Topic 4 - Materials - Spec Flashcards
What is the Density Equation?
ρ = m / V ρ - Density (kg m*-3) m - Mass (kg) V - Volume (m*3)
Define Upthrust?
Weight of Fluid Displaced Calculate the weight of the fluid an object displaces to calculate the Upthrust
What is Stokes Law?
F = 6πaηv F - Viscous Drag (N) η - Viscosity of the fluid (Pas) r - Radius of the Object (m) v - speed of the object moving at terminal velocity (ms-1)
What does the Stokes Law Equation only apply to?
Only applies too: - Small spherical objects - Moving At slow speeds - With Laminar Flow (absence of Turbulent Flow)
What is Viscosity dependant upon?
Temperature
What is the CORE PRACTICAL to determine the viscosity of a liquid?
1 - Fill a wide, clear tube with the liquid you want to investigate, know the density of the liquid 2 - Put a rubber band half way down the tube so that the ball bearing when it passes is at its terminal velocity 3 - Place two more elastic bands below the first, so that the distance between them is equal 4 - Measure the diameter and the radius of the ball bearing 5 - Drop ball into the tube, start stopwatch when the ball reaches the first band and record the time it reaches the other bands. 6 - Repeat three times. 7 - If the ball falls close to the walls re-do that test. 8 - Calculate the average time taken for each ball bearing to fall between the elastic bands, calculate the terminal velocity by using the average time and distance between bands. 9 - Calculate the viscosity of the liquid, using this equation… η = 2r²g(ρ-σ) / 9v η = Viscosity of the liquid r = Radius of the ball bearing g = gravitational field strength ρ = density of the ball bearing σ = Density of the liquid v = Terminal Velocity
What is the Hooke’s Law?
F = kx F = Force (N) X = exetension (m) K = Spring Constant (N m*-1)
What is the (Tensile or Compressive) Stress Equation?
Force / Cross Sectional Area
What is the (Tensile or Compressive) Strain Equation?
Change in Length / Original Length
What is Young’s Modulus Equation?
Stress / Strain
What happens at the Elastic Limit Point?
Material starts to behave plastically, from this point the material would no longer return to its original shape.
What happens at the Yield Point?
Here the material suddenly starts to stretch without any extra load. This is the point at which a large amount of plastic deformation takes place with a constant or reduced load.
What happens at the Limit of Proportionality?
- Occurs when the line is no longer straight and is bent. - At this point the material stops obeying Hooke’s Law - But would still Return to its original shape if the load was removed
What does a happens with a Straight Line on a Stress Strain Graph?
A straight line through the origin shows the material IS OBEYING HOOKES LAW. - Gradient = Young’s Modulus
A strong material has a…
High breaking stress
Stiff materials are difficult to…
Stretch or Compress, they have a large Young’s Modulus
Interpret Force-Extension and Force-Compression graphs.
Materials that are strong have a HIGH STRESS materials that are weak have a LOW STRESS
Materials that are Stiff have a SMALL STRAIN materials that are less stiff have a HIGH STRAIN.
Where is the Limit of Proportionality on this graph?
Point P is the Limit of Proportionality
Where is the Elastic Limit on this Stress / Strain Graph?
Point E on the Graph
Where is the Yield Point on this Stress / Strain Graph.
Point Y on the graph is the Yield Point.
Show the Elastic and Plastic Reigon on a Stress / Strain graph.
What is meant by the term Breaking Stress?
When the stress becomes so great that atoms seperate completely and the material breaks.
What is the UTS point on a Stress / Strain graph.
UTS (Ultimate Tensile Stress) the maximum stress that the material can withstand.
CORE PRACTICAL - Find the Young Modulus, you need a very Long Wire.
1 - Set up the apparatus shown
2 - Test wire should be thin and as long as possible
3 - Find the cross-sectional area of the wire using a micrometer use the equation πr²
4 - Clamp the wire to the bench, start with a small weight to straighten the wire
5 - Measure the distance between the fixed end of the wire and the marker (unstretched length)
6 - By increasing the weight the wire stretches and the marker moves
7 - Increase the weight in equal steps, recording the marker each time, calculate the extension by subtracting the original length the the extended length
8 - Use the results from the experiment to calculate the stress and strain of the wire and plot a stress / strain curve.
How do you calculate the elastic strain energy energy on a Stress / Strain Graph?
The area underneath the linear part of the stress - strain graph gives the strain energy
Energy per unit vol = 0.5 x stress x strain
- If the line is not linear then you calculate the number of squares under the line and make an ESTIMATION.
