Solids and fluids

• No net force is acting on it, though forces may still be present.

• The existing forces cancel out, being equal in size and opposite in direction.

What does it mean if an object isn’t moving?

• Solids have tightly vibrating molecules with long-range order.

• Liquids have more mobile molecules with short-range order, while gases have high-energy, widely spaced molecules that move freely.

What are the key features of solids, liquids, and gases?

• An elastic object returns to its original shape when balanced forces are removed.

• Skin behaves elastically, but blu-tack does not.

What is elasticity in materials?

• Stress is the force per unit area acting on an object.

• It can be tensile (pulling/stretching) or compressive (pushing/squishing).

What is stress in materials?

• Strain is the measure of deformation due to stress.

• Tensile strain stretches an object; compressive strain shortens it.

What is strain in materials?

• Dimensions perpendicular to tensile or compressive stress change proportionally.

• This is due to material deformation responding in all directions.

What happens perpendicular to the direction of stress?

• Hooke’s law applies for small deformations where stress is proportional to strain.

• Young’s modulus measures material stiffness and is independent of object size or shape.

What is Hooke’s law and Young’s modulus?

• Shear stress is a tangential force causing layers of material to slide past one another.

• Shear strain is the resulting deformation, increasing with greater applied force.

What is shear stress and shear strain?

• Bulk stress is the pressure change applied to a volume.

• Bulk strain is the resulting volume change, and bulk modulus measures resistance to compression.

What is bulk stress and strain?

• They show material behaviour from elastic to plastic deformation.

• The elastic limit is the point beyond which permanent deformation occurs.

What are stress-strain curves?

• It has a steep initial slope (elastic region), a yield point (plastic region), and a final fracture point.

• Ductile materials deform before breaking, absorbing energy.

What does a ductile stress-strain curve look like?

• It starts elastic, becomes plastic at higher stress, and ends with fracture.

• Seen in biological tissues, which resist small stresses but yield under large ones.

What is a J-shaped stress-strain curve?

• Pressure is force per unit area, acting equally in all directions in fluids.

• A sphygmomanometer measures blood pressure using external pressure on arteries.

What is pressure and how is it measured in medicine?

• Density is mass per unit volume and affects how pressure builds in fluids.

• Greater density means more mass in the same space, influencing pressure.

What is density and how is it related to pressure?

• Pressure is the same in all directions at a given depth and acts perpendicular to surfaces.

• According to Pascal’s principle, pressure in an enclosed fluid is transmitted equally throughout.

How do fluids exert pressure?

• Absolute pressure is measured from vacuum; gauge pressure is relative to atmospheric pressure.

• Atmospheric pressure adds to any additional pressure measured inside a system.

What is gauge and absolute pressure?

• A fluid exerts an upward force equal to the weight of displaced fluid.

• Objects float or sink depending on whether their weight is less or more than the buoyant force.

What is buoyancy and Archimedes’ principle?

• Molecules at the surface of a liquid experience a net inward force, creating tension.

• It helps objects like needles or insects rest on water and forms spherical bubbles to minimise surface area.

What is surface tension?

• It is the upward movement of liquid in a narrow tube due to adhesion and surface tension.

• It plays roles in plant nutrient transport and blood flow in small vessels.

What is capillarity?

• A concave (negative) meniscus means adhesion is stronger than cohesion.

• A convex (positive) meniscus means cohesion is stronger than adhesion.

What do the shapes of menisci indicate?

• For incompressible fluids, flow rate remains constant through all cross sections.

• A narrower pipe means faster flow to conserve volume per time.

What is the continuity equation for fluids?

• In steady, incompressible, low-viscosity flow, energy is conserved.

• Pressure decreases where velocity increases and vice versa.

What is Bernoulli’s principle?

• Viscosity is a fluid's internal resistance to motion; higher viscosity means more force is needed to move.

• Viscosity increases flow resistance in long, narrow tubes or thicker fluids.

What is viscosity and how does it affect fluid flow?

• Reynolds number predicts flow type: low values mean laminar (smooth) flow; high values mean turbulent (chaotic) flow.

• It depends on fluid density, velocity, characteristic size, and viscosity.

What is Reynolds number and how does it relate to flow type?

• Free diffusion involves slow, random motion from high to low concentration.

• Osmosis is water moving through a semipermeable membrane; active transport uses energy to move substances, sometimes against concentration gradients.

What are types of molecular transport phenomena?