Force and Pressure | Class 8 | Science | Chapter 3 | Maharashtra State Board
Explore the concepts of force and pressure in physics, covering types of forces (contact and non-contact), Newton's laws of motion, inertia, and the principles of pressure in solids, liquids, and gases. Understand the role of friction, balanced and unbalanced forces, buoyancy, and atmospheric pressure with practical examples and experiments.
Questions & Answers
1. Write proper word In the blank space:
a. The SI unit of force is the ………..
(dyne, newton, joule)
Ans. The SI unit of force is the newton.
b. The air pressure on our body is equal to the …………. pressure.
(atmospheric, sea bottom, space)
Ans. The air pressure on our body is equal to the atmospheric pressure.
c. For a given object, the buoyant force in liquids of different ………… is ………….. .
(the same, density, different, area)
Ans. For a given object, the buoyant force in liquids of different density is the same.
d. The SI unit of pressure is ………………
(N/m³, N/m², kg/m², Pa/m²)
Ans. The SI unit of pressure is N/m².
2. Make a match.
Group 'A' | Group'B' |
| a. Higher pressure |
2. Blunt knife | b. Atmospheric pressure |
3. Sharp needle | c. Specific gravity |
4. Relative density | d. Lower pressure |
5. Hectopascal | e. Same pressure in all direction |
Ans.
Group 'A' | Group'B' |
| e. Same pressure in all direction |
2. Blunt knife | d. Lower pressure |
3. Sharp needle | a. Higher pressure |
4. Relative density | c. Specific gravity |
5. Hectopascal | b. Atmospheric pressure |
3. Answer the following questions in brief.
a. A plastic cube is released in water. Will it sink or come to the surface of water?
Ans.
It will come to the surface of water.[Note: This is because its density is less than that of water. When it floats, the unbalanced force acting on it is zero.]
b. Why do the load carrying heavy vehicles have large number of wheels?
Ans.
The pressure produced by a given force depends on the area of the surface on which the force acts. The greater the surface area, the less is the pressure produced. Load carrying heavy vehicles have large number of wheels so that the load (weight, force) is distributed over large surface area of the wheels in contact with the road. Hence, the pressure decreases and the tyres do not burst.
4. Why does it happen?
a. A ship dips to a larger depth in freshwater as compared to marine water.
Ans. The buoyant force acting on a body is proportional to the density of the fluid in which the body is immersed. The density of freshwater is less than that of marine water. Hence, the buoyant force on a body in freshwater is less than that in marine water. Therefore, a ship dips to a larger depth in freshwater as compared to marine water.
b. Fruits can easily he cut with a sharp knife.
Ans.
It is easy to cut vegetables. fruits with a sharp knife. A blunt knife does not work here.
The force exerted perpendicularly on a unit area is called ‘pressure’ Pressure = Force ÷ Area on which the force is applied
Presently we are considering only the force acting on an area in a direction perpendicular to it.
c. The wall of a dam is broad at its base.
Ans.
The pressure at a point in a liquid is proportional to the height of the liquid column above it. Hence, the pressure of water in a dam is much greater at the bottom of the dam than at the top.
To withstand this high pressure, the wall of a dam is made stronger and thicker (broad) at the base than at the top.
d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Ans.
When passengers sit or stand in a stationary bus, they are in a state of rest. When the bus suddenly speeds up, the lower parts of their body in contact with the bus acquire the speed of the bus.
The upper parts of their body, however, continue to be in the state of rest due to inertia. Hence, they are thrown in the backward direction.
5. Complete the following tables.
a.
Mass (kg) | Volume (m³) | Density (kg/m³) |
350 | 175 | - |
- | 190 | 4 |
Ans.
Using the formula,
density = mass ÷ volume
Mass (kg) | Volume (m³) | Density (kg/m³) |
350 | 175 | 2 |
760 | 190 | 4 |
b.
Density of Metal (kg/m³) | Density of water (kg/m³) | Relative Density |
- | 10³ | 5 |
8.5 × 10³ | 10³ | - |
Ans.
Using the formula,
relative density = density of a metal ÷ density of water
Density of Metal (kg/m³) | Density of water (kg/m³) | Relative Density |
5 × 10³ | 10³ | 5 |
8.5 × 10³ | 10³ | 4 |
c.
Weight (N) | Area (m²) | Pressure (Nm-²) |
- | 0.04 | 20,000 |
1500 | 500 | - |
Ans.
Using the formula,
pressure = weight ÷ area
Weight (N) | Area (m²) | Pressure (Nm-²) |
800 | 0.04 | 20,000 |
1500 | 500 | 3 |
6. The density of a metal is 10.8 × 103 kg/m³. Find the relative density of the metal.
Ans.
Given:
Density of the metal = 10.8 × 10³ kg/m³
Density of water = 10³ kg/m³
Relative density of the metal = ?
Relative density of a substance
= density of the substance ÷ density of water
= 10.8 × 10³ kg/m³ ÷ 10³ kg/m³ = 10.8
The relative density of the metal = 10.8.
7. The volume of an object is 20 cm3 and the mass is 50 g. The density of water is 1 gm-³. Will the object float on water or sink in water?
Ans.
To determine whether the box will float or sink in water, we can use the principle of buoyancy, which states that an object will float if its density is less than the density of the fluid it is placed in.
1. Calculate the Density of the Box:
The density (ρ) of an object is given by the formula:
ρ = mass ÷ volume
Given:
Mass of the box = 500 g
Volume of the box = 350 cm³
The density of the box is:
ρ box = 500g ÷ 350 cm³ = 1.43 g/cm³
2. Compare with the Density of Water:
The density of water is approximately 1 g/cm³.
Since the density of the box (1.43 g/cm³) is greater than the density of water (1 g/cm³), the box will sink in water.
8. The volume of a plastic-covered sealed box is 350 cm³ and the box has a mass 500g. Will the box float on water or sink in water? What will be the mass of water displaced by the box?
Ans.
1. Calculate the Density of the Box:
The density (ρ) of an object is given by the formula:
ρ = mass ÷ volume
Given:
Mass of the box = 500 g
Volume of the box = 350 cm³
The density of the box is:
ρ box = 500 g ÷ 350 cm³ ≈1.43 g/cm³
2. Compare with the Density of Water:
The density of water is approximately 1 g/cm³.
Since the density of the box (1.43 g/cm³) is greater than the density of water (1 g/cm³), the box will sink in water.
3. Calculate the Mass of Water Displaced:
According to Archimedes' principle, the mass of water displaced by the box is equal to the mass of the box if the box is completely submerged.
Therefore, the mass of water displaced by the box is:
Mass of water displaced = Volume of box × Density of water
= 350 cm³ × 1 g/cm³ = 350 g
Thus, the box will sink, and the mass of the water displaced by the box is 350 g.
E X T R A
1. Rewrite the sentences after filling the blanks
1. The tendency of an object to remain in its existing state is called its …………. .
Ans.
The tendency of an object to remain in its existing state is called its inertia.
2. Pressure = ……………
Ans.
Pressure = Force ÷ Area on which the force is applied
3. 1 bar = ……………….. N/m.
Ans.
1 bar = 10^5 N/m².
4. 1 atmosphere = ………….. Pa.
Ans.
1 atmosphere = 101 × 10³ Pa.
5. The SI unit of density is …………. .
Ans.
The SI unit of density is kg/m³.
2. Write proper word In the blank space:
1. According to Archimedes’ principle, the magnitude of the force of buoyancy acting on a body is …………. .
(Vρg, Vρ/g, Vρm, mρg)
Ans.
According to Archimedes’ principle, the magnitude of the force of buoyancy acting on a body is Vρg.
2. The pascal is the unit of ……………. .
(velocity, pressure, mass, force)
Ans.
The pascal is the unit of pressure.
3. Keeping the surface area constant, if the applied force is doubled, the pressure …………….. .
(becomes double, remains the same, becomes four times, becomes half)
Ans.
Keeping the surface area constant, if the applied force is doubled, the pressure becomes double.
3. State whether the following statements are True or False:
1. The density of water is 1000 g/cm³.
Ans. False. [The density of water is 1000 kg/m³ (or 1 g/cm³)]
2. Force and weight have the same units.
Ans. True.
3. Atmospheric pressure at sea level is about 106 dynes/cm².
Ans. True.
4. The buoyant force due to a liquid is proportional to the acceleration due to gravity.
Ans. True.
5. Atmospheric pressure increases with altitude.
Ans. False. (Atmospheric pressure decreases with altitude.)
6. Pressure due to a given force is directly proportional to the area on which the force acts.
Ans. False. (Pressure due to a given force is inversely proportional to the area on which the force acts.)
7. When a body is completely immersed in a liquid, the buoyant force acting on it due to the liquid is proportional to the volume of the liquid displaced by the body.
Ans. True.
8. The density of a material is useful to determine its purity.
Ans. True.
9. One tends to slip over a banana peel on the street and one can slip due to mud are events that occur due to reduced friction.
Ans. True.
10. Frictional force is electromagnetic in origin.
Ans. True.
4. Identify the odd term:
1. Density, Pressure exerted by a gas, Mass, Force.
Ans. Force. (Force is a vector quantity; other quantities are scalar quantities.)
2. Lactometer, Hydrometer, Voltmeter, Submarine.
Ans. Voltmeter. (Its working is not based on Archimedes’ principle. The working of a lactometer, hydrometer and submarine is based on Archimedes’ principle.)
5. Answer the following questions in one sentence:
1. Which of the following has more inertia? A ₹ 10 coin and a ₹ 1 coin.
Ans. A ₹ 10 coin has more inertia than a ₹ 1 coin.
2. Name the physical quantity expressed in pascal.
Ans. Pressure is expressed in pascal.
3. State the SI unit of pressure.
Ans. The SI unit of pressure is N/m2, also called the pascal.
4. Name the property of a liquid due to which it exerts an upward force on an object immersed in it.
Ans. Buoyancy is the property of a liquid due to which it exerts an upward force on an object immersed in it.
5. Name the principle used in designing ships and submarines.
Ans. Archimedes’ principle is used in designing ships and submarines.
6. What is specific gravity?
Ans. The specific gravity of a substance is another name used for relative density, i. e., the ratio of the density of the substance to the density of water.
7. State any one factor on which the pressure exerted by a liquid at a point inside the liquid depends.
Ans. The pressure exerted by a liquid at a point inside the liquid depends on the density of the liquid.
8. State any one factor on which the buoyant force due to a liquid depends.
Ans. The buoyant force due to a liquid depends on the density of the liquid.
9. Name the device used to determine the purity of a sample of milk.
Ans. The lactometer is used to determine the purity of a sample of milk.
10. Name the device used to determine the density of a liquid.
Ans. The hydrometer is used to determine the density of a liquid.
11. Name two instruments whose working is based on Archimedes’ principle.
Ans. Working of the lactometer and hydrometer is based on Archimedes’ principle.
6. Answer the following questions:
1. Give three examples to show that a force acts on two bodies through an interaction between them.
Ans.
Consider a car at rest on a level (plane) road. If it is pushed from behind, it moves in the forward direction.
Iron nails get attracted to the poles of a magnet and stick to the magnet.
The moon revolves around the earth.
2. What is a contact force? Give one example.
Ans. A force that acts through a direct contact of two objects or via one more object, is called a contact force.Example: If a ball at rest on the ground is kicked, it starts moving.
3. What is a non contact force? Give one example.
Ans. A force that acts between two objects even if the two objects are not in contact, is called a non contact force.Example: The earth revolves around the Sun.
7. Write short notes:
1. Buoyant force.
Ans.
When a body is immersed partially or completely in a liquid, the liquid exerts forces on all sides of the body. This force is perpendicular to the surface of the body and equals the product of pressure and area at that point.
The resultant of all these forces acts upward. It is called the upthrust or buoyant force.
The buoyant force is proportional to (i) the volume of the liquid displaced by the body (ii) the density of the liquid (iii) the acceleration due to gravity. Its magnitude equals the magnitude of the weight of the liquid displaced by the body.
2. Applications of Archimedes’ principle.
Ans.
The working of a lactometer, a device used to determine the purity of a sample of milk, and a hydrometer, a device used to determine the density of a liquid, is based on Archimedes’ principle. The extent to which a lactometer floats (or sinks) depends on the density (and hence purity) of the milk. The same thing is true for a hydrometer. The greater the density of a liquid, the less is the extent to which a body sinks in it.
Archimedes’ principle is used in design of ships and submarines. A submarine is provided with large tanks at the front and the back. Its weight can be increased by filling the tanks with sea water or air from compressed air reservoirs. The weight can be decreased by pumping out water from the tanks by forcing compressed air in them. By controlling the weight, it can be made to sink or rise to the surface as desired.
The density of a body that floats or sinks in water or kerosine can be determined by. Archimedes’ principle.4. The density of kerosine can be determined by Archimedes’ principle, using a body of material that is not affected by water and kerosine.
8. Give scientific reasons:
1. The tiles are placed over a slushy patch of ground to help cross It.
Ans.
Tiles have greater area than the area of our feet.
The weight of the person crossing the slushy patch is exerted over a large area of the tiles.
Therefore, there is a decrease in the pressure and hence the tiles do not sink much in the slushy patch of ground. This helps to cross the slushy patch of ground.
[Note: If there were no tiles, the feet will come in direct contact with the slushy ground. The area of the feet being less, the weight of the person will act over a smaller area. Therefore. there will be more pressure and hence the feet will sink into the slushy ground.]
2. Drawing pins have flattened heads.
Ans.
The head of a drawing pin is flattened and the other end is pointed.
When enough force is applied to the head of the pin, the pressure due to the force on the pointed end increases tremendously and the pin can be easily inserted in the drawing board.
When we press the flattened end, the force applied spreads over a larger area. This reduces the pressure of the reaction force acting on the thumb. Hence, the thumb is not injured.
If the head of the pin is sharp, then the pressure due to the force would be more and hence the pressure of the reaction force would also be more and the sharp end would prick the thumb causing injury.
3. An iron nail sinks in water but a ship made from iron floats on water.
Ans.
An iron nail sinks in water because its density is more than that of water.
A ship made from iron, due to the particular shape given to it, displaces a large amount of water so that the buoyant force acting on the ship due to water balances the weight of the ship. Hence, the ship floats on water.
4. A piece of iron sinks in water but floats on mercury.
Ans. The density of iron is more than that of water but less than that of mercury. Hence, a piece of iron sinks in water but floats on mercury.
5. A sheet of metal that sinks in water can float if shaped like a pan.
Ans.
A sheet of metal sinks in water because its density is more than that of water.
If the sheet is shaped like a pan, it can displace a large amount of water such that the buoyant force on the pan due to water balances the weight of the pan. Hence, it can float on water.
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