Science & Technology · Chapter 08

35 solved questions

Chapter 8: Pressure

Every question in this chapter, answered and explained — step-by-step solutions drawn live from the chapter library across 8 question sections.

Very Short Answer Questions8 Short Answer Questions6 Give Reason Questions6 Differentiate Between3 Long Answer Questions4 Numerical Problems3 Diagram-Based Questions2 Application / Conceptual Questions3

Very Short Answer Questions

1SEE Boardqb-q0

What is thrust?

Ans.Thrust is a force that acts perpendicular to the surface of an object. Its SI unit is Newton (N).

2SEE Boardqb-q1

Define pressure and write its SI unit.

Ans.Pressure is the thrust acting per unit area of a surface (P = F/A). Its SI unit is Pascal (Pa), equal to N/m^2.

3SEE Boardqb-q2

What is a fluid?

Ans.A substance that can flow easily, such as a liquid or a gas, is called a fluid.

4SEE Boardqb-q3

State Pascal's law.

Ans.Pascal's law states that when a force is exerted at a point in an enclosed liquid, the pressure generated is transmitted equally in all directions throughout the liquid.

5SEE Boardqb-q4

What is upthrust?

Ans.The net upward force exerted by a fluid on an object partially or completely immersed in it is called upthrust. Its SI unit is Newton (N).

6SEE Boardqb-q5

State Archimedes' principle.

Ans.When an object is partially or completely immersed in a liquid, the upthrust on it is equal to the weight of the liquid displaced by it.

7SEE Boardqb-q6

State the law of floatation.

Ans.For an object to float in a liquid, the weight of the object must be equal to the weight of the liquid it displaces.

8SEE Boardqb-q7

What is a hydraulic machine?

Ans.A hydraulic machine is a force-multiplying device based on Pascal's law, using a small piston and a large piston connected through an incompressible liquid.

Short Answer Questions

1SEE Boardqb-q8

Write any two applications of Pascal's law in daily life.

Ans.Two applications are (i) hydraulic brakes, which use a small pedal force to stop a vehicle, and (ii) hydraulic lifts, which use a small input force to lift heavy objects like cars.

2SEE Boardqb-q9

Write any two applications of Archimedes' principle.

Ans.Archimedes' principle is applied in (i) the design of ships and submarines to control floating and sinking, and (ii) the working of a hydrometer, which measures the density of a liquid.

3SEE Boardqb-q10

Mention the forces acting on a floating object and their directions.

Ans.Two forces act on a floating object: its weight, which acts downward due to gravity, and the upthrust (buoyant force) of the liquid, which acts upward. For the object to float, these two forces must be equal and opposite.

4SEE Boardqb-q11

Why is upthrust said to depend on the volume of liquid displaced?

Ans.As more of an object gets submerged, it displaces more liquid, and the upthrust increases proportionally. Once the object is fully submerged, the volume displaced stays constant, so upthrust also stays constant even if the object goes deeper.

5SEE Boardqb-q12

Explain why a hydraulic machine can lift heavy loads with a small force.

Ans.In a hydraulic machine, the pressure applied on the small piston is transmitted equally to the large piston (Pascal's law). Since pressure = force/area, and the large piston has a much bigger area, the same pressure produces a much larger force on it. This is why a small input force can lift a heavy load.

6SEE Boardqb-q13

What is the difference between real weight and apparent weight?

Ans.Real weight is the weight of an object measured in air, while apparent weight is the weight of the same object measured while it is submerged in a liquid. Apparent weight is less than real weight because of upthrust.

Give Reason Questions

1SEE Boardqb-q14

Why is a special type of oil used in hydraulic brakes?

Ans.A special brake oil is used because it does not compress easily and can transmit pressure effectively from the pedal piston to the brake pistons, allowing the vehicle to stop quickly and reliably.

2SEE Boardqb-q15

Why does a wooden cork pushed below water come back to the surface when released?

Ans.When the cork is submerged, the upthrust acting on it becomes greater than its weight. When the finger is removed, this excess upward force pushes the cork back up to the surface, where it floats.

3SEE Boardqb-q16

Why do we feel lighter while floating in water?

Ans.We feel lighter because water exerts an upthrust on our body in the upward direction, opposite to our weight. This upthrust reduces the net downward force we feel, making us feel lighter.

4SEE Boardqb-q17

Why does a stone submerged in water feel heavier when it comes out of the water?

Ans.Inside water, upthrust acts on the stone and reduces its apparent weight. Once it comes out of the water, this upthrust disappears, so the full real weight of the stone is felt, making it feel heavier.

5SEE Boardqb-q18

Why is it easier to float in the Dead Sea than in a normal swimming pool?

Ans.The water in the Dead Sea has a very high density (1240 kg/m^3) due to dissolved salt. Since upthrust is directly proportional to the density of the liquid, the Dead Sea exerts much more upthrust on a person, making it easier to float.

6SEE Boardqb-q19

Why is a boat likely to sink if more passengers climb aboard than its maximum capacity?

Ans.As passengers board, the boat sinks lower to displace more water and generate more upthrust to balance the added weight. But once the boat sinks to its rim, it cannot displace any more water, so upthrust cannot increase further. Beyond this point, added weight exceeds upthrust and the boat sinks.

Differentiate Between

1SEE Boardqb-q20

Differentiate between pressure and upthrust.

Pressure	Upthrust
Thrust acting per unit area of a surface.	Net upward force exerted by a fluid on an immersed object.
Can act in any direction.	Always acts vertically upward.
SI unit is Pascal.	SI unit is Newton.

2SEE Boardqb-q21

Differentiate between the reason for a steel pin sinking and a steel plate floating on water.

Steel Pin (Sinks)	Steel Plate (Floats)
Small surface area, so it displaces very little water.	Large flat surface area, so it displaces a lot of water.
Upthrust generated is less than its weight.	Upthrust generated equals its weight.
Sinks in water because net force is downward.	Floats because net force is zero.

3SEE Boardqb-q22

Differentiate between transmission of pressure in solids and in liquids.

Solids	Liquids
Molecules do not move; pressure is not transmitted throughout the object.	Molecules can move; pressure is transmitted equally in all directions.
Applied force stays localized to the point of contact.	Applied force spreads instantly through the enclosed liquid.

Long Answer Questions

1SEE Boardqb-q23

Explain, with a figure, the cause of the production of upthrust.

Ans.When an object is immersed in a liquid, the liquid exerts pressure on all its surfaces. Since pressure increases with depth, the pressure (and hence the force) on the lower surface of the object is greater than the pressure on its upper surface. The forces on the left and right (or front and back) side surfaces are equal and opposite, so they cancel out. Subtracting the smaller upward-facing force on the top from the larger upward-facing force on the bottom gives a net resultant force in the upward direction. This net upward force is called upthrust.

2SEE Boardqb-q24

A person is trying to lift a stone of 500 N using the smaller piston of a hydraulic machine by applying a force on the larger piston. What suggestion would you give so the load can be lifted easily? Explain with a figure.

Ans.The person is applying force on the wrong piston. In a hydraulic machine, the load should be placed on the LARGER piston and the input force should be applied on the SMALLER piston, not the other way around. Since A2/A1 (large piston area / small piston area) is always greater than 1, a small force on the small piston produces a much larger force on the large piston, according to F2 = F1 x (A2/A1). If the stone is placed on the large piston, a much smaller input force on the small piston will be enough to lift it easily.

3SEE Boardqb-q25

State Archimedes' principle and explain how it applies to the floating of ships.

Ans.Archimedes' principle states that when an object is partially or completely immersed in a liquid, the upthrust acting on it is equal to the weight of the liquid displaced by it (U = V x rho x g).

4SEE Boardqb-q26

Explain the working mechanism of a hydraulic brake system with a figure.

Ans.In a hydraulic brake system, the pipe connecting the master cylinder and the brake cylinders is filled with special brake fluid. In four-wheeled vehicles, the piston of the master cylinder is connected to the brake pedal; in two-wheelers, it is connected to the brake lever. Brake cylinders are located at the wheels, and their pistons are connected to brake pads or brake shoes. When the driver applies a small force on the master cylinder's piston, the pressure created is transmitted equally through the brake fluid, and a large force is generated on the piston of the brake cylinder. This large force presses the brake pads against the wheel disc, stopping a fast-moving vehicle almost instantly.

Numerical Problems

1SEE Boardqb-q27

In a hydraulic lift, the cross-sectional areas of the small piston and large piston are 0.25 m^2 and 5 m^2 respectively. Calculate the force required to lift a car of mass 1200 kg.

Ans.Given: A1 = 0.25 m^2, A2 = 5 m^2, mass of car m = 1200 kg

Required: Force F1 on the small piston

2SEE Boardqb-q28

A hydraulic machine constructed with syringes has piston A (area 0.5 cm^2) and piston B (area 5 cm^2). A force of 20 N is applied on piston A. How much force must be applied through piston B to balance it?

Ans.Given: Area of piston A (A1) = 0.5 cm^2, Area of piston B (A2) = 5 cm^2, Force on piston A (F1) = 20 N

Required: Force F2 on piston B

3SEE Boardqb-q29

Pressure of 30000 Pa is generated in the liquid of a hydraulic lift. If the cross-sectional area of the piston used to lift a weight is 0.1 m^2, how much load can it lift?

Ans.Given: Pressure P = 30000 Pa, Area A = 0.1 m^2

Required: Load (force) F that can be lifted

Diagram-Based Questions

1SEE Boardqb-q30

Draw and label a diagram showing the direction of upthrust on a cube immersed in water.

Ans.Draw a cube fully submerged in a beaker of water. Mark pressure arrows pointing inward on all six faces, with longer arrows on the bottom face (greater pressure, greater depth) than on the top face (lesser pressure, lesser depth). Label the net resultant force as an upward arrow labelled 'Upthrust'.

2SEE Boardqb-q31

Draw and label the diagram of a hydraulic machine showing the input and output pistons.

Ans.Draw a U-shaped or connected cylinder system with a small piston on one side (label: small piston, area A1, input force F1) and a large piston on the other side (label: large piston, area A2, output force F2). Show the incompressible liquid filling the connected cylinders, and label the formula F2 = F1 x (A2/A1) below the diagram.

Application / Conceptual Questions

1SEE Boardqb-q32

Additional SEE Practice — Q1. Two identical balloons are released, one filled with air and the other with hydrogen. What difference do you expect and why?

Ans.The hydrogen-filled balloon will rise into the air, while the air-filled balloon will simply fall or stay at the same level. This is because hydrogen is much less dense than the surrounding air, so the weight of air displaced by the hydrogen balloon (upthrust) is greater than the balloon's own weight, causing it to rise. The air-filled balloon has almost the same density as the surrounding air, so there is little or no net upward force.

2SEE Boardqb-q33

Additional SEE Practice — Q2. An object is floating just below the surface of water. If more and more salt is dissolved in the water, what happens to the object, and why?

Ans.As salt is dissolved, the density of the water increases, so the upthrust exerted on the object also increases (upthrust is proportional to the density of the liquid). Since the object's weight stays the same but the upthrust on it keeps increasing, the object will gradually rise higher until it floats mostly above the surface, similar to how an egg floats better in salty water than in plain water.

3SEE Boardqb-q34

Additional SEE Practice — Q3. When a person puts a lactometer into milk, the entire narrow stem sinks into the milk. What conclusion can be drawn?

Ans.A lactometer sinks more when the density of the liquid is low, and floats higher when the density is high. Since the entire stem sinks, it means the milk has a lower density than pure milk should have. This suggests that the milk may have been mixed with water, since water has a lower density than milk and would reduce the overall density of the mixture.

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