Chapter 3 Floating and Sinking PDF

FOR ONLINE USE ONLY DO NOT DUPLICATE Chapter Three Floating and sinking Introduction...

FOR ONLINE USE ONLY DO NOT DUPLICATE Chapter Three Floating and sinking Introduction LY In Standard Three, you learnt about matter and its properties. You also learnt about floatation of matter in the air. In this chapter, you will learn about the characteristics of objects which float on or sink in water. You N will also learn about the law of floatation and its application in marine transportation. In addition, you will make models of objects which O float and objects which sink in water. The competencies developed will enable you to apply the law of floatation in constructing models of vessels used in water transport. SE Objects which float on or sink in water U Think 1. Objects which float on water E 2. Objects which sink in water N There are various objects in our living environment. These objects differ LI 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, N objects like stones, glass, tiles, wax, and some metallic objects sink in O water. Characteristics of objects which float on or sink in water R Think FO Reasons for objects to float on or sink in water 20 SCIENCE AND TECHNOLOGY STD 7.indd 20 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 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, LY 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 N kerosene is less dense than water. O Density is the mass of an object per unit volume. Formula Mass Density = Volume SE The SI unit of mass is kilogram (kg), and that of volume is cubic metres ( ) 3 (m3). Therefore, the SI unit of density is kilogram per cubic metres kg / m. U Another SI unit of density is gram per cubic centimetres ( g / cm ). 3 E Example 1 N Find the density of an object with a mass of 640 grams and volume of 800 cubic centimetres. LI Solution Formula N Given: Mass Mass = 640 g Density = Volume O 640g Volume = 800 cm3 Density = 800cm3 R g FO Required: Density of an object. Density = 0.8 cm3 The density of the object is 0.8 g/cm3. 21 SCIENCE AND TECHNOLOGY STD 7.indd 21 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Example 2 Find the density of an object having a mass of 1 000 kilograms and volume of 100 cubic metres. Solution Formula Given: Mass Density = Mass = 1 000 kg Volume LY 1000 kg Volume = 100 m3 Density = 100 m3 N Required: Density of an kg Density = 10 m3 O object. The density of the object is 10 kg/m3. SE 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 U 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. E (b) Shape of the object Hollowed objects are filled with air. Hence their average densities are N less than water. These objects float on water because their masses are LI smaller than solid objects, which are not hollowed. Examples of hollowed objects include ships, canoes, balls, empty gallons and boats. N (c) Relationship between the upthrust force and the gravitational O 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 R 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 FO force is less than the gravitational force. 22 SCIENCE AND TECHNOLOGY STD 7.indd 22 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Upthrust Tennis ball Gravitational force Water Figure 1: The relationship between the upthrust and gravitational forces LY 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 N gravity towards its center. Upthrust is associated with the mass of water O displaced by a floating or sinking object. The gravitational force is also called weight. The SI unit of the gravitational SE force is newton, abbreviated by N. This force is measured using a spring balance as shown in Figure 2. U Holder E N LI Spring balance N O R FO Load Figure 2: Measuring weight using a spring balance 23 SCIENCE AND TECHNOLOGY STD 7.indd 23 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 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, Weight = Mass × Acceleration due to gravity or W =m× g LY Example N Find the weight of a substance with the mass of 10 kilograms and the acceleration due to gravity of 10 metres per second squared. O Solution Formula Given: Weight = Mass × Acceleration due to gravity m = 10 kg g = 10 m / s2 W=m×g SE W = 10 kg × 10 m / s2 Required: Weight (W). W = 100 kgm / s2 U W = 100 N Therefore, the weight of the object is 100 N. E Note: N 1. If the gravitational force is greater than the upthrust force, the object sinks. LI 2. If the gravitational force is less than the upthrust force, the object will be pushed vertically upwards. N 3. If the gravitational force is equal to the upthrust force, the object floats. O Activity 1: To identify objects which float on and those which sink in water Materials: nail, eraser, leaf, sharpener, metal spoon, plastic spoon, R stone, pencil, small pieces of paper, feather, tennis ball, FO and a dish or a bucket or a bowl, and water Procedure 1. Take a dish, a bucket, or a bowl; then, pour enough water into it as shown in Figure 3. 24 SCIENCE AND TECHNOLOGY STD 7.indd 24 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 2. Put one object after another into the bowl with water as shown in Figure 3. LY N O Figure 3: Objects which float on water and objects which sink in water 3. Observe the objects you have placed on water. Which objects float on water and which objects sink in water? SE 4. After observation, record your results in Table 1. Write “floats” for the object that floats and “sinks” for the object that sinks in water. U 5. For each result, explain why the object floated or sank in the water. Table 1: Objects which float on or sink in water E Object Results Explanation N Leaf LI Nail Eraser N Sharpener O Metal spoon Plastic spoon Stone R Pencil FO Feather Small pieces of paper 25 SCIENCE AND TECHNOLOGY STD 7.indd 25 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Exercise 1 Answer all questions in Sections A and B. Section A 1. Fill in the column by writing “floats” or “sinks” on placing the object in water. Table 2: Objects which float on or sink in water S/N Object Results LY (a) Razor blade placed vertically (b) Stainless plate N (c) Porcelain cup (d) “Sufuria” O (e) Dhow (f) Pencil Section B SE 2. Define density. U 3. Explain why a ship floats on water while a needle sinks in water. 4. Write two characteristics of the objects that float on water. E Archimedes’ principle N It is easier to pull a submerged object. For example, it is easier to pull LI 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 N air. Figure 4 shows the relationship between upthrust and the increase in water level when an object is submerged in water. This relationship was O 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 R 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 FO 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. 26 SCIENCE AND TECHNOLOGY STD 7.indd 26 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE LY N Measuring cylinders O Stone in water SE Water U Stone in the air Figure 4: The weight of stone in the air and in water E Example N A stone weighs 100 newtons in the air. If the stone is immersed in a LI fluid, it weighs 80 newtons. Find the upthrust force acting on the stone. N Solution Given: O Weight of the stone in the air = 100 N Weight of the stone in the fluid = 80 N R Formula Upthrust = Weight of the stone in the air – Weight of the stone in the fluid FO Upthrust = 100 N – 80 N Upthrust = 20 N Therefore, the upthrust is 20 N. 27 SCIENCE AND TECHNOLOGY STD 7.indd 27 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Experiment 1: Verification of Archimedes’ principle Aim: Verifying Archimedes’ principle Materials: spring balance, stone, Eureka Can full of water, thread, knife and an empty plastic bottle Procedure 1. Cut the top part of an empty plastic bottle and tie the thread to the LY upper section of the remaining part. Then, use a spring balance to measure its weight. Write its weight in your exercise book. 2. Using the spring balance, measure the weight of the stone in the air N as shown in Figure 5 (a). Write its weight in your exercise book. 3. Place a cut plastic bottle tied with thread under the spout of the O Eureka Can. 4. Immerse the stone suspended from the spring balance into the its weight in your exercise book. SE Eureka Can filled with water as shown in Figure 5 (b). Write down 5. Take the cut plastic bottle with the displaced water as shown in Figure 5 (c). Measure and record its weight in your exercise book. U E N LI N O Stone Eureka Can Stone in the in water Spout R air Plastic bottle Thread FO (a) (b) (c) Figure 5: Verification of Archimedes’ principle 28 SCIENCE AND TECHNOLOGY STD 7.indd 28 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Results The weight of the stone in the air in Figure 5 (a) is ___________ newtons. The weight of the stone immersed in water in Figure 5 (b) is__________ newtons. The weight of the displaced water in Figure 5 (c) is___________ newtons. The upthrust force is__________ newtons. LY Conclusion When an object floats or sinks in water, it displaces an amount of water whose weight equals the weight lost by the object. The loss in weight of N the object is equal to the upthrust force. When an object floats or sinks in the fluid, the weight of the displaced fluid is equal to the upthrust force. O The law of floatation SE The magnitude of the upthrust force depends on the volume of the submerged part of the floating object. Therefore, if an object is floating, with part of its body submerged, the depth of the submerged part does not affect upthrust. U The upthrust force on the object is equal to the weight of the fluid displaced by the object. For an object to float, the upthrust force must be equal to the gravitational force. If the gravitational force is greater than the upthrust E force, the object sinks. N The law of floatation states, A floating body displaces its own weight of the fluid in which it floats. LI Experiment 2: Verification of the law of floatation N Aim: To prove that a floating body displaces fluid of equal weight O to that of the floating body Materials: spring balance, four wooden blocks of different weights, R Eureka Can, empty plastic bottle, thread, knife and water Procedure: FO 1. Cut the top part of the bottle and tie the thread to the upper section of the remaining part. Then, measure its weight using a spring balance as shown in Figure 6 (a). Record its weight in Table 3. 29 SCIENCE AND TECHNOLOGY STD 7.indd 29 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 2. Tie each piece of wood using thread and measure its weight as shown in Figure 6 (b). Record its weight in Table 3. 3. Fill the Eureka Can with water and place the cut plastic bottle tied with thread under the spout of the Eureka Can as shown in Figure 6 (c). 4. Immerse the first piece of wooden block into the Eureka Can. 5. Measure the weight of the cut plastic bottle with displaced water and record its weight in Table 3. LY 6. Pour out the displaced water from the cut plastic bottle and refill the Eureka Can with water. 7. Repeat steps 4 up to 6 for each piece of wooden block. N 8. Use the weight of the empty cut bottle to find the weight of the displaced water for each piece of wooden block. Record their weights O in Table 3. SE U E N LI N Eureka Can O R FO Cut bottle Wooden block (a) (b) (c) Figure 6: Verification of the law of floatation 30 SCIENCE AND TECHNOLOGY STD 7.indd 30 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE Table 3: Verification of the law of floatation Piece of Weight of cut Weight of Weight of cut Weight of wooden plastic bottle wooden block plastic bottle with displaced block displaced water water 1. 2. 3. 4. Results LY What is the relationship between the weight of the wooden block and the weight of displaced water? Conclusion N Write the conclusion of this experiment. O Application of the law of floatation in water transport (a) The law of floatation is used in constructing water transport vessels. For SE example, a ship is heavy, but it does not sink in water. The reason is that the weight of the water displaced by the ship is equal to the weight of the ship, which is partially submerged as shown in Figure 7. U E Submerged part of ship N LI Water N O Figure 7: Ship floating on water R Other examples of water transport vessels are submarines. Submarines are different from ships because they can sink in or FO 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 31 SCIENCE AND TECHNOLOGY STD 7.indd 31 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 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. Activity 2: Constructing a model of a canoe, boat or ship LY Materials: plastic bottles, a plastic gallon, wood or metal, straws, a rope, manila paper or box, sticks or small nails, hammer, N glue and pictures of canoe, boat, and ship O Procedure 1. Choose a type of the model that you will construct. 2. Draw a picture of the model that you will construct. SE 3. Identify materials you will use to construct your model. 4. Cut pieces of materials according to the model diagram and its dimensions and connect them using glue, rope or small nails. U 5. Submit your model and place it in the Science and Technology learning corner. E Exercise 2 Answer all questions in Sections A and B. N Section A LI Choose the most correct answer and write its letter in the box provided. 1. What causes an object to float or sink? N (a) Length of the object (b) Width of the object O (c) Density of the object (d) Height of the object 2. If the density of an object is less than that of water, what will happen R if you drop the object into water? FO (a) It will sink. (b) It will float. (c) It will sink and float. (d) It will sink or float. 32 SCIENCE AND TECHNOLOGY STD 7.indd 32 26/07/2021 12:00  FOR ONLINE USE ONLY DO NOT DUPLICATE 3. The density of an object is mass per unit __________. (a) weight (b) height (c) depth (d) volume 4. The submarine sinks in the sea if its density is__________. (a) less than that of water (b) greater than that of water (c) equal to that of water LY (d) greater or less than that of water Section B N 5. Why do objects float on or sink in water? 6. What is the relationship between the weight of an object floating on O a fluid and upthrust? 7. State Archimedes’ principle. SE 8. Explain how the law of floatation is used in constructing a ship. 9. What is the relationship between the weight of a floating piece of wooden block and the weight of the water displaced when the piece of wooden block is immersed in water? U 10. Find the density of an object having a mass of 180 kg and a volume of 90 cubic metres. 11. The density of a substance is 80 kilograms per cubic metres. If its E mass is 240 kilograms, calculate its volume. N Vocabulary LI Fluid a substance which can flow easily Gravitational force is the force of attraction that exist between any two N masses Immiscible liquids liquids which do not mix e.g. water and kerosene O SI unit is a scientific method of expressing magnitude or quantity of phenomena R Submarine a watercraft that is capable of propelling itself beneath the water as well as on the water surface FO 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 33 SCIENCE AND TECHNOLOGY STD 7.indd 33 26/07/2021 12:00