- Prepared by Shakil Raiman Topic: 6: Fluids - Prepared by Shakil Raiman

6.1 Density Density is defined as mass per unit volume Unit: Kg/m3

6.2: Relative Density It is defined as the ratio of density of an object to the density of a fluid. It has no unit

6.3: Pressure It is defined as force per unit area. Unit: Nm-2 or Pa (Pascal) Pressure in Liquid = hg

6.4: Upthrust in Fluids When an object is immersed in fluid, the upthrust is equals to the weight of the displaced fluid. The upthrust is the difference between the force due to liquid pressure at the bottom surface and top surface of an object. Unit: N

6.5: Flotation An object will float in a fluid if the upthrust – that is, the weight of the fluid it displaces – is equals to its weight.

6.6: Moving Fluids –Streamline flow or laminar flow. While fluid flows, if there is no mixing of layers or no vortices or no eddies are formed, this is called streamline flow or laminar flow. Check on the board.

6.7: Moving Fluids –Turbulent flow While fluid flows, if there is mixing of layers or vortices or eddies are formed, this is called turbulent flow. Check on the board.

6.8: Viscosity The viscosity of a fluid relates to its stickiness and thus to its resistance to flow. Syrup and engine oil are very viscous. Runny liquids such as water and petrol and all gasses have low viscosity. For the same fluid, if temperature increases viscosity decreases.

6.9: Drag The drag on an object moving in a fluid is the opposing force due to its stickiness with the fluid or viscosity of the fluid. When a sphere of radius r, moves slowly through a fluid with velocity v, the viscous drag on it is stated by Stokes’ Law. Viscous drag, Where  = coefficient of viscosity. F =6𝜋rv

6.10: Acceleration and Terminal Velocity When a sphere is released and allowed to fall freely in a fluid, it is subjected to three forces: its weight, W, the upthrust, U and viscous drag, F For accelerating downward, W>U+F For terminal velocity, W=U+F For decelerating downward, W<U+F

6.11: Measuring viscosity using Stokes’ Law By measuring the terminal velocity of a sphere falling through a fluid it is possible to determine the coefficient of viscosity of the fluid. For a sphere of radius r and density s falling through a fluid of density f and viscosity  with a terminal velocity v, the equilibrium equation: U+F=W