The comprehensive approach in Kerala Syllabus 9th Standard Physics Textbook Solutions Chapter 5 Buoyant Force Notes Questions and Answers English Medium ensures conceptual clarity.
Std 9 Physics Chapter 5 Notes Solutions Buoyant Force
SCERT Class 9 Physics Chapter 5 Notes Solutions Kerala Syllabus Buoyant Force Questions and Answers
Class 9 Physics Chapter 5 Let Us Assess Answers Buoyant Force
Question 1.
If an object weights 0.2 kgwt in air and 0.19 kgwt when fully immersed in water,
a) What is the loss of weight of the object in water?
b) What is the buoyant force felt on the object?
Answer:
a) Weight in air (Wair) = 0.2kgwt
Weight in water (Wwater) = 0.19kgwt
Loss of weight in water = Wair – Wwater
= 0.2 kgwt – 0.19 kgwt = 0.01 kgwt
b) The buoyant force acting on the object is equal to the loss of weight when immersed in the fluid.
Therefore, Buoyant force = Loss of weight = 0.01 kgwt
Question 2.
A copper sphere and an iron sphere of the same diameter are immersed in water. Is the buoyant force experienced on both the same? Justify your answer. (density of copper is 8900 kg/m3, the density of iron is 7800 kg/m3)
Answer:
Yes. Since both spheres have the same diameter, they will have the same volume. Therefore, both spheres displace the same volume of water.
Buoyant force = weight of displaced fluid.
Question 3.
Write down the reasons for the following statements.
a) Toy balloons filled with helium gas rise in the air and balloons filled with carbon dioxide go down.
b) A ship plunges less in water when the weight is unloaded at the port.
Answer:
a) A balloon filled with helium will float in air. A balloon filled with CO2 will sink in the air. Because the density of helium is lighter than carbon dioxide, so carbon dioxide sink while helium floats.
b) When a ship is loaded with weight, it displaces a volume of water equal to the weight of that load due to the principle of buoyancy. When weight is unloaded from the ship at the port, the total weight of the ship decreases. As a result, the ship displaces less water because it needs to displace only the volume equal to the reduced weight.
Question 4.
The use of life jackets in boats is mandatory. Explain it in terms of buoyant force.
Answer:
When a person wears a life jacket in boats, it helps to save their life. Because the jacket adds volume and displaces more water than the person alone would displace. The jacket is filled with materials (like foam) that have a lower density than water, which helps it float. For a person in water, the weight of their body pulls them down. The life jacket increases the volume of water displaced, which increases the buoyant force acting on the person. If the buoyant force is greater than the person’s weight, they will float.
Question 5.
The density of saturated salt water is 1025 kg/m3. Calculate its relative density.
Answer:
Relative Density = \(\frac{\text { Density of Saturated Salt Water }}{\text { Density of Water }}\)
= \(\frac{1025 \mathrm{~kg} / \mathrm{m}^3}{1000 \mathrm{~kg} / \mathrm{m}^3}\)
=1.025
Question 6.
The illustration shows how an object is suspended in air and dipped in a liquid in different ways using a spring balance.
a) What is the reason for the difference in buoyant force in figure 2 and 3?
b) What is the reading of the spring balance in figure 4? Why?
Answer:
a) In Figure 2 the object is not fully submerged. Only a small amount of fluid is displaced. The weight of the displaced fluid, or the buoyant force experienced by it, will be less. In the third figure, the object is completely submerged. It displaces more fluid. The buoyant force will be higher.
b) 30 N. In three and four figures, the objects are completely submerged in water. Hence the buoyant force felt is equal.
Question 7.
An object weighing 800 N in air, floats on water. What is its weight in water? What is the weight of the displaced water?
Answer:
Weight of the object in air = 800 N
Weight of the object in water = 0
Since the object is floating, the weight of the displaced water is equal to the weight of the object. Therefore, the weight of the displaced water is also 800 N.
Class 9 physics Chapter 5 Extended Activites Answers Buoyant Force
Question 1.
Make a hydrometer using a test tube, lead shot, wax and calibrated paper. Mark the relative density of water by immersing it in pure water. Compare this value with the relative density of other liquids.
Answer:
An example of making a hydrometer is given below
Materials:
- Test tube
- Lead shot (for weight to make the test tube float upright)
- Wax (to seal the test tube)
- Calibrated paper (to mark the scale)
- Ruler (for measuring)
- Pure water (for calibration)
- Other liquids (Example: oil, saltwater, etc.)
- Glue
Procedure:
Test tube preparation
- Place a small amount of lead shot in the bottom of the test tube. This provides weight to ensure the test tube floats upright when immersed.
- Melt a bit of wax and carefully pour it over the lead shot to seal it inside the test tube. Wait till wax hardens before proceeding to next step
Hydrometer calibration
- Fill a tall container with pure water. Gently place the test tube into the water. It should float upright because of the weight of the lead shot.
- With the test tube floating, note the point where the water surface touches the test tube. This is the reference point for pure water (relative density = 1).
- Cut a small strip of calibrated paper (or simply mark the tube with a fine marker). Place it inside the test tube against the glass, or glue it on the outside. Mark the water level as “1” for pure water.
Testing Other Liquids
- Remove the test tube from the water and gently immerse it in another liquid like oil, alcohol, or saltwater.
- Notice how far the test tube sinks in each liquid. The relative density of a liquid determines whether the test tube floats higher (higher density than water) or sinks further (lower density than water).
Compare the new water line to the original one from the pure water test. You can then label these new positions with relative densities:
- If the tube sinks deeper, mark a reading above 1. (lower density)
- If it floats higher, mark a reading below 1. (higher density)
Observation
Relative density of liquids:
Water: 1.0
Saltwater: > 1.0
Oil: > 1.0
Question 2.
Make a lactometer. Find the relative density of milk in a few milk pails at your nearest milk society. Compare it with the society’s milk price chart.
Answer:
(Hints)
Materials:
- A small test tube
- Lead shot (sand or small stones for weight)
- Wax or tape to seal the tube
- Marker (to mark levels)
- Pure water
- Milk
Procedure
- Put some small weights (like sand or stones) into the bottom of the test tube.
- Seal the test tube with wax or tape so no liquid can get in.
- Place the test tube in a container of pure water.
- Mark the water level on the test tube with a marker or paper. This is the “water level.” Take the lactometer to your milk society.
- Dip it into different milk pails and see how deep it floats compared to water.
- If it floats higher than in water, the milk is denser (good quality).
- If it sinks more, the milk has lower density (may be watered down).
- Check the milk society’s price chart. The price of higher density milk is usually higher.
Buoyant Force Class 9 Notes Questions and Answers Kerala Syllabus
Question 1.
What do you observe?
Answer:
Bottle floats on water surface
Try to immerse the bottle to the bottom of the bucket with your hand. We can feel the upward force exerted by water from bottom, on our hands.
Question 2.
After fully immersing this bottle in water, remove your hand from the bottle. What do you observe?
Answer:
Bottle rises to the surface of water.
Question 3.
What made the plastic bottle rise to the surface of the water?
Answer:
It is because of the upward force exerted by water on the bottle.
Question 4.
Does the bottle experience force of gravity? If yes, in which direction?
Answer:
Yes.
In the downward direction.
Question 5.
Why doesn’t the bottle remain immersed in water?
Answer:
Even though the bottle experiences a gravitational force in downward direction, an upward force of greater magnitude is exerted by water.
Question 6.
When a bucket of water is drawn from a well, a decrease in weight is felt until it reaches the surface of the water. What is the reason for this weight loss?
Answer:
When water is lifted from the well, the weight of the bucket and water is felt downwards, while the upward force exerted by the water is felt in the bucket. But once the bucket of water reaches the surface of water, the strength of the upward force obtained from the water disappears. Therefore, decrease in weight is felt until the bucket reaches the surface of the water.
Activity
Fill three-fourth of a bucket with water. Try lifting a mug filled with water from the bottom of the bucket as shown in the figure. Take it out of the water.
Question 7.
What is the difference felt in the weight of the mug when it is inside and outside the water in the bucket?
Answer:
Weight of mug is less inside water compared to its weight outside the water.
Question 8.
What could be the reason?
Answer:
A small force is enough to lift the cup from inside the water to the surface of the water. More force is required to lift it upwards from the surface of the water. Liquid exerts an upward force on objects in the liquid. An upward force is experienced by the objects in liquids. This upward force is not only felt in liquids.
Question 9.
What happens to a helium-filled balloon when it is set free in the atmosphere? What is the reason?
Answer:
A helium-filled balloon, when it is set free in the atmosphere, rises upwards. Even though a helium-filled balloon has weight; it rises upwards because the upward force exerted by the atmosphere is greater than the weight of the balloon.
Question 10.
Find and record in your science diary instances where you experience a buoyant force in everyday life.
· Air bubbles rising from the water
Answer:
- Air bubbles rising from water
- Ships float on the surface of the water
- Ice floats on water.
- The smoke goes up.
- The steam goes up.
- Life jackets float on the surface of water.
Question 11.
Take a spring balance calibrated in newton, a trough filled with three-fourths of water, stone, an iron block and a glass slab.
Measure the weight of stone, iron block and glass slab using the spring balance and write it down in table given below. Then immerse each of them in water. Find their weight in water and complete the table.
| Object | Weight in air (W1)N | Weight in water (W2) N | Loss of weight in water buoyant force (W1 – W2) N |
| Stone | |||
| Iron block | |||
| Glass slab |
Answer:
| Object | Weight in air (W1)N | Weight in water (W2) N | Loss of weight in water buoyant force (W1 – W2) N |
| Stone | 6 N | 5 N | 1 N |
| Iron block | 10 N | 8.6 N | 1.4 N |
| Glass slab | 8 N | 6.8 N | 1.2 N |
Activity
Take water, saturated salt water and glycerine in separate beakers.
Question 12.
Find the buoyant force felt by a 100 g slotted weight in these liquids using a spring balance and complete the table.
Weight of slotted weight in air W1 = mg
= 100 g × 9.8 m/s2
= 0.1 kg × 9.8 m/s2
= 0.98 N
| liquid | Weight of slotted weight in liquid N | Loss of weight (buoyant force) N |
| Saturated salt water | ||
| Water | ||
| Glycerine |
Answer:
| liquid | Weight of slotted weight in liquid N | Loss of weight (buoyant force) N |
| Saturated salt water | 0.68 N | 0.3 N |
| Water | 0.78 N | 0.2 N |
| Glycerine | 0.58 N | 0.4 N |
Question 13.
What may be the reason for the difference in the loss of weight in different liquids ?
Answer:
It is due to the difference in densities of different liquids.
Let’s examine whether the loss of weight experienced by an object in different liquids is related to the density of the liquid.
Density of a substance is its mass per unit volume.
Density of some familiar liquids are given in the table below.
| Liquid | Density kg/m3 (approximately) |
| Water | 1000 |
| Saturated salt water | 1025 |
| Kerosene | 810 |
| Glycerine | 1260 |
| Coconut oil | 920 |
Question 14.
In which liquid did the slotted weight experience the greatest buoyant force?
Answer:
Glycerine
Question 15.
In which liquid did the slotted weight experience the least buoyant force?
Answer:
Kerosene
Question 16.
Which liquid given in the table has the highest density?
Answer:
Glycerine
Question 17.
Why did the slotted weight experience the greatest buoyant force in glycerine?
Answer:
Among the given liquids, glycerine has the highest density. As the density of liquid increases, the buoyant force experienced by it increases.
Question 18.
As the density of the liquid increases, the buoyant force felt by the object Increases/decreases
Answer:
Increases
Buoyant force varies with the density of a liquid.
Question 19.
Explain the following situations and record in the science diary.
• An egg sinks in freshwater but floats on saturated salt water.
• As a ship moves from freshwater lake to sea, it rises further. No one completely drowns in the Dead Sea.
Answer:
a) An egg experiences more buoyant force in saturated salt water which is denser than freshwater.
So it sinks in freshwater and floats on saturated salt water.
b) The density of sea water is greater than freshwater, it exerts more buoyant force on ships. So
the ship rises further as it moves from freshwater lake to sea.
c) Due to its higher density, it exerts more buoyant force.
Activity to find out if objects of same weight experience equal buoyant force in the same liquid
Take an iron block and a copper block of equal weight. Complete the table by finding their weights in air and water.
| Object | Weight of object in air (W1) N | Weight in object in water (W2) N | Buoyant force (W1 – W2)N N |
| Copper block | 20 N | 18 N | 2 N |
| Iron block | 20 N | 17 N | 3 N |
Question 20.
Which object experienced more buoyant force? iron block/ copper block
Answer:
Iron block
Question 21.
Which has greater volume?
Iron block/copper block
Answer:
Iron block
It is clear that the buoyant force experienced by an object fully immersed in a liquid depends on the volume of the object.
Factors influencing buoyant force experienced by an object fully immersed in a fluid are:
density of the fluid
volume of the object
Activity
Take a stone and a glass slab. Find the buoyant force experienced by them in water. Immerse them one after the other in water taken in an overflow jar. Collect the water over-flowed in a beaker. Find the weight of the water overflowed (water displaced by the object).
| Object | Weight in air (W1) N | Weight in water (W2) N | Buoyant force (W1 – W2) N | Weight of displaced water N |
| Stone | ||||
| Glass slab |
Answer:
| Object | Weight in air (W1) N | Weight in water (W2) N | Buoyant force (W1 – W2) N | Weight of displaced water N |
| Stone | 6 N | 5 N | 1N | 1N |
| Glass slab | 8 N | 6.8 N | 1.2 N | 1.2 N |
Question 22.
What is the relation between the buoyant force experienced by the stone and glass slab in the water and the weight of the water displaced by each of them?
Answer:
Buoyant force is equal to weight of liquid displaced by the object
Question 23.
What is the relation between the weight of an object in the air and the weight of the water displaced by it?
Answer:
The weight of the object and the weight of the liquid displaced by the object are equal. The buoyant force experienced by an object immersed in a liquid is less than the weight of the object. But the buoyant force acting on a floating object is equal to the weight of the object.
Question 24.
Make a boat with a piece of aluminium foil of approximate 15 cm length and 10 cm breadth. Place it on the surface of water.
What do you observe?
Answer:
When the aluminium foil is in the shape of a boat it displaces water equivalent to its weight and it floats on the surface of water.
Question 25.
Place a slotted weight of mass 100 g inside the boat. The boat will not sink. Why?
Answer:
When the slotted weight is placed in the boat, it displaces more water. The weight of the displaced water is equivalent to the weight of the boat and the slotted weight together.
Question 26.
Roll the same aluminum foil firmly and try to place it on the surface of the water.
Answer:
The rolled-up aluminum foil sinks into the water. When the aluminum foil was rolled up, it could only displace water less than its weight. The buoyant force equal to weight of the foil was not obtained. So the foil sinks down in the water.
Inference
The volume of water displaced by a boat-shaped aluminium foil is greater than that displaced by a rolled aluminium foil.
Question 27.
Now can you clear the doubt why a ship made of iron floats on water while a nail made of iron sinks in water as we have seen in the beginning of the chapter.
Answer:
The buoyant force experienced by the iron nail is less than its weight. So the iron nail sinks in water. Its volume is very small even if it is completely submerged, only a small amount of water can be displaced. Since the ship has a larger volume, it is possible to displace water equal to its weight. So the ship will float in the water. The same thing happens when the ships are loaded.
Question 28.
A boat on a lake displaces 6000 kg of water. Calculate the buoyant force experienced by the boat.
Answer:
Weight of water displaced =Buoyant force
= 6000 kgwt = 6000 × 9.8 = 58800 N
Question 29.
If an object weighs 0.45 kgwt in air and 0.31 kgwt when fully immersed in water, find the following.
a) The loss of weight of the object.
b) The buoyant force.
c) Weight of water displaced
Answer:
a) Loss of weight of the object = 0.45 kgwt – 0.31kgwt = 0.14 kgwt
b) Buoyant force =0.14 kgwt = 0.14 × 9.8 = 1.372 N
c) Weight of water displaced =1.372 N
All liquids do not have the same density
Question 30.
Based on the data given in table 3, classify kerosene, coconut oil, glycerine and saturated salt water by comparing with the density of water and complete the table.
Answer:
| Denser than water | Less dense than water |
| Saturated salt water Glycerine |
Kerosene Coconut oil |
The density of water is 1000 kg/m3. In scientific studies, there are instances where the densities of substances are often compared with the density of water.
Question 31.
The density of copper is 8900 kg/m3. What is its relative density?
Answer:
Density of copper = 8900 kg/m3
Density of water = 1000 kg/m3
Relative density of copper = \(\frac{\text { Density of copper }}{\text { Density of water }}\)
= \(\frac{8900 \mathrm{~kg} / \mathrm{m}^3}{1000 \mathrm{~kg} / \mathrm{m}^3}\) = 8.9
Question 32.
What is the density of mercury if its relative density is 13.6?
Answer:
Relative density of mercury =13.6
Density of water = 1000 kg/m3
Density of mercury =13.6 × 1000 = 13600 kg/m3
Devices that work on the principle of floatation
Hydrometer
Hydrometer is a device used to measure the relative density of liquids.
Using a hydrometer find the relative density of various liquids available. Answer the following questions based on your observations.
Question 33.
What is the reading of the hydrometer in water?
Answer:
1
Question 34.
If the hydrometer is placed in a liquid denser than water, will the reading be above or below 1?
Answer:
The reading will be Below 1.
The relative density values calibrated on the hydrometer increase as it goes down [as shown in figure given below.
Therefore, the relative density of liquids in which the hydrometer sinks lesser, will be higher.
It is a crime to sell adulterated (water – added) milk.
Question 35.
How will you know that water is added to milk? What is the device used for it?
Answer:
Lactometer. A lactometer sinks more in water-added milk.
Lactometer
Lactometer is a device used to measure the relative density of milk.
Basically it is a hydrometer.
Question 36.
Will the reading be the same if a lactometer is placed in pure milk and water-added milk? Why?
Answer:
No
The density of water-added milk will be less than pure milk. Therefore lactometer sinks more in water-added milk, and it shows a reading of less than one.
Question 37.
Is it in pure milk or fat-removed milk that a lactometer sinks more? Why?
Answer:
The density of fat-removed milk is less than pure milk. Therefore lactometer sinks more in fat-removed milk.
Question 38.
Using a lactometer, how will you identify whether water is added to milk?
Answer:
When water is added to milk, the lactometer sinks more Adulteration of food is a crime, like the sale of water-added milk. It is also a social menace.
Question 39.
Prepare a poster and display it on the school notice board to conscientize society.
Answer:
Pure substances are a gift of nature. Human greed fuels adulteration.
Say not to adulteration. Lead a healthy life.