Class 10 Science Lesson 8

Exercise

1. Choose the correct options for the given questions.

(a) On which law/principle is a lactometer based?

Answer: (ii) Archimedes’ principle

(b) What is the condition for flying a hydrogen balloon upwards in the air?

Answer: (iv) weight of balloon < weight of air displaced

(c) In a hydraulic machine, if the cross-sectional area of the larger piston is twice that of the smaller piston, what is the correct group of weights on the pistons to keep the machine balanced?

Answer: (ii) W₁ = 5 N, W₂ = 10 N

(d) When an object is suspended using spring balance in air, water, and glycerin, the weights are W₁, W₂, and W₃ respectively. What will be the weight of the object in those mediums in increasing order?

Answer: (iii) W₃ < W₂ < W₁

(e) When a ship enters a river from the sea, it sinks more than before. What is the reason for this?

Answer: (ii) the density of seawater is more than that of river water

(f) Which one of the following statements is correct for a hand pushing a ball into the water?

Answer: (i) equal pressure acts on all parts of the ball under the water

(g) If the same magnitude of upthrust acts on three cubical balls made of different materials in water, which one of the following quantities is equal for them?

Answer: (iii) volume

(h) What is the upthrust when the cork shown in the figure is placed in the liquid?

Answer: (ii) 3 N

2. Differentiate between:

(a) Pressure and Upthrust

(b) Steel pin sinking in water and a steel plate floating on water

(c) hot air balloon sinking and rising in the air

3. Give a reason for:

(a) A special type of oil is used in hydraulic brakes.

Ans: Hydraulic oil is used because it is incompressible and transmits pressure efficiently through the brake system. Also, it has a high boiling point, preventing vaporization under high heat.

(b) A wooden cork dipped below the surface of the water by applying force with a finger comes to the surface when the finger is removed.

Ans: The cork is less dense than water, so it experiences a greater upward buoyant force. When the external force is removed, this unbalanced upthrust pushes it to the surface.

(c) We feel lighter while floating on water.

Ans: Water exerts an upward buoyant force equal to the weight of the water displaced. This force reduces the apparent weight, making us feel lighter.

(d) On lifting a stone submerged in water, it feels heavier when it comes out of the water.

Ans: When submerged, the stone experiences upthrust which reduces its apparent weight. Once out of water, the upthrust disappears, and we feel the full weight.

(e) It is easier to float in the Dead Sea than in a swimming pool (the density of water in the Dead Sea is 1240 kg/m³).

Ans: Dead Sea water is denser, so it provides greater upthrust for the same volume displaced. This makes it easier for our body to stay afloat.

(f) If more passengers climb a boat than its maximum capacity, the boat is likely to sink.

Ans: When the boat’s weight exceeds the buoyant force (maximum water it can displace), the balance is lost, and the boat sinks.

4. Answer the following questions:

(a) State Pascal’s Law.

Answer: Pascal’s law states that when pressure is applied to a confined fluid, the pressure is transmitted equally in all directions through the fluid.

(b) Write any two applications of Pascal’s Law in daily life.

Answer:

1. Hydraulic brakes in vehicles
2. Hydraulic lifts used in service stations

(c) What is a hydraulic machine?

Answer: A hydraulic machine is a device that uses the principle of Pascal’s law to multiply force and perform mechanical work using a liquid under pressure.

(d) State Archimedes’ Principle.

Answer: Archimedes’ principle states that a body partially or fully immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by it.

(e) What is upthrust?

Answer: Upthrust is the upward force exerted by a fluid on any object placed in it. It opposes the weight of the object.

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

Answer:

1. Weight of the object acts downward.
2. Upthrust from the fluid acts upward.

(g) State the law of floatation.

Answer: An object floats in a fluid if the weight of the object is equal to the weight of the fluid displaced by it.

(h) Write any two applications of Archimedes’ Principle.

Answer:

1. Designing ships and submarines to float.
2. Determining the purity and density of metals.

(i) Explain, with a figure, the cause of the production of upthrust.

Answer: Upthrust is produced because of the difference in fluid pressure at different depths. When an object is immersed in a fluid, the pressure at the bottom surface is greater than at the top surface, as pressure in a fluid increases with depth. This pressure difference causes a net upward force, which is called upthrust or buoyant force.

This force acts against gravity and helps objects float or reduces their apparent weight in a fluid.

(j) When a stone is dropped into the water, it sinks. Does this happen because of the absence of upthrust?

Answer: No, the stone does not sink due to the absence of upthrust. In fact, every object submerged in a fluid experiences upthrust. The stone sinks because its weight is greater than the upthrust provided by the water. Since the density of the stone is much higher than that of water, it displaces a smaller amount of water and receives less buoyant force, causing it to sink.

(k) A person is trying to lift a 500 N stone on the smaller piston of a hydraulic machine by applying a force on the larger piston. What suggestion would you give? Explain with figure.

Answer: In a hydraulic machine, Pascal’s Law applies — it states that pressure applied to a confined fluid is transmitted equally in all directions. The pressure generated on one piston is transferred through the fluid to the other piston.

In this case, the person is trying to lift a heavier object (500 N) placed on the smaller piston by applying force on the larger piston, which is not the correct method.

Suggestion:

The person should place the heavier object (500 N stone) on the larger piston and apply the force on the smaller piston.

 Reason:

• In a hydraulic system, a small force applied on a smaller piston can lift a larger load on a larger piston, because:

where:

• F1is the applied force
• A1 is the area of the smaller piston
• F2 is the output force (in this case, lifting the stone)
• A2 is the area of the larger piston

– If the heavy object is on the smaller piston, the person would need to apply even more force on the larger piston, defeating the purpose of using a hydraulic machine, which is to multiply force.

To make the work easier, place the heavy object on the larger piston and apply a smaller force on the smaller piston, allowing the hydraulic system to lift the stone efficiently.

(l) Substances with a density greater than the density of the liquid sink in it. Is this always true? Justify with an example.

Answer: This statement is generally true, but not always. While an object with higher density than the fluid usually sinks, there are exceptions where the shape and volume of an object also matter. For example, a steel ship floats in water despite steel being denser than water. This is because the ship’s overall average density, including the air inside it, is less than that of water. Hence, floatation depends on the average density of the object, not just the material.

(m) Two balloons, one filled with air and the other with hydrogen, look identical. What difference can be noticed when they are released into the air? Explain with reasons.

Answer: When released, the hydrogen-filled balloon will rise rapidly, while the air-filled balloon may stay near the ground or slowly descend. This is because hydrogen is lighter (less dense) than air, so it experiences a greater upthrust compared to its weight. In contrast, an air-filled balloon has nearly the same density as the surrounding air, so it doesn’t receive enough upthrust to rise significantly.

(n) An object is floating just below the surface of the water. If more salt is added to the water, what change will occur in the position of the object? Explain with reasons.

Answer: When salt is added to water, the density of water increases. As a result, the upthrust on the object increases because the same volume of denser water weighs more. Therefore, the object now needs to displace less water to balance its weight, and it will float higher than before, possibly even partially above the surface.

o) What are the readings shown by the weighing machine given in figures (a) and (b)? Explain with reason.

Answer: The reading in figure’a’ will be 25N when a wood of 5N is added because displaced water is still in the pot. The readings in the figure ‘a’ will be 28N when wood of 8N is added because displaced water is still in the pot.

The reading in the figure ‘b’ will be 30N when adding wood of either 5N or 8N because displaced water has gone out of pot.

(p) When a person puts a lactometer into the milk, the whole part of the narrow stem sinks into the milk. What conclusions can be drawn from this event?

Answer: If the narrow stem of the lactometer sinks, it indicates that the density of the milk is lower than normal, likely due to adulteration with water. A lactometer floats higher in pure milk because of its higher density. When diluted, the milk can no longer provide the same upthrust, causing the instrument to sink deeper.