Experiment+Buoyancy

Today we are going to talk about fish...



But not just fish...but rather a term called buoyancy (the ability or tendency to float in water or air or some other fluid.)

What can you tell me about fish? Where do they live? How do they move around?

[|Fish]  are a bit like birds or flying insects: They have built-in­ mechanisms that allow them to move up and down and side to side in their environment.



But the methods that let fish do this are closer to the principles behind manmade flying machines than to natural flying methods. Most fish rise and sink in the water the same way a [|helium-filled balloon]  or a [|hot air balloon]  rises and sinks in the air.



To see how this works, you need to understand the various forces at work in the air and underwater. While these environments seem very different to us, water and air are actually very similar.

Both are **fluids **, substances with mass but no shape.



On Earth, an object immersed in a fluid (such as a fish or a person) experiences two major forces:
 * The downward pull of **gravity**
 * The upward push of **buoyancy**

<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Particles at the lower levels are pushed down by the weight of all the particles above them. The particles at the upper levels have less weight above them. <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;"> <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">Consequently, there is always greater pressure below an object than above it, so the fluid constantly pushes the object upward.



<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">To go up, a fish must reduce its overall density by increasing its volume without significantly increasing its mass. Most fish do this with something called a **<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">swim bladder **<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">. A swim bladder is just an expandable sac, like a <span style="background-color: #ffffff; color: #005288; font-family: arial,helvetica,clean,sans-serif; text-decoration: none;">[|human lung] <span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">. To reduce its overall density, a fish fills the bladder with oxygen collected from the surrounding water via the gills. When the bladder is filled with this oxygen gas, the fish has a greater volume, but its weight is not greatly increased. This allows the fish to float or sink.

<span style="background-color: #ffffff; color: #333333; font-family: arial,helvetica,clean,sans-serif;">To stay at a particular level, a fish fills its bladder to the point at which it displaces a volume of water that weighs what the fish weighs. In this case, the forces of buoyancy and gravity cancel each other out, and the fish stays at that level.



What other things should we test? A rock? A piece of wood?

Boat making

(look up boat plans online with class)