Buoyancy - Free Educational videos for Students in K-12

Buoyancy - By MITK12Videos

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00:07	we know that objects fall towards the earth because gravity
00:10	acts on them . But if gravity acts on everything
00:13	, why doesn't everything sink ? Imagine a submarine that
00:18	is stationary underwater . We know gravity is acting on
00:22	it , but the sub isn't sinking . So there
00:24	must be a force opposing gravity . We call this
00:28	force buoyancy . In this video we will examine buoyancy
00:33	and why some things float while others don't . 1st
00:37	. Let's review density . Before we move on to
00:39	buoyancy , different materials have different densities . It's a
00:44	measure of how much mass there isn't a given volume
00:47	and is determined by the following formula , density equals
00:50	mass , divided by volume . Density is an intrinsic
00:54	property , which means it depends on the material but
00:57	not the shape or size of the object . So
01:00	while a gold bar may weigh more than a gold
01:02	coin , they have the same density . And while
01:05	a kilogram of rocks and a kilogram of feathers have
01:08	the same weight , they have very different volumes and
01:10	therefore very different densities . Mhm . Generally rocks and
01:15	metals are more dense than water and sink , while
01:19	styrofoam and wood are less than than water and float
01:24	. So we see the density affects buoyancy , but
01:28	it can't possibly be everything . Many modern ships are
01:31	made of tons of metal that are more dense than
01:33	water , but the ship's still float . The boats
01:36	are floating because they are not just a solid block
01:39	of metal . Their hull is made of metal ,
01:41	but the inside is full of air and room for
01:43	people in cargo . The ship , with all the
01:46	space inside displaces a greater volume of water than if
01:49	it were squished into a block , in which case
01:51	it would actually sink . Therefore , we now know
01:55	that both the volume of the fluid displaced by the
01:57	object and the density of the fluid play a role
02:00	in defining buoyancy . With this in mind , let's
02:03	take a closer look at how things actually float Here
02:07	, we're placing three identical wooden blocks and three liquids
02:10	with different densities . The liquids are honey water and
02:16	rubbing alcohol . Notice how the more dense and liquid
02:19	is , the higher the block floats . Hence we
02:23	see the importance of the liquid density on the buoyancy
02:26	. Also note that the higher the blocks float ,
02:29	the less liquid volume is being displaced . This demonstrates
02:33	how to displace volume plays a part on buoyancy as
02:35	well . So , up to now we know that
02:38	buoyancy opposes gravity . Buoyancy depends on the density of
02:41	the fluid , and buoyancy depends on the submerged volume
02:44	of the floating object . Our committee is an ancient
02:48	greek scientist found . The buoyancy force is proportional to
02:52	the density of the fluid and the volume of the
02:53	fluid displaced by the object . If this buoyancy force
02:57	is greater than the gravitational force acting on the object
03:00	, it floats . The exact equation found for the
03:03	buoyancy force is given by F . B equals G
03:06	. Times G times B . Where F B .
03:09	Is the buoyancy force D . Is fluid density .
03:13	G is gravitational acceleration , envy is displaced volume .
03:18	Let's use our comedians equation to figure out how much
03:21	weight a cargo ship can carry . Our cargo ship
03:24	is 250 m long , 30 m wide and goes
03:28	10 m below the water surface . Multiplying these together
03:32	, we know the volume displaced by the cargo ship
03:35	is 75,000 m3 . Multiplying by the density of water
03:39	and gravitational acceleration , we get that the buoyancy force
03:42	acting on the ship is 735 million newtons , or
03:47	about £165 million . That means that the ship could
03:52	hold £165 million pounds of weight before it sinks .
03:56	That's the equivalent of 8000 elephants , or even 170
04:00	jumbo jets . We can also use buoyancy to explain
04:03	some interesting experiments that you can try at home .
04:08	For example , one of these eggs is fresh and
04:10	one was accident left out of the refrigerator . How
04:13	can we use buoyancy to figure out which is which
04:16	? Well , one floats and one sinks what's going
04:19	on ? This is a bit tricky . While X
04:22	shells look solid , they're actually porous . That means
04:26	that the shell is covered in small hole over time
04:29	. As the egg starts to rock , the eggs
04:32	liquid leaves through these holes and is replaced by air
04:36	. When this happens , the volume of the X
04:39	. Stays the same while the X mass is decreasing
04:42	. Therefore the density decreases . After enough time ,
04:47	the X density becomes lower than the water's density and
04:51	the egg starts to flow . Here's another experiment that
04:56	you can try at home . What happens when you
04:58	dropped raisins into soda ? Sometimes they're on the bottom
05:02	of the glass while other times throughout the top what
05:05	is going on soda has carbon dioxide gas , which
05:10	escapes as bubbles . Raisins are more dense in water
05:14	than initially sink . Since raisins have wrinkled skin .
05:17	The bubbles can get trapped on the surface as they
05:19	try to escape . These bubbles are buoyant in water
05:23	because they're low density and cause the raisins to float
05:27	after the raisins reached the surface , the bubbles pop
05:30	and the raisins sink again . This process repeats and
05:33	the raisins dance and thats buoyancy . Mhm .

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