Whether something floats or sinks depends on its , not its .

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Key concepts of floating and sinking
Students’ alternative conceptions of floating and sinking
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Updated April 25, 2017

By Michael Merry

Whether an object sinks or floats depends on the density of the object and the fluid in which it is immersed. An object that is denser than a fluid will sink in the fluid while an object that is less dense will float. A floating object is said to be buoyant. The classical Greek inventor Archimedes was first to understand that buoyancy is a force and stated so in an important principle that bears his name. Archimedes' Principle states that any object immersed in or floating in a fluid is buoyed up by a force equal to the weight of displaced fluid.

Consider an iron ball of volume 1 cc (cm-cubed) immersed in water. Find the densities of iron and water from tables in a chemistry handbook or textbook.

Note that the density of iron (7.87 g per cm-cubed) is much higher than the density of water (1 g per cm-cubed.)

Determine the buoyant force acting on the iron ball by multiplying the density of water by the displaced volume of water: 1 gram / cm-cubed x 1 cm-cubed = 1 g. The iron ball weighs 7.87 g, which is greater than the buoyant force, and therefore the iron ball sinks.

••• Matt Cardy/Getty Images News/Getty Images

Consider a balloon that contains 10,000 cubic feet (ft-cubed) of helium gas. Near the Earth's surface and at a temperature of 68 degrees Fahrenheit, the density of helium is about 0.02 pounds (lbs) per foot cubed (ft-cubed), and the density of air is about 0.08 lbs per ft-cubed.

Calculate the weight of displaced air, as follows: 10,000 ft-cubed x 0.08 lbs /ft-cubed = 800 lbs. Calculate the weight of helium in the balloon: 10,000 ft-cubed x 0.02 lbs / ft-cubed = 200 lbs.

Note, according to Archimedes' principle, that the air exerts a buoyant force of 800 lbs on the balloon. Because the helium in the balloon weighs only 200 lbs, the balloon will rise up if the total weight of balloon and equipment is less than the difference between the weight of air and the weight of helium, which is 600 lbs. As the balloon rises, the weight of displaced air decreases due to decreasing air density. The balloon will stop rising when its weight is balanced by the buoyant force of the air.

Floating and sinking is a common activity in early years classrooms. Students’ ideas about floating and sinking are intriguing. The strategies for developing their understandings discussed in this topic are examples of the probing, investigative and challenging activities that characterise effective science teaching and learning.

Key concepts of floating and sinking

The activities in this topic are designed to explore the following key concepts:

Early years

Whether something floats depends on the material it is made of, not its weight.
Objects float if they are light for their size and sink if they are heavy for their size.
An object can be light for its size if it contains air, such as a hollow ball.
Materials with a boat shape will float because they effectively contain air.
Water pushes up on objects with an upthrust force.

Middle years

Objects float if the upthrust force from the water can balance their weight (gravity force).
Objects float depending on their density compared to water; for an object to float its density needs to be less than that of water.
Objects float when air is enclosed in an object; their density is lowered, thereby increasing the likelihood of floating.
The upthrust depends on the amount of water displaced.
Objects float better in salt water (density of salt water is greater than that of pure water).
Water surfaces have a cohesive force (surface tension) that makes them act like a ‘skin’.
Small, dense objects (e.g. a pin; a water spider) can ‘float’ on the surface of water without breaking it, due to surface tension effects.

Scientific terms associated with floating and sinking

force: a push or a pull.
density: amount of mass per unit mass of an object (i.e. the concentration of mass, or how ‘heavy for its size’ an object is). The density of water is 1 kg per litre.
pressure: amount of force applied per unit area. At a given pressure, twice the area will experience twice the force.
Archimedes’ principle: A floating object will experience an upthrust force from water, equal to the weight of water displaced (pushed aside). It will sink into the water until it reaches the point where the weight of the water pushed aside equals its own weight. For an object that is floating, the mass of the material equals the mass of water that is displaced by the object (1 kg = 1 L of water). Dense objects cannot displace enough water to provide an upthrust force to counterbalance their weight, so they plummet below the surface. Objects made of material denser than water (e.g. a boat made of iron) can still float if they contain air so that the mean density is less than that of water.

Students’ alternative conceptions of floating and sinking

Research into student’s ideas about this topic has identified a number of non-scientific conceptions.

Students will have views about at least three aspects of floating and sinking that differ from science views. These alternative views centre around the questions:

What do we mean by ‘floating’?
What determines whether something will float or sink?
What causes things to float (i.e. what are the forces involved in floating)?

Interviews reveal that students can attach different meanings to the term ‘floating’ and that these meanings vary depending on the context (such as observing real objects in water as opposed to viewing line drawings). The students still seem to be at the formative level with respect to this idea and there are likely to be students in most classrooms whose understanding of ‘floating’ differs from scientists. Some students could become confused if teachers do not recognise this.

Students have a range of views about why some things float while others sink. Younger students (7-10 years) often do not realise that there could be a single explanation. Their response is to give explanations for individual materials. The explanations offered could be described as partial explanations. They focus on specific aspects such as lightness or heaviness and fail to take into account other aspects (such as size) needed to formulate a general rule that would explain all cases. Very few students seem to have an understanding of flotation that approximates that of scientists. Others realise that they do not really know why things float or sink, but they appear interested to know.

A number of students think that the length of floating material, or the depth of water underneath or on top of an object, affects flotation level. Some further believe that floating material will sink if the part above the water is cut off, or if it has vertical holes put through it. After initial experiences with reshaped nonfloating material, almost all students realise that non-floating material can be shaped to float.

View the full topic, including the activities (.pdf)

Demonstrate how the distribution of molecules in a substance determines its density.
Investigate the relative densities of liquids and the relative densities of solids.
Predict, test and explain relative density by investigating the interactions of liquids and solids.
Demonstrate understanding of the relationship between density and buoyancy by building a boat.

Materials

see individual activities for materials.

Background

Density, Mass & Volume

Simply put, density is how tightly “stuff” is packed into a defined space.

For example, a suitcase jam-packed with clothes and souvenirs has a high density, while the same suitcase containing two pairs of underwear has low density. Size-wise, both suitcases look the same, but their density depends on the relationship between their mass and volume.
Mass is the amount of matter in an object.
Volume is the amount of space that an object takes up in three dimensions.

Density is calculated using the following equation: Density = mass/volume or D = m/v.

Let’s compare three familiar substances to explore the concept of density. If we take the same volume (one cubic centimetre) of foam, wood and concrete, we can see that each has a different mass.

Less Dense, More Dense

If something is heavy for its size, it has a high density. If an object is light for its size it has a low density.

A pebble is heavy for its size, compared to a piece of popcorn which is light for it’s size. Imagine a big bowl of popcorn, compared to a big bowl of pebbles, which would feel heavier?

It is easy to estimate relative densities if you keep either the volume or the mass of two objects the same.

If you filled one bag with a kg of feathers and another with a kg of lead you would see that the feathers take up much more room, even though both bags have the same mass. This because feathers are less dense, they have less mass per volume. If you made a copper cube and an aluminum cube of the same volume and placed one in each hand, you would be able to feel that the copper cube would be heavier. Copper has more mass per volume than aluminum.

How can one substance have more mass per volume than another? There are a few possibilities:

Atoms of one substance might be a similar size yet have more mass than the atoms of another substance.
Atoms of one substance might be a similar mass but be smaller, so more of them fit within the same volume.
Atoms of one substance might be arranged in a way that allows more of them to fit in the same volume.

Any one or a combination of these explanations could be the reason why one substance has a higher density than another. In the case of copper and aluminum, their atoms are arranged similarly, but copper atoms are smaller and have more mass than aluminum atoms, giving it a higher density.

Density, Sinking and Floating
Why do some things float, while others sink? You might expect heavier objects to sink and lighter ones to float, but sometimes the opposite is true. The relative densities of an object and the liquid it is placed in determine whether that object will sink or float. An object that has a higher density than the liquid it’s in will sink. An object that has a lower density than the liquid it’s in will float.

You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking. For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Even though it’s lighter, the piece of clay has a higher density than water and therefore sinks. Even though it’s heavier, wax has a lower density than water, so the big candle floats.

Sinking and floating applies to liquids too. For example, if you add vegetable oil to water, the oil floats on top of the water because the oil has a lower density than the water.

Buoyancy and Archimedes’ Principle
The ancient Greek philosopher Archimedes found that when an object is submerged in water, it pushes aside (or displaces) an amount of water with the same mass as the object.

The water pushes upward against the object with a force (buoyancy) equal to the weight of water that is displaced.

Let’s explore Archimedes’ principle by dropping a bowling ball into a tub of water. When the ball is submerged in the water, it displaces its volume in water. According to Archimedes’ principle, the water can “push back” with a force equal to the weight of the water that has been displaced.

A litre of water has a density of 1 kilogram per litre (1 kg/L), so a bowling ball’s worth of water (4.5 L) can push back on the bowling ball with a force equal to 45 newtons (N). That’s the weight of a 4.5 kg mass. However, the weight of the ball is more like 55 N. That’s more than the buoyant force of the water it displaced, so it sinks.

A beach ball may have the same volume as a bowling ball, but it has a much smaller mass. When you a beach ball in a tub of water, it displaces the mass of water equal to its own mass—about 0.01 kg. If you were to try to push the beach ball down and displace more water, the water would push back with a force greater than the weight of the beach ball. The push of the water keeps the beach ball afloat.

Buoyancy is the upward force we need from the water to stay afloat. Buoyant forces are why we feel so much lighter when we are in a swimming pool. Our bodies are mostly water, so our density is fairly close to that of water. Because of this, an average person needs only a little bit extra buoyancy to float. A life jacket provides this extra lift.

Changing Density
You can change the density of a substance by heating it, cooling it, or by adding something to it. If an object sinks in water, it’s because the object has a higher density than the water. There are two possible ways to make that object float, however:

Increase the density of the water so that the water becomes denser than the object. For example, an egg will usually sink in a glass of water, because it is denser than water. Adding salt to the water increases the density of the water, allowing the egg to float. This experiment also works with people, but you need a lot of salt (try the ocean, or even better, the Dead Sea !)
Increase the volume of the object so that the object becomes less dense than the water. A great example of this is ice floating in water. Ice is formed by freezing water. When it freezes, it increases in volume as the water molecules move farther apart to accommodate the lattice structure of ice. Because the ice is now less dense than water, it floats. This phenomenon also explains why ships float even though they are made of steel. A ship is built in such a way that it encloses large amounts of open space. The ship still displaces its weight in water, but because of the way the ship is constructed, it takes up more space than the volume of the water it displaces, so it floats.

Vocabulary

Archimedes: Greek mathematician, physicist, engineer, inventor and astronomer (c. 287 BC–c. 212 BC).
Archimedes’ principle: Any object wholly or partially immersed in a fluid is buoyed by a force equal to the weight of the fluid that is displaced by the object. In other words, the buoyancy is equal to the weight of the displaced fluid.
buoyancy: The upward force that a fluid exerts on an object less dense than itself; the ability to float.
density: How closely packed together the molecules of an object or substance are.
displace: To push out of the way. For example, when an object goes into water, it displaces the water.
immiscible: Unable to be mixed together, like oil and water.
ironwood: The name for a large number of woods that have a reputation for hardness and high density.
mass: The amount of matter in a given space.
matter: The substance that makes up all physical things.
pumice stone: Lava froth known for its small mass and low density, despite looking like a rock.
weight: A measure of the force of gravity on an object.
volume: The amount of space a substance or object takes up.