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BackgroundDensity, Mass & Volume 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. 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 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:
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 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 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
VocabularyArchimedes: Greek mathematician, physicist, engineer, inventor and astronomer (c. 287 BC–c. 212 BC). Other ResourcesBrainPOP | Science | Matter & Chemistry | Measuring Matter EDinformatics | Mass, Volume, Density WatchKnowLearn.org | Buoyancy and Density ProTeacher Collection | Density
Learn what determines whether an object in water will float or sink. Encyclopædia Britannica, Inc.All sorts of objects can float in water regardless of their shape or solidity. Whether an object is flat or pointy- or hollow- or solid- does not affect its ability to float in water. What determines whether something floats or sinks in water? When an object enters water, it pushes out water to make room for itself. The object pushes out a volume of water that is equal to its own volume. This is called displacement. We observe displacement when we take a bath. As we get into the bathtub, the water level rises. Displacement happens when any object enters water. Two forces act on an object when it enters water: a downward force called gravity and an upward force called buoyancy. An object’s weight measures the downward force of gravity that acts on it. The upward force, or buoyant force, that acts on an object in water is equal to the weight of the water displaced by the object. Any object that is in water has some buoyant force pushing up against gravity, which means that any object in water loses some weight. If the object displaces an amount of water equal to its own weight, the buoyant force acting on it will be equal to gravity—and the object will float. But, if the object weighs more than the water it displaces, the buoyant force acting on it will be less than gravity, and it will sink. How compact, or closely packed, an object is determines how much water it will displace and, therefore, whether it will float or sink. This compactness is referred to as density. Density is mass per unit volume. If an object is more compact, or denser, than water, it will sink in water. If the density of an object is less than the density of water, the object will float in water. This piece of steel is denser than water. If steel is denser than water, then why does a steel ship float? A ship floats when it can displace water equal to its own weight. What’s more, a steel ship is not made of solid steel. It is hollow and contains a lot of air. Air is a much less dense substance than steel. Calculating the density of a steel ship with the formula, "density equals mass over volume" should show that the steel ship is less dense than water. The volume of the ship is so great that its ratio of mass to volume is less than that of water. The buoyant force pushing up on the ship is equal to the gravity pulling the ship down. This is why the ship floats. |