4 - Materials

Question 1

Define density

Answer 1

mass per unit volume (think equation)

Question 2

When an object is submerged in fluid, what determines the upthrust it experiences?

Answer 2

Upthrust = weight of fluid displaced

Question 3

What determines if an object sinks or floats?

Answer 3

Balance between weight and upthrust
Weight > upthrust, it will sink

Question 4

Why will an object with a density greater than that of the liquid it is submerged in always sink?

Answer 4

-Upthrust = weight
-So max upthrust = density of fluid x volume of object x g
-Weight of object = density of object x volume of object x g
-If object’s density is greater, then weight is always greater than upthrust so the object will sink

Question 5

What shaped objects does Stokes’ Law apply to?

Answer 5

Small spherical

Question 6

What type of flow is required for Stokes’ Law to apply?

Answer 6

Laminar

Question 7

What does Stokes’ Law allow to calulate?

Answer 7

Viscous drag force when falling through a viscous fluid at low speed with laminar flow

Question 8

State Stokes’ Law equation

Answer 8

F=6πηrv
η - Viscosity of fluid/Pas
r - Radius of sphere/m
v - Velocity of sphere/ms^-1

Question 9

Define elastic deformation

Answer 9

If an object returns to its original shape once deforming forces are removed

Question 10

Define plastic deformation

Answer 10

If an object does not return to its original shape once deforming forces have been removed - it will have permanent deformation

Question 11

Express Hooke’s Law

Answer 11

Extension of an elastic object is proportional to the force that is applied to it, up to the limit of proportionality

Question 12

Define limit of proportionalilty

Answer 12

When Hooke’s Law is no longer obeyed

Question 13

Define elastic limit

Answer 13

The point beyond which the object will no longer elastically deform and will instead deform plastically

Question 14

State the equation for Hooke’s Law

Answer 14

F=kΔx
k - Spring constant
x - Extension/m

Question 15

Define mechanical stress

Answer 15

Force experienced per unit area

Question 16

State the equation for stress

Answer 16

Stress = Force/Cross sectional area

Question 17

Units for stress

Answer 17

Nm^-2

Question 18

State the equation for strain

Answer 18

Strain = Change in length/Original length

Question 19

Units for strain

Answer 19

None - ratio of 2 lengths

Question 20

What does the Young Modulus of a material show?

Answer 20

Measure of how much force is required for a given extension, regardless of the object’s dimensions

Question 21

State the equation for Young’s Modulus

Answer 21

Young’s modulus = Stress/Strain

Question 22

Units for Young’s Modulus

Answer 22

Nm^-2

Question 23

Define breaking stress

Answer 23

Maximum stress that an object can withstand before fracturing

Question 24

Define yield point

Answer 24

The point beyond which the object will experience a large extension without substantial increase in the force applied

Question 25

What type of energy is stored in an object that has been stretched?

Answer 25

Elastic potential

Question 26

State 2 equations to find energy stored in a spring

Answer 26

E=0.5FΔx E=0.5kΔx^2

Question 27

What does gradient and area represent on a force-extension graph?

Answer 27

Gradient - k Area - Elastic potential energy in spring

Question 28

What does gradient represent on a stress-strain graph?

Answer 28

Young Modulus of the material

Question 29

Describe a brittle material

Answer 29

-Sustain lots of stress -High spring constant -After reaching elastic limit, snaps -Stores little elastic potential energy -Loading/unloading proportional to extension -No plastic region

Question 30

Describe a ductile material

Answer 30

-Can be drawn into a wire -Large plastic region -Durable -Extension the same for loading/unloading -Goes back to original length when all load is removed -If elastic limit is exceeded unloading will be parallel to loading (be permanently deformed)