Buoyant Force Buoyant force is the upward force that fluids exert on all matter A great example is how a rubber duck floats in a bath tub Buoyant Force is Caused by Differences in Fluid Pressure ID: 268607
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Slide1
Chapter 3.2M
Buoyant ForceSlide2
Buoyant force is the upward force that fluids exert on all matter.
A great example is how a rubber duck floats in a bath tub. Slide3
Buoyant Force is Caused by Differences in Fluid Pressure
Water will exert fluid pressure on all sides of an object. The pressure exerted on the sides of the object are equal and cancel each other out.
Because pressure increases with depth, the bottom pressure is greater than pressure at the top.
Because of this water exerts a net upward force on the object and this force is called buoyant force. Slide4
Determining Buoyant Force
Archimedes’ principle says that the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid that the object displaces (to take the place of).
That means if an object displaces 250 ml of water the weight of that water is about 2.5 N so the buoyant force on the object is about 2.5 N. Slide5
Weight Vs. Buoyant Force
An object will sink if it has a weight greater than the weight of the fluid that is displaced.
So an object will sink if it’s weight is larger than the buoyant force.
An object will float only if the buoyant force is equal to the object’s weight. Slide6
Sinking
Look at the bottom of pg. 69 – the rock on the bottom of the picture weights 75 N. It only displaces 5 L of water – that is 50 N. Because the rock weight more than the water it displaces it sinks. Slide7
Floating
The fish in the picture on pg. 69 weighs 12 N, it displaces water with a weight of 12 N so it will float.
The duck weighs 9 N and the buoyant force is 9 N. Slide8
Buoying Up
When the buoyant force on an object is larger than the objects weight the object is buoyed up or pushed up out of the water until weight of the water is equal to the weight of the object. Slide9
An Object Will Float or Sink Based on its Density
The rock in the example before sinks because it’s density is more than water. While the duck will float because it is less dense than water. Slide10
More Dense than Air
Most substances are more dense than air.
Few substances float in air.Slide11
Less Dense than Air
One substance less dense than air is helium.Slide12
The Mystery of Floating Steel
How do huge steel ships float?
The secret is the shape of the ship. If the steel were just a big block it would sink. Because they are built with a hollow shape that increases the volume of the ship and decreases the density. This makes the ship less dense than water and so it floats. Slide13
Density on the Move
Submarines can travel on the surface or under the water.
Submarines have special tanks that can be opened to allow sea water to flow in, increasing the density of the submarine and thus making it so the submarine will sink in the water.
Since a submarine can control it’s amount of water, it can control the density of the submarine and that will change the depth. Slide14
How is a Fish Like a Submarine?
Fish can also change their density. They have a swim bladder that can fill with gas in change the fishes density moving the fish up and down the water column. Sharks do not have a swim bladder so they must swim constantly to keep from sinking.