Density and Buoyancy

Density Which would have more mass? It depends on BOTH the size of the object AND the material contained inside.

Density Density is the amount of mass per unit volume of a material. Matter is made of atoms which are particles that have mass. Density depends on mass AND the number of particles packed into a given volume.

Density

Density The volume of air has fewer particles, thus less mass than the same volume of water. Thus, the density of air is less than the density of water. <

D = m V Density D – density, the unit is g / cm3 (solids) or g / mL (liquids) m – mass, the unit is g V – volume, the unit is cm3 (solids) or mL (liquids)

Density Density depends on the material an object is made from, NOT its size. An entire chocolate bar will have the same density as just one piece. It’s all made from the same material – chocolate.

Density Density depends on the mass of the particles. (more mass = more density) Density depends on the distance between particles. (more distance = less density) Gases are usually less dense than solids or liquids because particles are far apart.

Density

Density

Density To make something less dense, you can decrease the mass or increase the volume of an object. A solid steel ball sinks in water, but that same mass of steel made into a curved hull of a boat will float – the shape of the hull causes it to displace a greater volume of water than a solid ball.

Measuring Density Measure its mass. Measure its volume. Divide its mass by its volume.

Density is a Physical Property A physical property is something you can measure without changing the composition of the material. When you measure an object’s mass and volume, you don’t change it into something else.

Pressure in a Fluid A fluid is a material that can flow and has no definite shape. Liquids and gases are fluids. Fluids can exert forces.

Pressure in a Fluid Pressure is the force per unit of area applied on the surface of an object.

Pressure in a Fluid Pressure depends on the area over which a force is exerted. If the area increases, pressure decreases.

Pressure in a Fluid Pressure depends on the size of the force. If the force increases, pressure increases.

Pressure in a Fluid The equation for pressure is: P = F ÷ A Pressure = force (newtons) area (meters squared)

Pressure in a Fluid The SI unit of pressure is the pascal, abbreviated Pa. A pressure of 1 Pa is equal to a force of 1 N applied over an area of 1 m2 1 Pa = 1 N / m2

Pressure in a Fluid For any liquid in a container of any shape, the pressure depends only on the height of the column of liquid above the surface where you measure the pressure.

Pressure in a Fluid The deeper underwater you go, the greater the pressure is on you. This is because as you go deeper, the column of water above you gets taller.

Pressure in a Fluid On April 10, 1963, the USS Thresher nuclear-powered attack submarine was lost at sea during deep-diving tests. All 129 aboard died. (8400 ft / 2600 m below sea level).

Pressure in a Fluid In a fluid, pressure is exerted on all points on the outside of an object in a direction perpendicular to the surface of the object at that point.

Pressure in a Fluid A gas, like any fluid, exerts a pressure on an object. That pressure depends on the height of the gas above the object.

Pressure in a Fluid Atmospheric pressure is the force exerted per unit area by air particles. As you climb, or increase your elevation, atmospheric pressure decreases. mb = millibar (100 Pa)

Pressure in a Fluid Atmospheric pressure at sea level is caused by a large weight of gas, but is difficult to notice because there is an equal, internal pressure pushing out from the inside of your body.

Buoyant Force The buoyant force is the net upward force exerted on an object in a fluid by the surrounding fluid. It acts against gravity. Buoyant force is the DIFFERENCE between the pressure above and the pressure below.

Buoyant Force In a fluid, pressure increases with depth. The force of the fluid pushing up on the bottom of an object suspended in water is always greater than the force of the fluid pushing down on the top of the same object.

Buoyant Force The size of the buoyant force does NOT depend on depth.

Buoyant Force The buoyant force does NOT change as an object goes deeper because it’s a DIFFERENCE between the pressure above and the pressure below. Both those increase so the buoyant force stays the same.

Buoyant Force 5 N 10 N 10 N 15 N

Buoyant Force At the fish goes deeper, the force on its back increases. However, the force on its belly also increases to match and keep the fish suspended in the water. The net buoyant force is always the same, no matter what the depth.

Buoyant Force Archimedes’ Principle states that the buoyant force on an object is equal to the weight of the fluid the object displaces. This is why objects feel lighter in water!

Why do objects sink or float?

Why do objects sink or float? An object will float if the density of the object is LESS than the density of the fluid. Fluids exert pressure on any object in the fluid. Gravity and the buoyant force act on the object in opposite directions.

Why do objects sink or float? A submerged object displaces a volume of fluid equal to its own volume. A floating object displaces a volume of fluid equal to the volume of the portion underwater.

Why do objects sink or float? If the upward buoyant force is less than the object’s weight, the net force on the object is downward. The object accelerates downward because of the unbalanced force.

Why do objects sink or float? Archimedes’ Principle… Archimedes’ (287 B.C.E. to 212 B.C.E.) observed that an object immersed in water sinks or floats depending on the WEIGHT OF THE WATER it displaces.

Why do objects sink or float? Negative Buoyancy – (sinking) when the object weighs MORE than the weight of the water displaced

Why do objects sink or float? Positive Buoyancy – (floating) when the object weighs LESS than the weight of the water displaced.

Why do objects sink or float? Neutral Buoyancy – (hovering wherever you put an object) when the objects weighs the SAME as the weight of the water

The Buoyant Force & Density An object floats if it is LESS dense than the fluid around it.

The Buoyant Force & Density Almost all metals have a density greater than water (1 g/mL), so they sink.

The Buoyant Force & Density BUT, the density of a metal ship is LESS than water because a large volume of the ship is filled with air.

Floating & Sinking in the Atmosphere Air is a fluid and it exerts pressure on things around it. An object will float in air only if the buoyant force from air pressure on an object is large enough.

Floating & Sinking in the Atmosphere Helium gas is less dense than nitrogen gas and oxygen gas, which are the main components of air. When a balloon is filled with helium, it is less dense than the air around it, and so the balloon floats.

Floating & Sinking in the Atmosphere Whether a hot-air balloon rises or falls can be controlled by changing the density of the air inside the balloon.

Floating & Sinking in the Atmosphere A burner heats the air in the hot-air balloon. Heat makes the air molecules move farther apart from each other. The hot air inside becomes less dense than the air outside the balloon, and it floats.

Floating & Sinking in the Atmosphere To bring the balloon back down, cool the air. The air particles inside come closer together, and the balloon’s total density is more than the air outside, so it sinks.

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REVIEW 1. How does the density of an object floating on a fluid compare to the density of the fluid? A. The floating object is less dense. B. The floating object is equally dense. C. The floating object is denser. D. The density cannot be compared.

REVIEW 2. The force keeping an object floating on a fluid is called A. Viscosity B. Buoyant force C. Natural force D. Surface tension

REVIEW 3. Pressure is the _____ per unit area. A. mass B. volume C. force D. pascal

REVIEW 4. Why does a hot air balloon float? A. The surrounding air is denser than the balloon. B. The surrounding air is less dense than the balloon. C. The balloon is denser than the surrounding air. D. The balloon has less mass than the surrounding air.

REVIEW 5. An object that is _____ than water will _______. A. less dense; sink B. denser; float C. less dense; lose mass D. denser; sink