Buoyancy & Flotation

Buoyancy is a force Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged. The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.

Volume and buoyancy The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. As you keep pushing downward on the ball, the buoyant force gets stronger and stronger. Which ball has more volume underwater? The one the right

Why is it easier to lift something that is submerged in water? The object has an apparent weight loss due to an upward force exerted on the object by the water. Why does buoyant force push upward when forces are exerted all around the object? Horizontal forces cancel out. Upward forces (against the bottom of the object) are greater than downward forces (against the top of the object) because the bottom of the object is deeper.

Flotation 2 forces acting on a ship. Gravity and buoyancy. How do Ships Float? https://www.youtube.com/watch?v=xniW3_afO-0

Float or Sink? When the weight of the submerged object… is greater than the buoyant force, the object will sink. When the weight of the submerged object is equal to the buoyant force, the object will float. When the weight of the submerged object is less than the buoyant force, the object will rise to the surface.

Archimedes’ Principal What happens to water level when something is placed in it? It rises In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object. The amount of displaced water is equal to the volume of the submerged object. A completely submerged object always displaces a volume of liquid equal to its own volume. https://www.youtube.com/watch?v=ijj58xD5fDI

Archimedes’ Principle A object weighs less in water than air. A 500 gram mass weighs about 5 N in air. Suppose the mass displaces 3 N of water when it is submerged. The buoyant force on the mass will also measure 3 N (pointing up). How much will the mass weigh in the water? The mass will weigh only 2 N in the water (5 N – 3N).

Flotation Flotation How can a ship be made of iron if a solid block of iron sinks? - A floating object displaces a weight of fluid equal to its own weight. The ship must be shaped to displace enough water to equal its own weight.

Weighted-base Stability Used for narrow-hull Ships. Weight in the bottom of the hull is used as “ballast” to stabilize the hull.

Leveraged Stability A wide hull is used for stability.

Center of gravity and buoyancy Center of gravity- The sum of the entire weight of a ship acting straight downward on the geometric center. Center of buoyancy- the center of the volume of water which the hull displaces.

Neutral Stability Ship will right itself as buoyancy shifts and pushes directly against the gravitational weight. Stiff ship – rights Itself very quickly. Tender ship – tend to be a little top heavy, which means that when it rolls one way it goes far that way, then rolls far the other way. It tends to rock back and forth.

Ship is in danger of capsizing because the weight on the right is increasing, while the buoyancy on the left is increasing as well, creating a tendency for the “heavy side” to push downward and the “more buoyant” side to push upward.

Plimsoll Lines Show the maximum depth to which a ship can be safely loaded in different zones and seasons.

Depth to which the hull is riding in the water. Draft Marks Show the Depth to which the hull is riding in the water.

Bill Nye - Buoyancy https://www.youtube.com/watch?v=ZvmChpDExxw

Vocab/key Words Buoyancy, net buoyant force, stability, gravitational force, center of gravity, buoyant force, center of buoyancy, draft marks, plimsoll marks, weight stability, leverage stability. You must define, give key words, sentence indicating you know what the word means and draw a picture for full credit. Quiz- next week