Topic 3 – Floating And Sinking – Science and Technology Grade Seven (VII)

Topic 3 - Floating And Sinking - Science and Technology Grade Seven (VII)

Topic 3 – Floating And Sinking – Science and Technology Grade Seven (VII)

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Objects which float on or sink in water

Think

1. Objects which float on water
2. Objects which sink in water

There are various objects in our living environment. These objects differ in mass and volume. Some of these objects float and others sink in water.

Objects like balloons, balls, leaves and canoes float on water. In contrast, objects like stones, glass, tiles, wax, and some metallic objects sink in water.

Characteristics of objects which float on or sink in water

Think: Reasons for objects to float on or sink in water

Floating or sinking of objects in water depends on the following:

(a)  Density of the object

Different objects have different densities. For example, a piece of metal rod has a different density from that of wood. The density of the piece of metal rod is greater than that of the piece of wood.

Likewise, the metal rod has a greater density than water. On the other hand, a piece of wood is less dense than water. When these objects are placed on water, the metal rod will sink in water whereas the piece of wood will float. Moreover, immiscible liquids such as water and kerosene have different densities.

When they are poured in a container, the less dense liquid will float on the more dense liquid. In this case, kerosene will float on water because kerosene is less dense than water.

Density is the mass of an object per unit volume.

Formula               Density = Mass/Volume

The SI unit of mass is kilogram (kg), and that of volume is cubic metres (m3).

Therefore, the SI unit of density is kilogram per cubic metres (kg / m3 ). Another SI unit of density is gram per cubic centimetres (g / cm3 ).

The density of water is one gram per cubic centimetres; that is, 1 g / cm3 or 1000 kg / m3. If the density of an object is greater than that of water, the object will sink in water. Objects which float on water are less dense than water. For example, a ship is large and heavy, but it floats on water because its average density is less than that of water.

(b) Shape of the object

Hollowed objects are filled with air. Hence their average densities are less than water.

These objects float on water because their masses are smaller than solid objects, which are not hollowed. Examples of hollowed objects include ships, canoes, balls, empty gallons and boats.

(c) Relationship between the upthrust force and the gravitational force

The tennis ball floats because the upthrust force exerted by water is greater or equal to the gravitational force as shown in Figure 1.

If the upthrust force is less than the gravitational force, the object sinks. For example, when a coin is placed on water, it sinks because the upthrust force is less than the gravitational force

The relationship between the upthrust and gravitational forces

In Figure 1, the upthrust force acts in the opposite direction to the gravitational force. The gravitational force is a natural force originating from the earth’s gravity towards its center. Upthrust is associated with the mass of water displaced by a floating or sinking object.

The gravitational force is also called weight. The SI unit of the gravitational force is newton, abbreviated by N. This force is measured using a spring balance as shown in Figure 2.

Measuring weight using a spring balance

The gravitational force is a product of mass (m) and acceleration due to gravity (g). The SI unit of the acceleration due to gravity is metre per second squared (m/s2).

Mass is the quantity of matter in an object. Its SI unit is kilogram (kg).

The gravitational force or weight is represented by W. The SI unit of weight is newton (N), which is equivalent to (kgm/s2). Therefore,

Topic 3 – Floating And Sinking – Science and Technology Grade Seven (VII)

 

Archimedes’ principle

It is easier to pull a submerged object. For example, it is easier to pull a bucket of water tied to a rope when drawing water from a deep well. Pulling starts to become harder as soon as part of the bucket is in the air.

Figure 4 shows the relationship between upthrust and the increase in water level when an object is submerged in water. This relationship was discovered by a Greek scientist, named Archimedes, who lived from 288 to 212 BC.

For example, the weight of a stone in the air is 40 N, but, when immersed in water, it weighs 20 N. Also, when the stone is sunk in water, the water volume increases from 20 mL to 40 mL.

The loss in weight of the stone is equal to the weight of the increased volume of water. Archimedes’ principle states, When an object is partially or totally immersed in a fluid, it experiences an upthrust which is equal to the weight of the fluid displaced by the object.

Other examples of water transport vessels are submarines. Submarines are different from ships because they can sink in or float on water. They can do so because they have tanks which can be filled with water or air.  If a submarine is required to sink, the tanks are filled with water. This causes the submarine to have a greater density than that of water.

Therefore, it sinks. When the submarine has totally submerged, the water in the tanks can be replaced with air. Thus, its density becomes less than that of water. Therefore, the submarine floats.

(b) Ships that conduct research on waves in deep water can move horizontally or vertically. This is made possible by filling water in the ship’s tanks when the ship is required to sink and removing water when the ship is required to float.

Topic 3 – Floating And Sinking – Science and Technology Grade Seven (VII)

Vocabulary

Fluid – a substance which can flow easily

Gravitational force is the force of attraction that exist between any two masses

Immiscible liquidsliquids which do not mix e.g. water and kerosene SI unit is a scientific method of expressing magnitude or quantity of phenomena

Submarine – a watercraft that is capable of propelling itself beneath the water as well as on the water surface

Sufuria – a flat based, deep sided, lipped cooking pot or container

Upthrust – is the upward force exerted by a fluid which enables the object to float

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