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Published byKerry Stevens Modified over 4 years ago

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Buoyancy What is buoyancy? The ability to float.

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**Which one is more likely to float?**

Buoyancy Which one is more likely to float? w w Roughly the same size h h This bowling ball Or this balloon? Your students probably already understand some of the basics…

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**Buoyancy Density = mass/volume**

Even though they are the same size (dimensions), the balloon will float because it is less dense than the fluid around it! Density = mass/volume

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Buoyancy “Any body wholly or partially immersed in a fluid experiences an upthrust equal to, but opposite in sense to, the weight of the fluid displaced.” THIS GUY “Any body wholly or partially immersed in a fluid experiences an upthrust equal to, but opposite in sense to, the weight of the fluid displaced.” According to legend… King Hiero II gave a goldsmith an amount of pure gold to construct a crown. After the crown was delivered the king became suspicious that the craftsman had stolen some of the gold and mixed the rest with silver. He asked Archimedes to determine if this was true without melting the crown. While taking a bath, Archimedes noticed that the level of the water in the tub rose as he got in, and realized that this effect could be used to determine the volume of the crown. The submerged crown would displace an amount of water equal to its own volume. By dividing the mass of the crown by the volume of water displaced, the density of the crown could be obtained. This density would be lower than that of gold if cheaper and less dense metals had been added. Archimedes then took to the streets naked, so excited by his discovery that he had forgotten to dress, crying "Eureka!" (meaning "I have found it!"). The test was conducted successfully, proving that silver had indeed been mixed in. Was Archimedes buoyant?!

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**Archimedes looked at forces**

Buoyancy Archimedes looked at forces Buoyancy: upward force on an object Gravity: downward force on an object Pressure: force all around an object Gravity The ability to float. Pressure Pressure Buoyancy

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**Archimedes principle simplified**

Buoyancy Archimedes principle simplified Downward force equal to the weight of the object A body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces Upward force (buoyant force) equal to the weight of the displaced fluid

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**Types of Buoyancy Buoyancy Positively Buoyant Neutrally Buoyant**

Negatively Buoyant

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**Buoyancy Remember these?**

Density = mass/volume When the density of the object equals the density of the water around it, the object is neutrally buoyant

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**Play with these concepts Archimedes loves neutrally buoyant ROVs**

Buoyancy Play with these concepts Archimedes loves neutrally buoyant ROVs

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**So how do we make our ROVs neutrally buoyant?**

Buoyancy and Balance So how do we make our ROVs neutrally buoyant? Once you drill holes in your frame, your ROV will be negatively buoyant

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Buoyancy Ballast Dynamic (soft): system that changes the amount of water displace inside the ballast chamber. (Think submarines) Static: ballast chamber does not change. ROV is designed to be neutrally buoyant and motors move it up and down. The volume of air stays the same.

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**What are some pros and cons to different materials?**

Buoyancy What types of materials could you use to make your ROV neutrally buoyant? What are some pros and cons to different materials? AND LOTS MORE!

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**Think about how you place buoyant materials**

Buoyancy and Balance What about balance? Build an ROV Interactive: Knowing the center of mass for a car, for example, is enough to estimate whether it can be tipped over by maneuvers on level ground. The center of mass of a boat must be low enough for the boat to be stable. Any propulsive force on a space craft must be directed towards the center of mass in order to not induce rotations. Tracking the trajectory of the center of mass of an exploding plane can determine whether or not it was hit by a massive object. Any rotating piece of machinery must have its center of mass on the axis of rotation if it is not to cause much vibration. Think about how you place buoyant materials

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**Buoyancy – going further**

*NOTE* Totally optional If your students are grasping the concepts of buoyancy – you can push them further by introducing some basic calculations.

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**Buoyancy – going further PVC Pipe specifications**

Diameter (cm) 3.340 4.216 4.826 6.033 Weight in Air (g/cm) 2.532 6.357 7.174 10.000 Volume per cm (cc) 8.762 13.960 18.292 28.581 Net Buoyancy (g/cm) [weight in air per cm – Volume per cm] -6.230 -7.603 Essentially, this figure gives us the “float” value of each cm of PVC Calculations provided by the MATE center

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**Buoyancy – going further**

Then, weigh your ROV! Fnet (apparent weight or wet weight) of ROV = ____________g

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**Buoyancy – going further**

If your ROV weighs 645 g, then you will need -645g of positive, buoyant force! How many cm of 2’’ PVC pipe would you need? ~ cm *You’d probably also want to add in the weight of the 2’’ PVC endcaps to the weight of your ROV before you did the calculation.

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**Buoyancy and Balance Buoyancy**

Let’s make these items positively, neutrally, and negatively buoyant and balanced!

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