Pressure- All about Pressure

Important questions and answers of Pressure

1. What is pressure? Write its SI unit?
Ans: Pressure is the force acting normally per unit area. Its SI unit is N/m² or Pascal.

2. Define 1 Pascal.
Ans: If F = 1N and A = 1 m², then pressure is defined as 1 Pascal.
Or,
1 Pascal is defined as the pressure exerted by 1 N force on the unit area (1 m²).

3. Define liquid pressure. Write its unit.
Ans: The pressure exerted by the liquid is called liquid pressure. Its unit is Pascal.

4. Write down three properties of liquid pressure.
Ans: Two properties of liquid pressure are:

1. Liquid pressure is directly proportional to the depth of liquid.
2. Liquid pressure is directly proportional to the density of liquid.
3. Liquid pressure is independent of the shape of container.

5. State Pascal law or define Pascal law.
Ans: Pascal law states that “When pressure is applied at a point on a liquid kept inside a closed container then the pressure is transmitted equally and normally in all direction”. This law is also called the principle of transmission of liquid pressure.

6. Name two instruments that work on pascal law. or what are the applications of Pascal law?
Ans: Two instruments that work on pascal law are:

1. Hydraulic brake
2. Hydraulic lift
3. Hydraulic press

7. What is the working principle of Hydraulic machines?
Ans: The working principle of Hydraulic machine is
Pressure on small piston = Pressure on the big piston

8. Why hydraulic machine is called a force multiplier, why?
Ans: According to pascal law,
Pressure on small piston = Pressure on the big piston

\text{i.e.}\ \frac{F_{1}}{A_{1}}=\frac{F_{2}}{A_{2}}\\
\text{or,}\ F_{2}= \frac{F_{1}\times A_{2}}{A_{1}}\\
\text{since,}\ A_{2} >A_{1},\\
\therefore F_{2}>F_{1}

So, a Hydraulic machine is a force multiplier.

9. What is Upthrust? Or Define Upthrust.
Ans: When a body is partially or completely immersed in liquid it experiences an upward force, which is called Upthrust. Its SI unit is Newton.

10. What are the factors on which Upthrust depends?
Ans: The factors on which Upthrust (U) depends are:

1. Density of liquid(d)
2. Volume of displaced water (V)
3. Acceleration due to gravity (g)

Note: Formula to calculate upthrust is U = Vdg.

11. What is density?
Ans: Density is defined as the mass of a substance per unit volume. Formula to calculate the density is:

\text{density}=\frac{\text{mass}}{\text{volume}}\\
\text{It's SI unit is kg/m}^{3}

12. States Archimedes’ principle.
Ans: It states that “When a body is partially or completely immersed in water it experiences an upthrust which is equal to the weight of the liquid displaced by it.”

13. Name the forces acting on a body immersed in liquid.
Ans: The forces acting on a body immersed in liquid are:

1. Weight of the body acting vertically downward.
2. Upthrust on body acting vertically upward.
   

12. State principle of floatation.
Ans: It states that “A floating body displaces liquid equal to its weight.”
Note: weight of floating object = weight of the liquid displaced

13. What is the relation between the density of body and floatation? Or, State the condition that determines the sinking or floating of the body when it is kept in the liquid?
Ans: The condition that determines the sinking or floatation of the body are:

1. When weight of the body is greater than upthrust or when density of body is greater than density of liquid, body sinks.
2. When weight of the body is less than upthrust or when density of body is less than density of liquid, body floats partially immersed.
3. When weight of the body is equal to upthrust or when density of body is equal density of liquid, body floats freely under liquid surface.
   

In short:

Let, W = weight of the body and U = Upthrust provided by liquid then,

• When W > U, it sinks
• When W < U, body floats partially above the surface of liquid.
• When W = U, body floats freely under the surface of liquid
  

16. Write down the working mechanism of the syringe.
Ans: At first the piston is pushed inward up to the mouth of the storage cylinder (barrel) and then the needle is inserted in a liquid medium (liquid medicine). When the piston is pulled outward, a partial vacuum is created inside the barrel which reduces the atmospheric pressure of the barrel. Liquid always from a region of high pressure to a region of low pressure. As a result, liquid start to fill inside the barrel. In this way syringe work.

17. Three objects with different densities A, B and C are in the water as shown in the figure. Answer the following questions.

a. Which object has a higher density than water?
Ans: Object C has a higher density than water.

b. If the mass of object A is 1 kg. How much kg of water is displaced by A?
Ans: A displaces 1 kg of water because object A is floating on water and follow the law of floatation i.e. mass of floating object = mass of liquid displaced

c. Which law is applicable for object B?
Ans: Law of floatation is applicable for B.

d. Which law is applicable for both objects A and C?
Ans: Archimedes’ law is applicable for both objects A and C.

e. What is the density of object B?
Ans: The density of object B is 1000kg/m³ or 1g/cm³.

f. Which objects displace water equal to its weight?
Ans: Objects A and B displace water equal to its weight.

18. Study the given diagram and answer the following question.

a. What is the weight of the stone in air?
Ans: Weight of stone in air = weight of the stone in water + weight of the liquid displaced
= 20 N + 5 N = 25 N

b. How much upthrust is exerted by water on the object?
Ans: Upthrust = weight of liquid displaced = 5 N

c. What is the mass of the stone?
Ans: Weight of stone in air = mass x acceleration due to gravity

\begin{aligned}or,\ 25&=m\times 9.8\\
or,\ m &= \frac{25}{9.8}\\
\therefore m &=2.55\ kg
\end{aligned}

d. On which law is the above experiment-based? State the law.
Ans: The above experiment is based on Archimedes’ principle. It states that “When a body is partially or completely immersed in liquid, it experiences an upthrust which is equal to the weight of liquid displaced.”

e. In which condition the weight of the stone will be more and why?
Ans: The weight of the object will be more in the air because it exerts less upthrust on the stone than water.

Note: More upthrust means more loss of weight of an object in a medium.

f. What will happen in the weight of the stone if the water of eureka can is replaced by saltwater?
Ans: The weight of the stone will be less if the water of the eureka can is replaced by saltwater.

Note: The density of saltwater is more than normal water which means salt water provides more upthrust on the stone. As a result weight loss by stone in salt water will be more than weight loss in normal water.

g. Calculate the mass of displaced water?
Ans: weight of displaced water = mass of displaced water x acceleration due to gravity.
or, 5 = m x 9.8
or, m = 5/9.8 = 0.51 kg

Therefore, mass of displaced liquid is 0.51 kg.

h. Determine the volume of displaced water.
Ans: Upthrust = weight of liquid displaced = Vdg
Or, 5 = V x 1000 x 9.8
Or, V = 0.000051 m³

Therefore, the volume of displaced liquid is 0.000051 m³

i. What is the volume of stone?
Ans: Volume of stone = volume of displaced water = 0.000051 m³ (since the stone is completely immersed in water)

19. Loaded ship sinks more than an empty one. Why?
Ans: The weight of a loaded ship is more than that of an empty ship. In order to displace more water equal to its weight, the loaded ship sinks more than the empty one.

20. An egg sinks in pure water but floats on saltwater. Why?
Ans: As we know that upthrust is directly proportional to the density of the medium. The density of saltwater is more than that of pure water, Due to this upthrust provided by saltwater on the egg is sufficient enough to make it floats whereas upthrust provided by pure water on the egg is not enough to make it float.

21. Write two instruments based on the law of floatation.
Ans: Hydrometer (used to measure the density of the liquid and relative density of substance)
Lactometer (used to measure the density of milk or purity of milk)

22. Write two applications of the law of floatation.
Ans: Two applications of the law of floatation are:

1. To make hydrometer and lactometer.
2. To make ship and submarines.
   

23. Why liquid is used in hydraulic machines? Or, By which two properties of liquid it is used in hydraulic machines? Ans: Liquid are used in hydraulic machines because of the following reasons:

1. Liquid cannot be compressed.
2. Liquid can transmit pressure equally and perpendicularly in all directions.
   

24. Write two conditions for a substance to float on a liquid.
Ans: Two conditions for a substance to float on liquid are:

1. Weight of substance must displace the liquid equal to its weight.
2. Density of substance must be equal or less than density of liquid.
   

25. Explain the working mechanism of the hand pump/ water pump.
Ans:

Working of water pump involves two-stroke: upstroke and downstroke

Upstroke: When the handle of a pump is pushed downward, the piston moves upward creating a partial vacuum between the piston valve (V1) and the foot valve (V2) due to which V2 opens and V1 remains closed. In this condition, water enters the barrel due to atmospheric pressure.

Downstroke: When the handle of a pump is pulled upward, the piston moves downward creating a small gap between V₁ and V₂. In this case, V₁ opens and V₂ remains closed due to water pressure and water is transferred to the upper part of the barrel and water comes out through the spout.