Lessons - Term Two

Density is the relationship between an object’s volume and its mass. Volume is the amount of space the object occupies, and mass is a measurement of the amount of stuff in the object, sort of like weight. An object with a lot of mass can have a low density if it is very big, and an object with little mass can have a high density if it is very small. Water has a property called uplift which means that water pushes up against objects that are placed on its surface. Gravity, however, tries to pull the object down through the water, just as uplift tries to push the object back up. If you place something in the water that is less dense than the water, it will float. However, if you drop something into the water that has a mass that is greater than the mass of the water, it will sink. Another background idea is displacement which is what occurs with ships in the water. The ships are made of materials that are denser than water, yet they float. This is because of displacement. The object (ship) is pushing water out of its way. Water doesn’t like being pushed around, so it pushes back against the object, trying to return to the space from which it was pushed. Thus, the effect of the water pushing against the ship makes the ship float.

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