Topic 4 - Materials - Spec

Question 1

What is the Density Equation?

Answer 1

ρ = m / V ρ - Density (kg m*-3) m - Mass (kg) V - Volume (m*3)

Question 2

Define Upthrust?

Answer 2

Weight of Fluid Displaced Calculate the weight of the fluid an object displaces to calculate the Upthrust

Question 3

What is Stokes Law?

Answer 3

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)

Question 4

What does the Stokes Law Equation only apply to?

Answer 4

Only applies too: - Small spherical objects - Moving At slow speeds - With Laminar Flow (absence of Turbulent Flow)

Question 5

What is Viscosity dependant upon?

Answer 5

Temperature

Question 6

What is the CORE PRACTICAL to determine the viscosity of a liquid?

Answer 6

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

Question 7

What is the Hooke’s Law?

Answer 7

F = kx F = Force (N) X = exetension (m) K = Spring Constant (N m*-1)

Question 8

What is the (Tensile or Compressive) Stress Equation?

Answer 8

Force / Cross Sectional Area

Question 9

What is the (Tensile or Compressive) Strain Equation?

Answer 9

Change in Length / Original Length

Question 10

What is Young’s Modulus Equation?

Answer 10

Stress / Strain

Question 11

What happens at the Elastic Limit Point?

Answer 11

Material starts to behave plastically, from this point the material would no longer return to its original shape.

Question 12

What happens at the Yield Point?

Answer 12

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.

Question 13

What happens at the Limit of Proportionality?

Answer 13

• 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

Question 14

What does a happens with a Straight Line on a Stress Strain Graph?

Answer 14

A straight line through the origin shows the material IS OBEYING HOOKES LAW. - Gradient = Young’s Modulus

Question 15

A strong material has a…

Answer 15

High breaking stress

Question 16

Stiff materials are difficult to…

Answer 16

Stretch or Compress, they have a large Young’s Modulus

Question 17

Interpret Force-Extension and Force-Compression graphs.

Answer 17

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.

Question 18

Where is the Limit of Proportionality on this graph?

Answer 18

Point P is the Limit of Proportionality

Question 19

Where is the Elastic Limit on this Stress / Strain Graph?

Answer 19

Point E on the Graph

Question 20

Where is the Yield Point on this Stress / Strain Graph.

Answer 20

Point Y on the graph is the Yield Point.

Question 21

Show the Elastic and Plastic Reigon on a Stress / Strain graph.

Answer 21

Question 22

What is meant by the term Breaking Stress?

Answer 22

When the stress becomes so great that atoms seperate completely and the material breaks.

Question 23

What is the UTS point on a Stress / Strain graph.

Answer 23

UTS (Ultimate Tensile Stress) the maximum stress that the material can withstand.

Question 24

CORE PRACTICAL - Find the Young Modulus, you need a very Long Wire.

Answer 24

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.

Question 25

How do you calculate the elastic strain energy energy on a Stress / Strain Graph?

Answer 25

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.