Pressure Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. Name the instrument that is used to measure pressure.

1. Odometer
2. Voltmeter
3. Ammeter
4. Barometer

Ans. The correct answer is d. Barometer

Explanation: A scientific instrument known as a "barometer" is used to measure air pressure. Because pressure changes with height, a barometer may also be used to determine the altitude of a region.

Ques. What is the SI unit of pressure?

1. Candela
2. Pascal
3. Joule
4. Kelvin

Ans. The correct answer is b. Pascal

Explanation: Pascal (Pa) is the SI unit of pressure named after famous scientist Blaise Pascal.

Ques. The working of hydraulic machines is based on which of the following laws?

1. Newton’s law of cooling
2. Ideal gas law
3. Pascal’s law
4. The universal law of gravitation

Ans. The correct answer is c. Pascal’s law

Explanation: According to Pascal's Law, a change in pressure at any point in a fluid is transmitted so that the same change happens everywhere throughout the fluid. This phenomenon is commonly used in hydraulic machines by changing cross-sectional areas at particular places.

Ques. The decrease in pressure takes place when

1. The area is decreased but not the force
2. Either force increases or area decreases
3. The force is increased but not the area
4. The area increases and the force decreases

Ans. The correct answer is d. The area increases and the force decreases

Explanation: Pressure on an object is given by

P = F/A

As a result of the equation mentioned above, we can conclude that pressure is inversely proportional to the area and directly proportional to the applied force. As a result, when the area increases or the force decreases, the pressure will decrease.

Ques. Pascal’s Law is valid only for _____.

1. Fluids
2. Metals
3. Water
4. Solids

Ans. The correct answer is a. Fluids

Explanation: Pascal's Law holds true for water, gases, and even other fluids. However, it does not apply to any other type of matter. As a result of the above statement, Pascal's Law is only applicable to fluids.

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Short Answers Questions [2 Marks Questions]

Ques. Define pressure.

Ans. Pressure is defined as the normal force acting per unit area of a surface. The SI unit of pressure is Pascal (Pa). It is given by the formula

P = F/A

Where

• P is the pressure exerted on the surface of an object
• F is the force acting on the surface
• A is the area of the surface

Ques. Define 1 Pascal.

Ans. Pascal is the SI unit of pressure. If a force of one Newton acts perpendicular to a one-meter square area, the pressure is said to be one pascal (1 Pa).

Ques. What is absolute pressure?

Ans. Absolute pressure is the pressure that is measured relative to zero pressure in the vacuum or air-free space of the universe. This reference pressure is the ideal or absolute vacuum.

Ques. What are the factors affecting pressure at a point in a liquid?

Ans. The pressure acting at a point located at depth h in a liquid is given by

P = ρgh

Where

• ρ is the density of the liquid
• g is the acceleration due to gravity
• h is the depth of the point in the liquid

So the pressure will change if there is a change in the depth, density of the liquid, and acceleration due to gravity.

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Long Answers Questions [3 Marks Questions]

Ques. Find the pressure exerted at the tip of a board pin of area 0.1 mm² if it is passed against the board with a force of 15 N.

Ans. Given

• Area of the tip of the board pin, A = 0.1 mm² = 0.1 x 10^-6 m²
• Force applied, F = 15 N

Pressure exerted on the tip of the board pin is given by

P = F/A

⇒ P = 15/(0.1 x 10^-6)

⇒ P = 15 x 10⁷ N/m²

Ques. Atmospheric pressure is nearly 100 kPa. How much force does the air in a room exert on the inside of a window pane that is of area 40 cm x 80 cm?

Ans. Given

• Pressure exerted on the window pane, P = 100 kPa = 10⁵ Pa
• Area of the window pane, A = 40 cm x 80 cm = 3200 cm² = 32 x 10^-2 m²

Force exerted by the air in the room on the window pane is given by

F = P x A

⇒ F = 10⁵ x 32 x 10^-2 N

⇒ F = 32000 N = 32 kN

Ques. How does temperature affect gas pressure?

Ans. The effect of change in temperature on the pressure of the gas is

• The particles of the gas are separated far away from each other. The kinetic energy of the gas molecule increases as the temperature of the container rises. This signifies that the particle collision rate increases.
• The pressure across the container wall rises due to the collision between the gas particles.
• As a result, when the temperature increases, the pressure of the gas will increase.

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Very Long Answers Questions [5 Marks Questions]

Ques. What are the practical applications of pressure?

Ans. The following are the applications of pressure

• Pins and nails are made to have pointed ends. When a force is applied over the head of a pin or a nail, it transmits a large pressure at its tip and hence easily penetrates into the wall.
• A sharp needle (a syringe of an injection) can easily penetrate the skin, but a wider object like a ruler cannot do so when pressed against the skin with the same force.
• It is difficult for a man to walk on the sand while a camel can easily walk on it, in spite of the fact that the camel is heavier than a man. This is because camel’s feet have a larger area than the feet of a man. Due to the larger area, the pressure is less.

Ques. Water is filled in a flask up to a height of 40 cm, the bottom of the flask is square with a side of 10 cm. If the atmospheric pressure is 1.01 × 10⁵ Pa then what force is exerted by the water on the bottom? Take density of water = 1000 kg /m³

Ans. Given

• Height up to which water is filled, h = 40 cm = 0.4 m
• Atmospheric pressure, Pa = 1.01 × 10⁵ Pa
• Side of the squared bottom of the flask, s = 10 cm = 0.1 m
• Density of water, ρ = 1000 kg/m³

Pressure at the bottom of the flask is given by

P_B = Pressure at the top of the flask (Pa) + ρgh

⇒ P_B = 1.01 × 10⁵ + (1000 x 9.8 x 0.4)

⇒ P_B = 1.05 x 10⁵ Pa

The force exerted by the water on the bottom is given by

F = P_B x Area of the bottom (A)

⇒ F = P_B x (Side)²

⇒ F = 1.05 x 10⁵ x 0.1²

⇒ F = 1050 N

Hence the force exerted by the water on the bottom is 1050 N.

Ques. A tank with a square base of area 2 m² is divided into two compartments by a vertical partition in the middle. There is a small hinged door with a face area of 20 cm² at the bottom of the partition. Water is filled in one compartment and an acid of relative density 1.5 in the other, both to a height of 4 m. What is the amount of force necessary to keep the door closed? Take g = 10 m s^-2

Ans. Given

• Area of the base of the tank, A = 2 m²
• Area of the small hinged door, a = 20 cm² = 20 x 10^-4 m²
• The density of acid, ρacid = 1.5 x 10³ Kg m^-3
• Height up to water and acid is filled in the tank, h = 4 m

Pressure exerted by the water at the base of the tank is given by

P_Water = ρ_Water x g x h

⇒ P_Water = 1000 x 10 x 4

⇒ P_Water = 4 x 10⁴ N/m²

The force exerted by the water at the door which is at the base of the tank is given by

F_Water = P_Water x area of the door (a)

⇒ F_Water = 4 x 10⁴ x 20 x 10^-4 = 80 N

Pressure exerted by the acid at the base of the tank is given by

P_Acid = ρ_Acid x g x h

⇒ P_Acid = 1.5 x 10³ x 10 x 4

⇒ P_Acid = 6 x 10⁴ N/m²

The force exerted by the acid at the door which is at the base of the tank is given by

F_Acid = P_Acidr x area of the door (a)

⇒ F_Acid = 6 x 10⁴ x 20 x 10^-4 = 120 N

Therefore, the net force acting on the door, F = F_Acid - F_Water

⇒ F = 120 - 80 = 40 N

Therefore, 40 N force is required to keep the door closed.

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