NCERT Class 9 Science Lab Manual – Archimedes’ Principle
To establish the relation between the loss in weight of a solid when fully immersed in
- tap water
- strongly salty water, with the weight of water displaced by it by taking at least two different solids.
- Fluids: Gases and liquids flow and are thus called fluids.
- Buoyancy: The upward force exerted by fluids on any body is called the buoyant force and this phenomenon is known as buoyancy.
- Thrust: The force acting on a body perpendicular to its surface is called thrust. S.I. unit is Newton.
- Pressure: The thrust per unit area is called pressure.
S.I. Unit = or Nm-2. This unit (Nm-2)is also called Pascal,
.’. 1 Pascal (Pa) = 1 Nm-2
- Weight of a body = Mass x acceleration due to gravity
W = mg
- When a body is immersed in water or liquid, the body displaces some liquid.
- The volume of liquid displaced = total volume of the solid.
- The mass of liquid displaced can be measured as:
Mass of liquid displaced = Volume x Density
M = V x D
- Weight of liquid displaced = Volume x Density x g (acceleration due to gravity)
W = V x D x g
- The body loses some weight when immersed in fluid, it can be found as follows:
Weight of the body in air = W1
Weight of the body when immersed in liquid = W2
- Hence loss in weight = W2 -W1
How much will be this upward force/buoyant force depends on the density of liquid in which it is immersed. The upthrust is more by denser liquids.
- Archimedes’ Principle: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
A spring balance, a metal bob, a cotton thread, an overflow can, a glass beaker, tap water, salty water, an iron stand.
A. Find the zero error and least count of spring balance:
- Take an iron stand and suspend a spring balance to it.
- Study the spring balance, its scale and its least count.
- Record your observations. If any error, record it as ‘x’ gf.
B. Find the weight of solid metal bob in air:
- Take a metal bob, tie thread to it and suspend on the hook of the spring balance.
- Record the weight of the metal bob in air. Let this weight be Wr
C. Find the weight of the metal bob immersed in tap water and record the apparent loss in weight
- Take an overflow can, fill it with water such that its water level touches the spout of the overflow can.
- Keep an overflow can under the spring balance such that the metal bob gets fully immersed in the water of
the overflow can.
- Keep a beaker whose weight P1 is recorded, at the mouth of the spout of overflow can.
- As soon as the metal bob is suspended in water the weight on spring balance scale is recorded. This loss in weight is due to buoyancy (W2).
- Collect the water that has overflown in a beaker till the last drop that comes out of the spout.
- Weigh the beaker with water (P2).
- Calculate the weight of the displaced water (P2-P1).
- Calculate the loss in weight of the metal bob when immersed in water.
D. Find the weight of metal bob immersed in salty water and record the apparent loss in weight.
- Prepare salty water by taking a 500 mL beaker and adding 300 mL of water in it and dissolving common salt till the saturated solution is obtained.
- Take the same metal bob and repeat the steps from 1 to 8 as given in procedure ‘C’
- Tabulate your observations.
Observations and Calculations
- Zero error in spring balance = (x) = 0 gf.
- Least count of the spring balance = 2 gf.
- Density of water = 1 g/cm3
- Weight of empty beaker P1= 100 gf.
- Hence it is proved that the weight of the water displaced by the metal bob is approximately equal to the apparent loss of weight of the metal bob in water. Archimedes’ principle is verified.
- The weight of water displaced by a given solid when immersed fully in strongly salty water is more than the weight of the water displaced when fully immersed in tap water.
- Carefully study the spring balance used for the experiment.
- Fill the overflow can above the mark of the spout, allow extra water to overflow through the spout without disturbing it. Use this overflow can for the experiment.
- Do not allow the suspended solid mass i.e. metal bob/stone to touch the base and sides of the overflow beaker.
Why can’t we call solids as fluids?
Solids cannot flow, so they are not fluids.
Can fluids exert pressure?
Yes, fluids exert pressure in all directions.
In what direction does the buoyant force on an object immersed in a liquid act?
The buoyant force acts in an upward direction.
PRACTICAL BASED QUESTIONS
What are fluids?
Anything that flows is called a fluid. All liquids and gases are fluids.
Why do you feel light while swimming?
While swimming, water (fluid), exerts an upward force on the body this is called upthrust or buoyant force, because of this buoyant force we feel light.
On what factor does the magnitude of the upward force depend?
The magnitude of the buoyant force depends on the density of the fluid.
Why does the plastic bottle float on water?
When the upward force acting on the bottle is more than the downward force acting on the bottle then it will float.
What is the downward force acting on the bottle immersed in water?
The downward force is due to the gravitational attraction of the earth.
A cork and iron nail have same weight. When immersed in water nail sinks but cork floats. Why?
This is because the density of cork is less than iron nail, it is also less than water density. Hence the upward force exerted by water on cork is much greater than the density of the cork, so it floats on water.
A boy swims in river water and in sea water. In which case he finds swimming easier?
It is easy to swim in sea water as it is denser and exerts more upthrust/buoyant force and makes floating easier.
State Archimedes’ principle.
When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Name the device used to determine the density of liquids.
The density of liquids can be determined by hydrometer, it is based on Archimedes’ principle.
Name the device used to determine the density/purity of a sample of milk.
What is relative density?
The relative density of a substance is the ratio of its density to that of water.
What is the unit of relative density?
It has no unit, as it is a ratio of similar quantities.
NCERT LAB MANUAL QUESTIONS
Why does the pointer of a spring balance move up when the stone suspended from it is immersed in water?
When the spring balance with stone is immersed in water, the stone experiences an upward force called buoyant force. It is due to this property the spring balance pointer moves up to show the loss in weight due to buoyancy.
State the factors on which buoyant force acting on an object immersed in a fluid depend.
The buoyant force acting on an object immersed in a fluid depends on the density of the fluid and the volume of the fluid displaced by a body.
What will be the effect on the apparent loss in weight of a stone if it is immersed in salty water instead of tap-water?
The stone when immersed in salty water will displace water with more mass than as compared to the tap-water (density of salty water is more as compared to tap-water).
State two precautions that should be observed while making use of an overflow-can.
The two precautions to be observed while making use of an overflow-can are:
- The overflow can should be filled up to the level where the water is about to overflow.
- The container should be placed on a smooth horizontal surface.
An object suspended from a spring balance is gradually lowered in an overflow-can. What will be the change in the apparent loss in the observed weight as the object is immersed in water?
The object when immersed in water will gradually go on decreasing its weight and displaces water. The apparent weight loss will increase as the object is immersed in water.
What is a fluid? Is it different from a liquid or from a gas or from a solid?
Fluid is a substance that can flow. Gases and liquids can flow but solids cannot, hence all liquids and gases are called fluids.
A spring balance calibrated in newton, reads 19.6 N. What will be its mass in grams at your place?
As w = mg,
19.6 = m x 9.8
Hence m = = 2 kg = 2000 g
You are given two measuring cylinders of least count 1.0 mL and 2.5 mL, respectively. Which one will you prefer to determine the density more accurately?
The measuring cylinder with the least count 1.0 ml will give more accurate reading.
Write two precautions that you will observe while measuring the volume of a liquid, with the help of a measuring cylinder.
The eye level and the liquid level should be the same and the lower meniscus of the liquid should be read as its volume.
Two bottles of equal volume are filled with glycerin and water respectively. Which of the bottle will be heavier? Give reason for your answer.
The bottle with glycerine would be heavier because its mass is greater than water. The density of the substance is directly proportional to the mass of the body. The density of glycerine is 1.26g/cc and that of water is 1g/cc.
Why is the density of water at 80 °C less than its density at 30° C?
The density of a liquid and a gas changes with temperature. With the increase in the temperature its density will decrease gradually.
Why is it easier to swim in sea-water rather than a swimming pool or river water?
The density of sea water is more than the pool or river water. The buoyant force exerted by the sea water on a body is more and it keeps the body more afloat and less immersed and helps in swimming.
In which liquid—glycerine or kerosene—the loss in the weight of a solid object when fully immersed in it will be more?
Loss in weight of a solid depends on the weight of the liquid displaced and density of liquid. As the density of glycerine is more than the kerosene the loss in weight of a body in glycerine will be more as compared to kerosene.
How can you perform this experiment for a solid whose density is lesser than that of the liquid used?
The same experiment for a body with density less than the liquid can be performed by using an object with known density, mass and volume as sinker can be tied to the light object. This way the density can be calculated.
What are the limitations of this experiment?
The recording of the readings needs lot of precision and the least count of the objects used should be the minimum. The thread used for tying the objects may also interfere in the accuracy.
Using this method, how would you find the density of a liquid?
Loss in weight of the solid object = weight of the liquid displaced by it.
= Volume of liquid displaced x density of liquid
Hence Density = weight of liquid displaced/volume of liquid displaced.
How will you select solid to perform this experiment using glycerine instead of water?
The solid should have more density than glycerine.
MULTIPLE CHOICE QUESTIONS (MCQs)
Questions based on Procedural and Manipulative Skills
When a body is immersed in water, the buoyant force will act on it
(a) in downward direction
(b) in all the directions
(c) in the upward direction
(d) all of these.
The fluid among the following is
(b) tap water
(d) all of these.
The weight of a cork in air is 20 gf, its weight in water will be
(a) 20 gf
(b) – 20 gf
(c) 0 gf
(d) 10 gf.
The S.I. unit of density is
(d) kg /cm3.
The S.I. unit of volume is
The formula to calculate density of a given substance is
(a) D =
(b) D =
(c) D =
(d) D =
Relative density of a substance depends upon
(a) mass of the substance
(b) volume of substance
(c) material of the substance
(d) none of these.
The relative density of water is
(a) 1 0
In S.I. unit, the maximum density of water is
(a)1 l kg/ m3
(b) 100 kg/ m3
(c) 1000 kg/ m3
(d) 1000 g/ m3.
The correct formula to find the volume of a sphere is
One kg.wt is equal to
(a) 9.8 N
(b) 980 N
(c) 1000 x 9.8 N
(d) 1000 x 980 N.
The SI unit of thrust is
A student lowers a body in a liquid filled in a container. He finds that there is a maximum apparent loss in weight of the body when
(a) it just touches the surface of the liquid
(b) it is completely immersed in the liquid
(c) it is partially immersed in the liquid
(d) it is partially immersed and also touches the sides of the container.
Questions based on Observational Skills
When a body is completely immersed in water, it loses weight, the loss in this weight
(a) is equal to the buoyant force acting on it
(b) is equal to the weight of water displaced
(c) is equal to the volume of water displaced
(d) none of these.
A given solid is weighed in air using a spring balance. It is then weighed by immersing it fully, in each of the given vessels filled with water. The weight indicated by the spring balance will be
(a) least in vessel C
(b) least in vessel B
(c) least in vessel A
(d) equal in all three vessels.
If WA , WB and WC represent the weights of the solid in the figure shown below, then
(a) WA = WB = WC
(b) WA WB = WC
(c) WB WA = WC
(d) WA WB WC.
The least count of the given spring balance is
(a) 1 gf
(b) 2 gf
(c) 5 gf
(d) 20 gf.
Three students A, B and C determined the volume of a solid by immersing it in water in the overflow cans set-up as shown. The result obtained will be wrong for
(a) Student A
(b) Student A
(c) Student A
(d) All three students
The least count of measuring cylinder is
(a) 4 ml
(b) 5 ml
(c) 2 ml
(d) 0.2 ml.
A solid metal body was immersed in pure water and then in salty water. The correct observation is
(a) it sinks more in pure water
(b) it sinks more in salty water
(c) it sinks at same level in both
(d) it floats in both.
The spring balance shown here is used to measure the mass of a given solid. The mass of solid is
(a) 112.5 g
(b) 118 g
(c) 120 g
(d) 125 g.
A boy added few cubes of ice to a glass tumbler containing 20 ml of water. He observed the water level till the ice melts completely in water. As per his observation when ice was added to water, the level of water
(c) remains the same
(d) depends on shape of ice.
Which one of the following is a fundamental quantity?
Questions based on Reporting and Interpretation Skills
When a solid is partially immersed in water, the volume of displaced water is
(a) equal to the weight of solid
(b) equal to the volume of partial solid
(c) is more than the volume of partial solid
(d) equal to the weight of partial solid.
When the density of an object is less than the liquid in which it is immersed then the
(a) object sinks in the liquid
(b) object half floats and half sinks
(c) object floats on the liquid
(d) none of these.
When the ice kept in water melts completely the level of water in a tumbler
(a) will increase
(b) will decrease
(c) remains the same
(d) depends on ice.
The iron nail sinks in water because
(a) the downward gravitational force pulls it down
(b) the upthrust acting on the nail is less
(c) the downward force acting on nail is greater than the upthrust of water
(d) the downward force is equal to the upward force.
The density of iron is more than water. The ship made of this iron floats on water due to
(a) less density of ship than water
(b) increased surface area increases the pressure
(c) the downward force is balanced by the upthrust
(d) all of the above.
A given solid is weighed in air using a spring balance. It is then weighed separately by immersing it fully in a vessel containing tap water and then in vessel containing salt solution. The reading of the spring balance would be
(a) least in air
(b) least in water
(c) least in salt solution
(d) equal in all three cases.
In the experiment to establish the relation between loss in weight of an immersed solid with the weight of water displaced by it, the upthrust experienced by the object in tap water and in salty water are Uw and Us respectively, then
(a) Uw = Us
(b) Uw >Us
(c) Us = 2Uw
(d) Uw <Us
The weight of the body felt in tap water and salty water are WA and WB respectively, then
(a) WA = WB
(c) WA < WB
(d) WB = 2WA
A body floats in liquid when the weight of the body is
(a) half the weight of the liquid displaced
(b) greater than the weight of the liquid displaced
(c) less than the upthrust of liquid
(d) less than the weight of the liquid displaced.
The buoyant force acting on a floating body is
(a) vertically downwards
(b) vertically upwards
(c) horizontal on both the sides
(d) both horizontal & vertical.
Weight of a solid bob in air is ION, and in liquid is 9N. The buoyant force experienced by the solid bob is
(a) 9 N
(b) 10 N
(c) 1 N
(d) 19 N.
The upthrust experienced by a body depends on
(c) acceleration due to gravity ‘g’
(d) all of these.
SCORING KEY WITH EXPLANATION
- (c) Buoyant force is the upward force acting on a body in fluid.
- (d) Anything that flows is a fluid.
- (c) In case of cork floating in the liquid, the weight of the cork is equal to the weight of the liquid displaced, i.e. wt of the cork = weight of the displaced liquid.
The apparent weight = weight of the cork – weight of displaced liquid
The apparent weight = 0
- (a) It is the SI unit for density.
- (b) It is the SI unit for volume.
- (a) This is the relationship of density, mass and volume of a substance
- (c) The material of the substance is important in finding the relative density.
- (a) The relative density of any material is a ratio of density of given material with that of water.
- (c) The density of water is maximum at 4 degree Celsius and is 1000 kg/ m3
- (b) It is the formula to calculate the volume of sphere.
- (a) 1 kg weight is 9.8N
- (b) Thrust is the upward force, unit of force is Newton.
- (b) More the body immersed in a liquid more will be the loss in the weight of a body.
- (b) The volume of displaced liquid = the volume of the body immersed in it similar to the mass of liquid displaced = the loss in mass of the immersed body
- (d) The shape of the container does not affect the loss in weight of a body but the density of liquid in which the body is immersed will affect the loss of weight of a body.
- (b) WB and Wc are the same because in both the cases the body is immersed in tap water but WA
is the weight in air.
- (b) Between 0 to 10 there are 5 markings, hence each marking represents 2 gf.
- (d) In all the three cans the level of water is not filled upto the overflow spout.
- (c) Between 10 to 20 there are 5 markings, hence each marking represents 2 ml.
- (a) The salty water is denser and the buoyant force exerted is more and hence the body floats more and sinks in pure water whose density 7 is less than salty water.
- (c) While reading the value on spring balance avoid parallax error.
- (a) On adding ice in water the level of water increases due to the surface area of ice and water displaced by ice.
- (d) Length is not a derived quantity it is a fundamental quantity.
- (b) The volume of displaced liquid = the volume of the body immersed in it.
- (c) The bodies with less density w.r.t. liquid, will float on the given liquid.
- (c) The level of water remains the same because the ice had displaced some water when it was floating in water and on melting the same amount of water gets added to the water.
- (c) The iron nail is denser and the buoyant force acting on it is less.
- (c) When the buoyant force balances the force exerted by a body in a liquid, then the body floats in the liquid.
- (c) The density of salty water is more as compared to air and tap water hence, the immersed body will exerience loss in salty water.
- (d) The density of salty water is more as compared to tap water hence, the upthrust by salty water is more.
- (b) The body loses more weight in salty water than tap water, hence the weight of body immersed in tap water is more.
- (c) If the upthrust exerted on the body immersed in a liquid is more, then the body will float.
- (b) Buoyant force is the vertically upward force acting on a body in fluid.
- (c) Loss in the mass experienced by a body when immersed in a liquid is equal to buoyant force.
- (d) The upthrust depends on the density of the fluid/body, ‘g’ and the volume displaced.