Materials Flashcards
(27 cards)
Upthrust
The upward force exerted on an object by the fluid it is immersed in.
Archimedes principle
Upthrust is equal to the weight of the fluid displaced
Pressure of fluid at depth
Depth x density x g
Laminar flow
Streamlines are parallel and constant. No crossing / abrupt changes
Turbulent flow
Streamlines frequently cross. They form vortices. Non continuous and constantly changing.
Laminar flow occurs when
The rate of flow is below a critical point
Turbulent flow occurs when
The rate of flow is above a critical point or an object is obstructing or moving through the liquid.
Viscosity
An internal friction present between two layers of a liquid which resists the flow of the liquid. The higher the viscosity, the harder it is for a fluid to flow, so it flows slower.
Stoke’s law
F = 6πηrv.
Where :
F = Drag in Newtons
η = coefficient of viscosity in pascal seconds
r = radius of ball bearing in metres
V = velocity of ball bearing in ms^-1
Conditions for Stoke’s law to be valid
- Object must be solid, smooth and spherical
- Object must be of uniform density
- Object must be sufficiently large compared to fluid molecules
- The fluid must have laminar flow around the object
- The fluid must be homogenous
Rubber bands under stress
- Rubber bands are made from polymer chains. At rest, the chains are tangled and knotted which requires force to undo.
- Eventually, the chains become un-knotted, and the force required to stretch the rubber band decreases.
- However, when stretched enough the chains become taught and the rubber band becomes stiffer again as you are pulling against molecular forces.
Stress
The force applied per unit cross sectional area.
= Force / Cross sectional area = F / A
Units : NM^-2
Sometimes presented as σ (sigma)
Strain
The change in length divided by the original length
= Change in length / original length = ∆L / L
Units : None. m/m = pure number
Sometimes presented as ε (epsilon)
Breaking stress
The amount of stress required to break a material
If a stress-strain curve stops, the material has broken.
Young Modulus
The ratio of tensile stress to tensile strain.
Equation: E = Fl / AΔl
Units: Pa
Tells us how easily a material can stretch and deform
The gradient of a stress strain graph
The young modulus of the material
The area under the stress strain graph
The energy stored per unit volume in the material
Tough
A material capable of absorbing lots of energy before breaking
Strong
A material can withstand a large applied force before breaking
Stiff
A material resists deformation in response to applied forces
Hookes Law
F = k x X
Where
F is change is force,
k is spring constant
X is extension
Equation for energy stored in wire during extension
E = 1/2 Fx
What is the limit of elasticity?
The point at which a material begins plastic deformation
What does it mean when springs are connected in series?
They are joined by either end
