Introduction
HSA Notes brings complete, exam-focused solved notes to help Class 11 (1st Year, HSSC-I) students score high marks in their FBISE Physics board exam.
This page covers Unit 6, Fluid Mechanics, prepared strictly according to the FBISE New Book/Course syllabus for the current academic session, covering upthrust, Archimedes’ principle, and the basics of fluid statics and dynamics, along with fully solved MCQs, short and comprehensive questions, numericals, and assignments from HSA Notes.
Fluid Mechanics
Definition: Fluid Mechanics is the branch of Physics that studies fluids, meaning liquids and gases, both at rest, known as fluid statics, and in motion, known as fluid dynamics. T
his unit is crucial not only for board exams but also for students planning to pursue engineering or medicine, since concepts like pressure, flow rate, and viscosity appear directly in those fields.
Upthrust and Archimedes’ Principle
Definition: Upthrust, also called buoyant force, is the upward force exerted by a fluid on an object immersed in it, opposing the weight of the object. Archimedes’ Principle states that this upthrust is equal to the weight of the fluid displaced by the object.
Explanation: Upthrust arises because pressure in a fluid increases with depth. An object submerged in a fluid experiences greater pressure on its lower surface than on its upper surface, and this pressure difference produces a net upward force, the upthrust.
Derivation: Consider a cylinder of cross-sectional area A and height h fully submerged in a fluid of density ρ_f. The buoyant force is derived as:
F_B = ρ_f g A h = ρ_f g V
where V = Ah is the volume of fluid displaced by the cylinder, and this result confirms Archimedes’ Principle, since ρ_f g V is exactly the weight of the displaced fluid.
Examples: A piece of wood floats because the upthrust acting on it equals its weight, while a stone sinks because its weight exceeds the maximum upthrust the fluid can provide.
Exercises Solved
This session’s Unit 6 exercise solutions from HSA Notes cover every question type set by FBISE, with full working shown for each answer.
Solved MCQs
- The principle used to find the density of an object using floatation is: A. Density
- According to Archimedes’ Principle, upthrust equals: A. Weight of displaced liquid
- A sphere falling through a viscous fluid under Stokes’ law eventually moves with: B. Constant velocity
- When two small drops combine into one larger drop, the new terminal velocity is calculated as: C. 32 cm/s
Short Responses
6.1 Why do athletes wear body-hugging suits?
Streamlined, body-hugging suits reduce drag by minimizing air resistance, since their smooth shape reduces turbulence around the athlete’s body during motion.
6.2 What is the difference between turbulent and streamline flow?
In streamline flow, fluid particles move in smooth, orderly layers without crossing paths, whereas in turbulent flow, the motion is chaotic and irregular, with fluid particles mixing unpredictably.
6.3 Why does water flow faster where a river narrows?
This follows from the equation of continuity, which states that for an incompressible fluid, the product of cross-sectional area and flow speed remains constant; a wider section of the river has slower flow, while a narrower section has faster flow.
Long Responses
6.1 Explain upthrust with examples.
Upthrust is the upward force exerted on an object by the fluid it is immersed in, equal to the weight of the fluid displaced. Wood floats because the upthrust it experiences equals its own weight, while a stone sinks because its weight is greater than the maximum upthrust the water can exert on it.
6.2 State and explain the principle of floatation.
An object floats in a fluid when the upthrust acting on it equals its own weight. This session’s HSA Notes solution derives this relationship in full and connects it to practical applications such as ships, submarines, and hydrometers.
Numericals
6.1 A fog droplet has a terminal velocity v_t = 2.98 mm/s, and the fluid has a coefficient of viscosity η = 1.9 × 10⁻⁵ kg/m·s. Find the radius of the droplet.
Using the terminal velocity formula:
v_t = [2(ρ_obj − ρ_fl) r² g] / 9η
Solving for r gives r = 5.1 × 10⁻⁶ m.
Assignments
Assignment 6.1 Iron has a density of 7.8 g/cm³ and appears 200 N lighter when weighed in water. Find:
(a) The volume of the iron object → V = 0.0255 m³
(b) The weight of the object in air → Weight in air = 1950 N
HSA Notes Features
- Complete solved MCQs based on the FBISE paper pattern
- Short and comprehensive questions with clear answers
- Solved numericals with full working and correct units
- Solved assignments matching the current session’s syllabus
- Solved textbook exercises covering every question
- Full Fluid Mechanics notes for Class 11, based on the FBISE new book/course, available in PDF form at hsanotes.com
Why This Unit Matters
Fluid Mechanics connects directly to real-world applications that students will encounter again in higher studies, particularly in engineering and medicine.
Available Across All FBISE Institutions
These notes are built for students across all FBISE-affiliated schools and colleges, including Islamabad Model Colleges, F.G. Colleges, Army Public Schools & Colleges, Punjab Colleges, Dr. A.Q. Khan Colleges, Fazaia Inter Colleges, IMCB, IMCG, and Sir Syed Colleges.
Join Online Classes
Join HSA Notes online classes to strengthen your concepts in Fluid Mechanics and every other unit of Class 11 Physics, with regular past paper practice to help you aim for 90 plus marks in your FBISE board exam this session.
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Frequently Asked Questions
What is upthrust and how is it related to Archimedes’ Principle?
Upthrust is the upward force a fluid exerts on an immersed object, and Archimedes’ Principle states that this force equals the weight of the fluid displaced by the object.
Why does an object float or sink in a fluid?
An object floats when the upthrust acting on it equals its weight, and sinks when its weight exceeds the maximum upthrust the fluid can provide.
What is the equation of continuity used for?
It explains why fluid speed increases in narrower sections of a pipe or river and decreases in wider sections, based on the principle that the product of area and velocity remains constant for an incompressible fluid.
Are these Fluid Mechanics notes updated for the current FBISE session?
Yes, these notes are prepared according to the current FBISE New Book/Course syllabus for this session’s Class 11 Physics exam.
Conclusion
With Unit 6, Fluid Mechanics, fully covered, Class 11 FBISE students now have a solid understanding of upthrust, Archimedes’ Principle, and the basics of fluid flow, along with fully solved MCQs, questions, numericals, and assignments for this session. Practice these solved exercises alongside past papers, and consider joining our online classes for guided, exam-focused preparation, to build the accuracy and confidence needed for your board exam.
Best of luck for your Class 11 Physics exam!