Zusammenfassung der Ressource
Materials
Anlagen:
- Fluid Properties
- Fluids Basics
- A fluid is a substance that can 'flow'.
This primarily refers to liquids or
gasses. Some solids, such as sand in an
hourglass show liquid qualities
- Density
- Density tells us how much 'stuff' there
is inside a liquid (or solid) It is a
measure of the mass per unit volume
- We can calculate the
density of an object (p)
using its mass (m) and
its volume (v)
- Upthrust
- When an object is submerged in a fluid,
it feels an upwards force called upthrust.
Upthrust is the 'weight of fluid
displaced'.
- If we calculate weight and
upthrust, then take one from
the other, we can find a
resultant force
- If an object is completely
submerged, we can find upthrust
by multiplying the volume of the
object (V) by the density of the
fluid (p) and g (9.81 on earth)
- If a point is reached where
upthrust and weight are equal,
we can say that the object is
floating
- We use the idea of floating at different depths in
a instrument called the hydrometer. We use this
to calculate the densities of different liquids by
allowing a object of constant weight to sink, until
it floats. Where the object floats, we read off the
scale to get the density of the liquid
- Fluid Movement
- Fluids can flow in 2 ways: Laminar
flow or Turbulent flow
- Turbulent Flow
- Turbulent flow is alot more chaotic than laminar flow.
It happens at faster rates of flow, or after the fluid
passes an obstacle. It can unpredictable velocities,
that are always changing, as well as eddies and
whirlpools
- Laminar Flow
- Laminar flow (streamlined flow) happens when a
fluid is flowing at a constant velocity at any given
point. It usually occurs in slow moving fluids
- It is important to streamline things like cars and aeroplanes to
minimise the amount of turnbulent flow. This is because
turbulent flow will increase resistive forces, increasing fuel
consumption, increasing costs, which is bad.
- Stoke's Law
- The viscosity of a fluid is a measure of
how runny it is. A greater viscosity
means its less runny, which means there
will be a greater force of viscous drag.
- Viscosity is inversely proportional to
temperature. This means that as
temperature increases, a fluid will get
less viscous
- Newton devised an equation to find the
coefficient of viscosity of a fluid. This value is a
numerical value that will indicate how resistant
to flow a fluid is
- Stoke then came up with a formula to find
how much viscous drag (f) a flowing object
will experience, given its radius (r), terminal velocity
(v) and coefficient of viscosity.
- It is important to know that this equation
only works for small, slow moving spherical
objects falling in laminar flow
- Terminal Velocity
- From stokes law, we can derive an equation to find
the terminal velocity of a falling object.
- When an object is freely falling, we can
describe a few force changes that act on
it.
- Initially, weight > upthrust + viscous drag
- As theres a resultant force, the object
will accelerate downwards. As velocity
increases viscous drag increases (from
stokes law)
- Eventually weight = upthrust + viscous drag
- This means no more acceleration, due
to a 0 resultant force, meaning
terminal velocity has been reached
- Solid Material Properties
- Hookes Law
- When a force acts on a material, itll be deformed in some way. If
its stretched, the force is a tensile force, and if the sample is
compressed the force is a compressive force
- Hooke's Law states that the force needed to extend
a spring is proportional to the extension, uptil a
certain point called the limit of proportionality.
- Hookes law can be described mathematically as the
force needed to extend a spring (f) is proportional to
its extension (x) and its spring constant (k)
- We can find the spring constant of a spring
from the gradient of a force extension graph
- Stress, Strain and the young
Modulus
- Sress
- The stress in a material is a measure of the force within a
material sample taking into account the cross sectional
area.
- It allows comparisons to be made between
materials of different sizes
- We use the force exerted on the sample (F) and
the cross sectional area (A) to calculate the
stress
- Srain
- The strain is a measure of the extension of a
material, taking into account its original
length.
- This, again, allows comparisons to be made
between materials of different sizes
- We can calculate the strain using
the extension and the original
length
- The young Modulus
- The young modulus is a stiffness constant of a
material, taking into account the stress and
strain in a material
- This means that different samples of the
same material will have the same young
modulus making it a property of a material
- This idea gives us a measure of
how much a material deforms
when forces are applied to it
- We can only find the young modulus in the
straight part of a stress strain graph, where
the material is still obeying hooks law
- Stress- Strain
graphs
- Elastic Strain
Energy
- Elastic strain energy is the amount of energy stored inside a material
when it is deformed.
- It can be calculated using the force and the
extension
- We can also find it from the area under the
graph of a force extension graph