MCQ : Gravitation (Floatation) - 1 - Class 9

Density is one factor affecting upthrust (buoyant force). If the density of the substance is more than the density of the fluid, then the upthrust will be less and thus the substance sinks. If the density of the substance is less than the density of the fluid, then the upthrust will be more and thus the substance float.

Archimedes' Principle:
According to Archimedes' principle, when a body is submerged in a liquid, it experiences an upward buoyant force equal to the weight of the liquid displaced by the body. This principle helps in understanding the behavior of objects in fluids, such as water.
Effect on Apparent Weight:
When a body is submerged in a liquid, its apparent weight, which is the weight perceived by an observer, undergoes a change. The apparent weight is the difference between the actual weight of the body and the buoyant force acting on it.
The effect on the apparent weight depends on the relationship between the weight of the body and the buoyant force:
1. If the weight of the body is less than the buoyant force, the body will float and the apparent weight will be less than the actual weight. In this case, the body is buoyed up by a force greater than its own weight, resulting in a decrease in the apparent weight.
2. If the weight of the body is equal to the buoyant force, the body will be in equilibrium and the apparent weight will be equal to the actual weight. In this case, the body neither sinks nor floats, and the apparent weight remains unchanged.
3. If the weight of the body is greater than the buoyant force, the body will sink and the apparent weight will be greater than the actual weight. In this case, the body is pulled down by a force greater than its own weight, resulting in an increase in the apparent weight.
Conclusion:
Based on Archimedes' principle, the apparent weight of a body submerged in a liquid will either decrease or increase depending on whether the body floats or sinks. Therefore, the correct answer is option A: decreases.

Upthrust = apparent loss of weight of object = weight in air - weight in liquid.

weight in air= W
Upthrust ,U= W/4
weight in liquid = weight in air- Upthrust
= W - W/4
=3W/4

 

Archimedes principle states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principal applies to both floating and submerged bodies and to all fluids, i.e. liquid and gases. If the body is denser than the fluid it will sink.

Given:
- Height of the liquid in the tank: h
- Depth of points A and B from the free surface: 3h/4
- Point A is closer to the wall than point B
Explanation:
- The pressure at any point in a fluid is directly proportional to the depth of the point from the free surface.
- Since point A is closer to the wall, its depth is greater than that of point B.
- Therefore, the pressure at point A will be greater than the pressure at point B.
Conclusion:
The correct answer is C: P_A = P_B.

 The correct answer is a

   Archimedes' principle, a physical law of buoyancy, states that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body.

When the heavy cylinder is slowly taken out of a dense liquid, the weight felt by the cylinder will change. Here's a detailed explanation of why the weight increases till it attains the weight in air:
- Weight in liquid: When the cylinder is fully submerged in the dense liquid, it experiences a buoyant force equal to the weight of the liquid displaced by the cylinder. This buoyant force partially cancels out the weight of the cylinder, making it feel lighter than its actual weight in air. So, the weight felt in the liquid is less than the weight in air.
- Weight as it comes out: As the cylinder is slowly taken out of the liquid, the buoyant force acting on it decreases gradually. This is because the amount of liquid displaced by the cylinder decreases as it comes out of the liquid.
- Weight increase: Since the buoyant force decreases, the net force acting on the cylinder increases. According to Newton's second law of motion (F = ma), an increase in net force leads to an increase in acceleration. In this case, the acceleration is upwards, opposite to the direction of gravity. As a result, the weight felt by the cylinder increases as it comes out of the liquid.
- Weight in air: Once the cylinder is completely out of the liquid, there is no more buoyant force acting on it. At this point, the weight felt by the cylinder is equal to its weight in air.
In conclusion, the weight felt by the cylinder increases as it comes out of the dense liquid until it attains the weight in air. Therefore, the correct answer is option D: increases till it attains the weight in air.

Reasons for making the walls of a dam at the bottom thick:


There are several reasons why the walls of a dam at the bottom are made thick, with the primary reason being the large pressure exerted by water at the bottom. This pressure increases with the depth of the water and can have significant consequences if not properly accounted for.
Other reasons include:
1. Strength and Stability: Thick walls provide the necessary strength and stability to withstand the immense pressure of the water. This prevents the dam from collapsing or being structurally compromised.
2. Water Load: The weight of the water creates a vertical load on the dam. By making the walls thick, the dam is better able to distribute and handle this load, ensuring its overall stability.
3. Prevention of Leakage: Thick walls help minimize the risk of water leakage through the dam. The pressure exerted by the water can cause seepage if the walls are not adequately thick, potentially leading to erosion and compromising the dam's integrity.
4. Resistance to External Forces: Thick walls provide resistance against external forces such as waves, currents, and impacts from floating debris. This further enhances the dam's ability to withstand various environmental conditions and potential hazards.
5. Longevity: The thick walls contribute to the long-term durability of the dam. By ensuring the structural integrity, the dam can effectively serve its purpose over an extended period without requiring frequent repairs or maintenance.
Overall, the thickness of the walls at the bottom of a dam is crucial for maintaining its stability, preventing leakage, and withstanding the immense pressure exerted by the water. These factors are essential for ensuring the safety and longevity of the dam structure.

Given:
Weight of the object in air = 10 N
Weight of the object in water = 8 N
To find:
Weight of the liquid displaced by the object
Explanation:
When an object is immersed in a fluid, it experiences an upward force called buoyant force. This force is equal to the weight of the fluid displaced by the object.
The weight of the object in air is equal to the force of gravity acting on it. Therefore, the weight of the object is 10 N.
When the object is fully immersed in water, it appears to weigh less because it experiences an upward buoyant force. The weight of the object in water is 8 N.
To find the weight of the liquid displaced by the object, we need to find the difference between the weight of the object in air and the weight of the object in water.
Weight of the liquid displaced = Weight of the object in air - Weight of the object in water
Weight of the liquid displaced = 10 N - 8 N
Weight of the liquid displaced = 2 N
Therefore, the weight of the liquid displaced by the object is 2 N.
Answer:
The weight of the liquid displaced by the object is 2 N. Option A is the correct answer.

Pressure on membrane is P=ρgh

Since the density of both membranes is same.

Since both the membranes are placed at same depth ,So pressure will be same on both of then .

P₂​/P₁​​=ρgh/ρgh​=1