Title: InkPhysics / Materials
On every note of this deck, unless a note says otherwise:
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Front: RECALL
State how to find the energy stored from a force-extension graph.
Back: The elastic strain energy equals the area under the force-extension graph.

Front: RECALL
State the unit of density.
Back: kg m^−3, kilograms per cubic metre.

Front: SPOT THE ERROR
Say what is wrong with this claim: Things float because they are light.
Back: Floating depends on density, not on being light. An object floats when the upthrust, equal to the weight of fluid displaced, can equal the object's weight; that needs its average density below the fluid's.

Front: RECALL
State the conditions for Stokes' law to apply.
Back: The sphere must be small and slow-moving, so that the flow around it stays laminar.

Front: RECALL
State the unit of the spring constant.
Back: N m^−1, newtons per metre.

Front: DEFINITION
Define: Tensile stress.
Back: The force applied per unit cross-sectional area: stress = F/A.

Front: DEFINITION
Define: Pressure.
Back: The force acting at right angles to a surface, divided by the area of that surface: p = F/A, measured in pascals.

Front: DEFINITION
Define: Young modulus.
Back: The ratio of tensile stress to tensile strain for a material within the limit of proportionality.

Front: DEFINITION
Define: Laminar flow.
Back: Smooth flow in layers that do not mix.

Front: RECALL
Explain why rubber is elastic but does not obey Hooke's law.
Back: It returns to its original length when unloaded, so the deformation is elastic, but the force is not proportional to the extension, so the graph is not straight and there is no single Young modulus.

Front: RECALL
State the unit of tensile stress.
Back: The pascal, Pa, equal to N m^−2.

Front: RECALL
State how to find the Young modulus from a stress-strain graph.
Back: The gradient of the initial straight-line region.

Front: SPOT THE ERROR
Say what is wrong with this claim: Hooke's law holds right up until the spring snaps.
Back: Hooke's law fails well before the spring snaps: beyond the limit of proportionality, force is no longer proportional to extension.

Front: RECALL
State the condition for terminal velocity of a falling sphere.
Back: Weight equals upthrust plus viscous drag, so the resultant force is zero and the velocity is constant.

Front: DEFINITION
Define: Ductile material.
Back: One that shows a large plastic deformation beyond its yield point before it fractures.

Front: DEFINITION
Define: Upthrust.
Back: The upward force on a body in a fluid, equal to the weight of fluid displaced by the body.

Front: RECALL
State how the viscosity of a liquid changes with temperature.
Back: The viscosity of a liquid falls steeply as its temperature rises.

Front: DEFINITION
Define: Derive the equation for hydrostatic pressure.
Back: A column of fluid of cross-sectional area A and depth Δh has volume AΔh, mass ρAΔh and weight ρAgΔh. Dividing that weight by A gives Δp = ρgΔh; the area cancels, which is why the container shape does not matter.

Front: DEFINITION
Define: Elastic limit.
Back: The point beyond which a stretched object no longer returns to its original length when the force is removed.

Front: RECALL
State the condition for floating.
Back: A body floats when it can displace its own weight of fluid before becoming fully submerged.

Front: DEFINITION
Define: Hooke's law.
Back: The force needed to stretch a spring is proportional to the extension, F = kΔL, up to the limit of proportionality.

Front: DEFINITION
Define: Hysteresis loop.
Back: The loop enclosed between the loading and unloading curves of a polymeric material; its area is the energy released as heat in one loading cycle.

Front: DEFINITION
Define: Breaking stress.
Back: The maximum stress a material can withstand before it fractures.

Front: DEFINITION
Define: Density.
Back: Mass per unit volume: ρ = m/V.

Front: DEFINITION
Define: Limit of proportionality.
Back: The point beyond which force is no longer proportional to extension, so Hooke's law ceases to apply.

Front: RECALL
State how the pressure at a point in a fluid depends on direction.
Back: It does not. At a point in a fluid the pressure acts equally in all directions, and always at right angles to any surface it meets.

Front: RECALL
Explain why a Young modulus test wire is long and thin.
Back: Both choices enlarge the extension, giving a smaller percentage uncertainty in ΔL.

Front: DEFINITION
Define: Polymeric material.
Back: A material built from long tangled molecular chains, such as rubber or polythene, whose stress-strain curve stiffens as the strain grows.

Front: DEFINITION
Define: Viscosity.
Back: The property of a fluid that measures its resistance to flow: the constant η in Stokes' law.

Front: RECALL
Explain how to convert a density in grams per cubic centimetre.
Back: Multiply by 1000: 1 g cm^−3 = 1000 kg m^−3.

Front: DEFINITION
Define: Tensile strain.
Back: The extension per unit original length: strain = ΔL/L, a ratio with no unit.

Front: DEFINITION
Define: Brittle material.
Back: One that fractures at the end of its straight-line region with no plastic deformation.

Front: SPOT THE ERROR
Say what is wrong with this claim: The spring constant tells you how stiff the material is.
Back: The spring constant k describes one particular object: change its length or thickness and k changes. The Young modulus is stress over strain, which cancels the geometry and characterises the material itself.

Front: RECALL
Explain how to read the permanent extension from a force-extension graph.
Back: Read where the unloading line, parallel to the original straight section, meets the extension axis.