Chapter 13 Section 3. Buoyancy

Objectives Explain the effect of buoyancy on the apparent weight of an object. Explain the relationship between the volume of fluid displaced by an object and buoyant force acting on the object according to Archimedes. Describe the relationship among density , fluid density, and whether and object sinks or floats in a fluid. Describe the relationship among object weight, buoyant force, and whether an object sinks or floats in a fluid.

Vocabulary Buoyancy Buoyant force Archimedes principle

Buoyancy Lab Please share your calculated buoyancies… The winner will be awarded a prize. Recap: Did you finish the questions? What did you learn about buoyant force? We’ll go through this lecture and when finished maybe a few questions will have been answered.

Buoyant Force Think about the observations you have made when in a swimming pool. Is it easier or more difficult to lift stuff up? Easier, right? What you were experiencing is buoyancy.

Buoyant Force What is buoyancy? It is the ability of a fluid to exert an upward force on an object placed in it. What is observed is an apparent loss of weight of an object in that fluid. & EVERY object in a fluid experiences it…to a varying degree. It is dependent upon several factors.

Buoyant Force Which direction does buoyant force act? This should be obvious. But how can you explain this? You must remember the principles of pressures in a fluid. That they grow with depth, dependent upon the type of fluid, & at any given depth they are equally spread out in all directions. Therefore, the forces increase as you go down so the force acting from the bottom will be greater…the buoyant force. Consider the illustrations…

Buoyant Force

PRESSURE & FORCES INCREASE Buoyant Force PRESSURE & FORCES INCREASE

Buoyant Force These forces are equal and opposite and are contributed by the fluid pressure. FORCES INCREASE

Buoyant Force FORCES INCREASE For an object in a fluid, the force opposing the buoyant force is weight. THIS FORCE IS LARGER: THIS IS THE BUOYANT FORCE

Buoyant Force So why does this object sink? Weight FORCES INCREASE So why does this object sink? It has to have an unbalanced net force acting on it.

Archimedes Principle This leads into Archimedes principle which allows us to determine the buoyant force. It states that the buoyant force is equal to the weight of the fluid that is displaced by the object. So if we put an object into a full glass of water, the weight of the water spilled out would = the buoyant force. But this only explains the buoyant force acting on the object. It doesn’t explain why some objects float and why others sink…completely.

Density & Buoyancy Why an object floats can be dealing with density of the object compared to the density of the fluid it’s placed in. Copy down this chart: An object will… Float Sink When the object’s density is less than the density of the fluid it’s in. When the object’s density is greater than the density of the fluid. Water has a density of 1 g/cm3. According to this fact, would the density of ice be higher or lower than water?

What is Density Who remembers the equation for density? What is density a measure of? How can density play a role in buoyancy when considering how the molecules are packed together?

Weight and the Buoyant Force

Weight and the Buoyant Force

Weight and the Buoyant Force

Density and Buoyancy: What do you know about the densities of each of these objects? Compared to water, the object’s density is… A: Lower B: Equal C: Higher

Using Density to Predict Whether Something Will Float or Sink.. Water has a density of 1g/cm3 Based upon this, which of these objects will sink, which will float? 1. Sink 2. Float 3. Float 4. Sink 5. Sink 6. Float

Density & Buoyancy This accounts for why oil floats in water (oil is less dense than water). But you can also determine if an object floats by analyzing the net forces acting on it. The two forces acting on an object in fluid is 1) the buoyant force and 2) its weight (mass x gravity). They work at the same time and in opposite directions.

Weight & Buoyancy An object will… Float (not sink) Sink You calculate the buoyant force by measuring the weight of the fluid displaced when the object is submerged. This can be done with devices like the spring scale. You then compare this to the weight of the object &… (copy down) An object will… Float (not sink) Sink When the buoyant force is greater than or = to the weight of the submerged object. When the buoyant force is smaller than the object’s submerged weight.

Volume Displacement Ending Volume Starting Volume The weight of the displaced water exactly equals the buoyant force acting on the ball…even if it sinks.

Weight and Buoyancy: Compare the net force acting on each of these objects. The buoyant force is… A: Equal to or slightly larger than the weight B: Equal to the weight C: Smaller than the weight

Do you have to calculate buoyant forces and weights to determine whether something will float or not? Who thinks yes? The fact of the matter is no you don’t, all the time. You can see this with your own eyes. If you were planning on building a ship you would really want to know this though. It always equals the weight of the displaced fluid. As you saw with the boats in your lab, the larger the volume of the boat, the more weight it could have in it…therefore the greater amount of buoyant force.

What Were the Common Boat Designs? Canoes Boxes Shallow Deep Rafts (circle) What was the most buoyant boat? The one that displaced the greatest amount of water! The one with the most volume!

So Why Are Freighter Ships So Wide?

And Density Can Be Manipulated To Overcome Changing Depths.

Foil Ship Lab… What was the point of the lab yesterday? We were trying to find the weight in which the ship would change from being buoyant, like in A, to sinking, like in B. As long as the boat floated the weight always equaled the buoyant force. This point at which the weight of your boat exceeded the buoyant force is what the maximum buoyant force for your ship was. So, how do you calculate the buoyant force?

Buoyancy: Summary Finish up your labs tonight (or with the remainder of the period). These are due tomorrow. Any questions? I want everyone to take a few minutes to write 3 major things that they learned today. Write these as a journal entry after your notes.

Foil Lab Recap When Objects Float When objects sink Buoyant Force = Weight When objects sink Buoyant Force < Weight Weight Buoyant force Start 0.03N Float 0.1N 0.5N 1.0N 1.5N 2.0N 2.5N Sink 2.6N Weight Maximum Buoyant Force Buoyant Force

Video on Buoyancy Dr Dad…ugh, bare with me here…