Chapter 3 - Force and Pressure
1. Write proper word in the blank space.
a. The SI unit of force is ________
(Dyne, Newton, Joule)
Ans: newton.
b. The air pressure on our body is equal to ________ pressure.
(Atmospheric, Sea bottom, Space)
Ans: atmospheric
c. For a given object, the buoyant force in liquids of different ________ is ________.
(the same, density, different, area)
Ans: density, different
d. The SI unit of pressure is ________
(N/m³, N/m², kg/m², Pa/m²)
Ans: N/m²
2. Make a match.
| A group | B group |
|---|---|
1. Fluid | a. Higher pressure |
2. Blunt knife | b. Atmospheric Pressure |
3. Sharp needle | c. Specific gravity |
4. Relative density | d. Lower pressure |
5. Hecto Pascal | e. Same pressure in all directions |
Ans:
| A group | B group |
|---|---|
1. Fluid | e. Same pressure in all directions |
2. Blunt knife | d. Lower pressure |
3. Sharp needle | a. Higher pressure |
4. Relative density | c. Specific gravity |
5. Hecto Pascal | 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:
(i) An object sinks if the buoyant force is smaller than its weight and floats if the buoyant force is larger than its weight.
(ii) The buoyant force acting on an object is large if the density of the liquid it is dipped in is large.
(iii) The density of plastic is less than that of water. Hence, the buoyant force exerted by water on the plastic cube will be greater than its weight, causing the plastic cube to come to the surface.
(iv) Hence, a plastic cube released in water will come to the surface of the water.
b. Why do the load carrying heavy vehicles have a large number of wheels?
Ans:
(i) The pressure exerted by a force is inversely proportional to the area on which it acts.
(ii) A large number of wheels are provided on the load-carrying vehicles to avoid putting a large amount of pressure on a single wheel.
(iii) The pressure gets equally distributed among the wheels and prevents the bursting of tires due to the large pressure.
c. How much pressure do we carry on our heads? Why don’t we feel it?
Ans:
(i) The atmospheric pressure of 101 × 10³ Pa is exerted on our body at sea level.
(ii) The cavities in our body are filled with air while veins and arteries are filled with blood. (iii) The pressure of the blood and the other fluids present in our body balances this atmospheric pressure.
(iv) Hence, we do not feel the pressure on our heads.
4. Why does it happen?
a. A ship dips to a larger depth in fresh water as compared to marine water.
Ans:
(i) The buoyant force on a body immersed in a liquid is directly proportional to the density of the liquid in which it is immersed.
(ii) As the density of freshwater is less than that of marine water, the buoyant force acting on the object placed in fresh water is smaller than that acting on the object in marine water.
(iii) Hence, a ship dips to a greater depth in fresh water as compared to marine water.
b. Fruits can easily be cut with a sharp knife.
Ans:
(i) The pressure exerted by a force is inversely proportional to the area on which it acts.
(ii) The area of the edge of a sharp knife is small.
(iii) As a result, a small force on the knife produces a large pressure at the point of contact, which helps us cut desired things easily.
(iv) If the knife is blunt, then less pressure is exerted at the point of contact due to the large surface area in contact.
(v) Hence, it is easy to cut vegetables and fruits with a sharp knife.
c. The wall of a dam is broad at its base.
Ans:
(i) The pressure increases as the depth of the liquid increases. Therefore, pressure is greatest at the base of the dam.
(ii) To withstand the pressure, the thickness of the wall is increased and the base is made broad so that the area of contact increases and the pressure exerted by the water on the walls of the dam is minimized.
Hence, the wall of a dam is broad at its base.
d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Ans:
(i) A body resists change in its state of rest or motion due to inertia.
(ii) When the bus is at rest, the passengers inside the bus are also at rest.
(iii) As the bus is put in motion, the portion of the passenger’s body that is in contact with the bus acquires velocity, but the upper part of the body tries to remain at rest.
(iv) As a result, passengers experience inertia at rest and get a backward jerk when the bus suddenly speeds up.
5. Complete the following tables.
1.
| Mass (kg) | Volume (m³) | Density (kg/m³) |
|---|---|---|
350 | 175 | ___ |
___ | 190 | 4 |
Ans:
(i) Given:
Mass (M) = 350 kg
Volume (V) = 175 m³
To find:
Density (D)
Solution:
We know that,
Density (D) = \(\large \frac {Mass\,(M)}{Volume\,(V)}\)
∴ Density (D) = \(\large \frac {350}{175}\)
∴ Density (D) = 2 kg/m³
(ii) Given:
Volume (V) = 190 m³
Density (D) = 4 kg/m³
To find:
Mass (M)
Solution:
We know that,
Density (D) = \(\large \frac {Mass\,(M)}{Volume\,(V)}\)
∴ Mass (M) = Density (D) × Volume (V)
∴ Mass (M) = 4 × 190
∴ Mass (M) = 760 kg
The complete table is as follows:
| Mass (kg) | Volume (m³) | Density (kg/m³) |
|---|---|---|
350 | 175 | 2 |
760 | 190 | 4 |
2.
| Density of Metal (kg/m³) | Density of water (kg/m³) | Relative Density |
|---|---|---|
___ | 10³ | 5 |
8.5 × 10³ | 10³ | ___ |
Ans:
(i) Given:
Density of water = 10³ kg/m³
Relative density = 5
To find:
Density of metal
Solution:
We know that,
Relative density = \(\large \frac {Density \,of\, metal}{Density \,of\, water}\)
∴ Density of water = Relative density of metal × Density of water
∴ Density of water = 5 × 10³ kg/m³
(ii) Given:
Density of metal = 8.5 × 10³ kg/m³
Density of water = 10³ kg/m³
To find:
Density of metal
Solution:
We know that,
Relative density = \(\large \frac {Density \,of\, metal}{Density \,of\, water}\)
∴ Relative density = \(\large \frac {8.5 \,×\, 10³}{10³}\)
∴ Relative density = 8.5
The complete table is as follows:
| Density of Metal (kg/m³) | Density of water (kg/m³) | Relative Density |
|---|---|---|
5 × 10³ | 10³ | 5 |
8.5 × 10³ | 10³ | 8.5 |
3.
| Weight (N) | Area (m²) | Pressure (Nm⁻²) |
|---|---|---|
___ | 0.04 | 20,000 |
1500 | 500 | ___ |
Ans:
(i) Given:
Area (A) = 0.04 m²
Pressure (P) = 20000 N/m²
To find:
Weight (W)
Solution:
We know that,
Pressure (P) = \(\large \frac {Weight\,(W)}{Area\,(A)}\)
∴ W = P × A
∴ W = 20000 × 0.04
∴ W = 800 N
(ii) Given:
Weight (W) = 1500 N
Area (A) = 500 m²
To find:
Pressure (P)
Solution:
Pressure (P) = \(\large \frac {Weight\,(W)}{Area\,(A)}\)
∴ P = \(\large \frac {1500}{500}\)
∴ P = 3 N/m²
The complete table is as follows:
| Weight (N) | Area (m²) | Pressure (Nm⁻²) |
|---|---|---|
800 | 0.04 | 20,000 |
1500 | 500 | 3 |
6. The density of a metal is 10.8 x 10³ kg/m³. Find the relative density of the metal.
Given:
Density of metal = 10.8 × 10³ kg/m³
Density of water = 10³ kg/m³
To find:
Relative density of the metal
Solution:
We know that,
Relative density = \(\large \frac {Density \,of\, metal}{Density \,of\, water}\)
∴ Relative density = \(\large \frac {10.8 \,×\, 10³}{10³}\)
∴ Relative density = 10.8
Ans: The relative density of the metal is 10.8.
7. Volume of an object is 20 cm³ and the mass is 50 g. Density of water is 1 gcm⁻². Will the object float on water or sink in water?
Given:
Volume of the object (V) = 20 cm³
Mass of the object (M) = 50 g
Density of water = 1 g/cm³
To find:
Will the object float on water or sink in water.
Solution:
Density of the object (D) = \(\large \frac {Mass\,of\,the\,object\,(M)}{Volume\,\,of\,the\,object\,(V)}\)
∴ Density of the object (D) = \(\large \frac {50}{20}\)
∴ Density of an object (D) = 2.5 g/cm³
Density of an object is greater than density of water, i.e., 2.5 g/cm³ > 1 g/cm³
∴ Object will sink in water.
8. The volume of a plastic covered sealed box is 350 cm³ and the box has a mass 500 g. Will the box float on water or sink in water? What will be the mass of water displaced by the box?
Given:
Volume of a plastic covered sealed box (V) = 350 cm³
Mass of a box (M) = 500 g
To find:
Will the box float on water or sink in water? Mass of water displaced by the box.
Solution:
Density of the box (D) = \(\large \frac {Mass\,of\,the\,box\,(M)}{Volume\,\,of\,the\,box\,(V)}\)
∴ Density of the box (D) = \(\large \frac {500}{350}\)
∴ Density of an object (D) = 1.43 g/cm³
As the density of the box is greater than the density of water, the box will sink in water.
According to Archimedes’ principle,
When the box is dipped in water, water of mass equal to the volume of the box is displaced.
∴ Mass of water displaced by the box = 350 cm³
Ans: The box will sink in water and the mass of water displaced by the box is 350 cm³.