Upthrust IN Fluids, Archimedes’ Principle AND Floatation PDF

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Selina Concise Chemistry Solutions Class 10 Chapter 10 Study of Compounds Nitric Acid...

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CHAPTER 5 (Part -1)

UPTHRUST IN FLUIDS, ARCHIMEDES’ PRINCIPLE AND FLOATATION --------------------------------------------------------------------------------------Upthrust When a body is partially or wholly immersed in a liquid, an upward force acts on it. This upward force is known as an upthrust or buoyant force. Unit of upthrust: newton(N) or kgf. Buoyancy The property of liquid to exert an upward force when a body is immersed in it, is called buoyancy. Upthrust can be demonstrated by the following experiment: Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with water. It floats with a large part of it above the surface of water and only a small part of it below the surface of water. Push the can into the water. You can feel an upward force and you find it difficult to push the can further into water. It is noticed that as the can is pushed more and more into the water, more and more force is needed to push the can further into water, until it is completely immersed. When the can is fully inside the water, a definite force is still needed to keep it at rest in that position. Again, if the can is released in this position, it is noticed that the can bounces back to the surface and starts floating again. Note: Like liquids, gases also have the property of buoyancy.

Condition for a body to float or sink in a fluid. 1

1. F’B > W or ρL > ρ The body floats partly immersed with only that much part of it inside liquid. 2. F’B = W or ρL = ρ The body will float with whole of it just immersed inside the liquid. Thus, for a floating body, net force acting downwards is zero. 3. F’B < W or

ρ > ρL The body will sink due to the net force acting

downwards. Bodies of density greater than that of liquid, sink in it, while bodies of average density equal to or smaller than that of liquid, float on it. Characteristic properties of upthrust • Larger the volume of body submerged in fluid, greater is the upthrust. • For same volume, inside the fluid more the density of fluid, greater is the upthrust. • The upthrust acts on the body in upward direction at the centre of gravity of the displaced fluid which is called the centre of buoyancy. Reason for Upthrust The thrust acting on the sides of walls of the body get neutralised because they are equal in magnitude and opposite in the directions. There is a pressure difference between the lower and upper faces of block. Since force is the product of pressure and area, The difference in pressure due to liquid is on the two faces causes a net upward force called upthrust on the body immersed in the fluid. Note If a thin sheet (or a lamina) is immersed in a liquid. the pressure on its both surfaces will be almost same, thus the upthrust exerted by the liquid on the thin sheet is negligible, causing the thin sheet to sink in to the liquid due to its own weight. 2

Formula for calculating upthrust When a body is immersed in a liquid, upthrust on it due to liquid is equal to the weight of the liquid displaced by the submerged part of the body. The upthrust is equal to the product of the volume (V) of body submerged in the liquid, the density (ρ) of the liquid, and the acceleration due to gravity (g). FB = Vρg Factors affecting the upthrust 1. Volume of body submerged in fluid. 2. Density of liquid in which the body is submerged. Effect of upthrust The effect of upthrust is that the weight of body immersed in a liquid appears to be less than its actual weight. Archimedes’ Principle Archimedes’ principle states that when a body is immersed partially or completely in a liquid, it experiences an upthrust, which is equal to the weight of the liquid displaced by it. --------------------------------------------------------------------------------------

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