4. Materials

Question 1

What is density, p

Answer 1

Density is the mass is the mass per unit volume of an object
p=m/V
where V= volume (m^3)

Question 2

What is upthrust, U?

Answer 2

The upward force an object has when it’s submerged in a fluid
So acts against weight

Question 3

What’s Archimedes’ Principle?

Answer 3

The upthrust force equal to the weight that the fluid displaces
E.g the water would weigh 70N if the box weren’t there, so the upthrust is 70N

Question 4

How do we calculate upthrust?

Answer 4

As U=weight of fluid displaced
U=mg , p=m/v so m=pv so…
U=p(fluid)vg

Question 5

How does upthrust actually work?

Answer 5

In a fluid, pressure. hits the object from all angles
- But the force at bottom >top
- results in a force upwards

Question 6

How does floating work?

Answer 6

In order for an equilibria, the weight of the object, only under water, must equal the upthrust
So the volume displaced will vary on how much it weighs
U=pvg
so if density of water= 1000
U=1000 x v x 9.81

Question 7

What is laminar flow?

Answer 7

Laminar flow is where the particles in a fluid move by smooth paths
- with no/little mixary
Tends to occur at lower speeds

Question 8

What is turbulent flow?

Answer 8

Turbulent flow is where particles mix between layers and form separate currents
- ‘chaotic’ movement
Tends to be at faster speeds

Question 9

What is Viscous drag force, F?

Answer 9

Viscous drag force is the force acting against the movement of an object in a fluid
- if an object was falling, there would be a drag against it
- its like air resistance in fluids
- so goes in opposite direction of flow
So for falling objects:
W=U+F at terminal velocity
They are equal as its not accelerating, it’s at a constant velocity
For rising objects:
U=W+F

Question 10

What is Stokes Law?

Answer 10

States that if the viscous drag force, F is experienced by small, spherical objects moving slowly with laminar flow:
F=6πηrv
where η(eta) is the viscosity of the fluid, pascal seconds (Pas) or (Nsm^-2)
where r=radius of sphere
where v=velocity of object

Question 11

What is viscosity, η dependant on?

Answer 11

Viscosity, η is temperature dependant
They also depend on pressure

Question 12

How does viscosity, η change in most fluids?

Answer 12

If temp increases, η of liquid decreases
This is because the particles are free moving warmer
so the object moves through it easier

Question 13

How does viscosity, η change in most gases?

Answer 13

If temp increases, η of gas increases
This is because it gets more ‘sticky’ so it’s harder to move

Question 14

How would you find the viscosity, η for a fluid when a small spherical ball is slowly falling through the liquid?

Answer 14

W=U+F
p(object)vg=p(liquid)vg + 6πηrv
Then rearrange for η

Question 15

What is Hooke’s Law?

Answer 15

States that the force needed to extend a spring is proportional to the extension of the spring
Force applied(N)=spring/ stiffness constant (Nm^-1) x extension(m)
F=k△x
where x=extension(m)

Question 16

When does a material obey Hooke’s Law?

Answer 16

A material only obeys Hooke’s law if it’s not past it’s limit of proportionality, P
So it has to only be a small force
The point limit of proportionality, P is the first point on the graphs

Question 17

What is the Elastic limit (E)?

Answer 17

Anything after E, the material will be permanently deformed
The second point on the graphs

Question 18

What happens to the boat if you throw an anchor in the water?

Answer 18

The weight will increase,
So as upthrust = weight in equilibrium
Weight will rise, so more of the ship will be taken out of the water to decrease the weight back and go back into equilibrium with the upthrust

Question 19

What is stress, σ?

Answer 19

Stress, σ means how much force per unit a material is having put on it
σ=F/A
where A=area(m^2)
σ (Nm^-2 or Pa)

Question 20

What is strain, ε?

Answer 20

Strain, ε means how much a material extends per unit length
Result of tensile force, so this is the same as tensile strain
ε=△x/x
where x=length(m)
so strain is just a proportion, so has no unit

Question 21

What are pascals, Pa?

Answer 21

Pascals is the unit of force per m^2
So =Nm^-2
So it’s pressure

Question 22

What is a tensile force?

Answer 22

Tensile forces pull an object to extend it, causing strain
The higher the tensile force = the more stress put on the material

Question 23

What is a compression force?

Answer 23

Compression force pushes and compresses an object (squash it)

Question 24

What is the yield point?

Answer 24

Beyond the yield point, the material has a sudden increased tension
As it’s atomic structure is significantly re-organised
- like a ‘slip’
The third point on the graphs

Question 25

What is the Ultimate tensile stress?

Answer 25

The max stress the material can take
- The highest point on the graph
The fourth point on the graphs

Question 26

What’s the breaking point?

Answer 26

The point where the material breaks in half
The fifth (last) point on the graphs

Question 27

What is the Young Modulus, E?

Answer 27

The Young Modulus, E tells us how stiff a material is
It’s the stress/strain
So E=σ/ε
E=Fx/A△x
In (Nm^-2 or Pa)
The higher YM, The more stiff the material is

Question 28

What is a stress - strain graph?

Answer 28

Stress (y axis)
Strain (x axis)
Compares how much a material is strained with a set amount of stress on it
Same thing as an force - extension diagram
Gradient = YM
- As YM=σ/ε
- only for the part that follows Hooke’s Law

Question 29

How do we find the elastic strain energy/ work done, E(el)?

Answer 29

We know work done=fd
- But Hooke’s Law tells us how the force is constantly changing with deforming metals
So its the avg force x extension
Which works out to be E(el)=1/2 x F△x
So this is the area under the graph is △E(el)
Since Hooke’s Law is F=k△x
△E(el)=1/2 x k(△x)^2 as well

Question 30

What is a Force - extension graph?

Answer 30

Force - y axis
Extension - x axis
Tells us the force needed to extend gets more and more past the limit of proportionality

Question 31

How can you tell which material is the strongest?

Answer 31

‘Strongest’ means which one can deal with the most stress without breaking
So the highest ultimate tensile stress = strongest

Question 32

How can you tell which material is the stiffest?

Answer 32

The highest gradient = most stiff
As it can take more stress without being strained much

Question 33

How can you tell which material is the most ductile?

Answer 33

The highest strain, the most ductile
As it can take the most extension without breaking

Question 34

What is a brittle material?

Answer 34

A material which can’t take much strain (extension)
So opposite of ductile

Question 35

What is elastic deformation?

Answer 35

Strain before the elastic limit
So the material isn’t permanently damaged

Question 36

What is plastic deformation?

Answer 36

Strain before the elastic limit
So the material is permanently damaged

Question 37

What is the work done when loading and unloading a material

Answer 37

There will be energy lost when unloading it but gone past elastic limit
The energy lost/ work is the loading are - unloading area under graph