/p>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row use_custom_gutter=”on” gutter_width=”1″ make_equal=”on” _builder_version=”4.16″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”50px|0px|0px|0px|false|false” use_custom_width=”on” width_unit=”off” custom_width_percent=”100%” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.16″ _module_preset=”default” custom_padding=”|5%||5%” custom_padding_tablet=”” custom_padding_phone=”|5%||5%” custom_padding_last_edited=”on|phone” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” background_color=”#ffffff” custom_margin=”50px||” custom_padding=”5%|5%|5%|5%” global_colors_info=”{}” theme_builder_area=”post_content”]Example:<\/strong>
\nAn anker weighs 220 kg. It displaces 100 liters of water. What is the minimum volume needed to lift this anker?<\/p>\n

Fresh water: <\/em>
\n220 – 100 = 120 liter<\/em><\/p>\n

Seawater: <\/em>
\n220 \u00f7 1.03 – 100 = 113.6 liter<\/em><\/p>\n

\nExplanation<\/summary>\n

Fresh water: The anker weighs 220 kg so to make it neutrally buoyant we need to displace 220 liters of fresh water. It already displaces 100 liters so we need 120 liters more.<\/p>\n

Seawater: The anker weighs 220 kg so to make it neutrally buoyant we need to displace 220\u00f7 1.03 = 213.6 liters of seawater. It already displaces 100 liters so we need 113.6 liters more.<\/p>\n<\/details>\n

<\/strong><\/p>\n

<\/strong><\/p>\n

Example:<\/strong>
\nMy camera and housing weighs 4.4 kg. How much weight do I need, to make it neutrally buoyant if the housing displaces 5.2 liter?<\/p>\n

Fresh water: <\/em>
\n5.2 – 4.4 = 0.8 kg<\/em><\/p>\n

Seawater: <\/em>
\n5.2 x 1.03 – 4.4 = <\/em>
\n5.4 – 4.4 = 1 kg<\/em><\/p>\n

\nExplanation<\/summary>\n

Fresh water: 5.2 liters = 5.2 kg pushing up. The weight is 4.4 kg so we need to add 0.8 kg more to make it neutral.<\/p>\n

Seawater: 5.2 liters = 5.2 x 1.03 = 5.4 kg pushing up. The weight is 4.4 kg so we need to add 1 kg more to make it neutral.<\/p>\n<\/details>\n

[\/et_pb_text][et_pb_text _builder_version=”4.22.0″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

If the question is asking for liters it is easiest to convert everything into liters. If the questions is asking for kg it is easiest to convert everything into kg.<\/p>\n

[\/et_pb_text][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” background_color=”#ffffff” custom_margin=”50px||” custom_padding=”5%|5%|5%|5%” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

Example:<\/strong>
A diver in full scuba gear weighs 85 kg. He displaces 90 liters of seawater. How many weights does he need to carry to become neutrally buoyant?
Convert to kg:<\/p>\n

Force down:<\/em>
85 kg + ??\u00a0<\/em><\/p>\n

Force up:<\/em>
90 x 1.03 kg = 92.7 kg<\/em><\/p>\n

85 kg + ?? = 92.7 kg<\/em>
92.7 kg – 85 kg = 7.7 kg<\/em><\/p>\n

\nExplanation<\/summary>\n

The water pushed the diver up with 90 x 1.03 = 92.7 kg. He weighs 85 kg so he needs 92.7 – 85 = 7.7 kg extra to become neutral.<\/p>\n<\/details>\n

 <\/p>\n

\"\"<\/a><\/p>\n

 <\/p>\n

Example:<\/strong>
A diver in full scuba gear weighs 95 kg. He displaces 90 liters of seawater. How much air does he need in his BCD to become neutrally buoyant?<\/p>\n

Convert to liters:<\/p>\n

Force down<\/em>
95 \u00f7 1.03 = 92.2 liters\u00a0 (92.2 liters of seawater weighs 95 kg !)<\/em><\/p>\n

Force up:<\/em>
90 liters + ??<\/em><\/p>\n

90 liters + ?? = 92.2 liters<\/em>
?? = 2.2 liters<\/em><\/p>\n

<\/em><\/p>\n

\nExplanation<\/summary>\n

The diver weighs 95 kg so he needs to be lifted up with 95 kg force to become neutral. For this he needs to displace 95 \u00f7 1.03 = 92.2 liters of seawater. He already displaces 90 liters of seawater with his body and equipment so he needs 2.2 liters more.<\/p>\n<\/details>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row make_equal=”on” _builder_version=”4.22.0″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”||||false|false” use_custom_width=”on” width_unit=”off” custom_width_percent=”100%” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.22.0″ _module_preset=”default” custom_padding=”||||false|false” custom_padding_phone=”|5%||5%” custom_padding_last_edited=”off|desktop” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” custom_margin=”5%||” custom_padding=”|||” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n

Buoyancy and pressure combined<\/h3>\n

Some questions say that an object is at a specific depth.
Does the depth matter? No.
If an object weighs 10 kilos at 12 meters, it weighs 10 kilos at 45 meters.
You still need the same volume to lift it.<\/p>\n

However, the volume in your liftbag will increase when it moves to the surface (because the pressure is less).
When asked \u201cWhat is the volume of the liftbag, upon reaching the surface\u201d or \u201cHow much air from the surface needs to be pumped in the liftbag\u201d then depth does matter.<\/p>\n

[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row make_equal=”on” _builder_version=”4.16″ _module_preset=”default” background_size=”initial” background_position=”top_left” background_repeat=”repeat” max_width=”100%” module_alignment=”center” custom_padding=”||||false|false” use_custom_width=”on” width_unit=”off” custom_width_percent=”100%” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.16″ _module_preset=”default” custom_padding=”|5%||5%” custom_padding_tablet=”” custom_padding_phone=”|5%||5%” custom_padding_last_edited=”on|phone” global_colors_info=”{}” custom_padding__hover=”|||” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”8b1f8231-1210-4c64-bae4-d177be5c655c” background_color=”#ffffff” custom_margin=”50px||” custom_padding=”5%|5%|5%|5%” global_colors_info=”{}” theme_builder_area=”post_content”]Example:<\/strong>
\nYou use a liftbag to lift an object that weighs 10 kg and displaces 2 liters of seawater.
\nThe object lies at 30 meter depth.
\nWhat will be the air volume in the liftbag upon reaching the surface?<\/p>\n

To lift 10 kg we need to displace<\/em>
\n 10 \u00f7 1.03 = 9.7 liter<\/em><\/p>\n

The object displaces 2 liter, so we need 7.7 liters more.<\/em>
\n At 30 meters the pressure is 4 Bar so the volume on the surface will be:<\/em><\/p>\n

4 x 7.7 liters = 30.8 liter<\/em><\/p>\n

<\/em><\/p>\n

Example:<\/strong>
\nAn object that weighs 12 kg and displaces 2 liters of water lies at 10 meter depth in seawater.
\nHow much air needs to be pumped from the surface into a liftbag to lift it?<\/p>\n

The object weighs 12 kg so we need to displace 12 \u00f7 1.03 = 11.65 liters of seawater to lift it. It already displaces 2 liter of seawater, so we need 9.65 liters in the liftbag.<\/em><\/p>\n

At 10 meters the pressure is 2 bar so the volume we need to pump down is:<\/em>
\n9.65 x 2 = 19.3 liter<\/em>[\/et_pb_text][\/et_pb_column][\/et_pb_row][et_pb_row _builder_version=”4.22.0″ _module_preset=”default” locked=”off” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.22.0″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_text _builder_version=”4.22.0″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”]<\/p>\n