Topic 4: Materials

Question 1

What is density?

Answer 1

Density is the mass per unit volume of an object.

ρ=mv

Question 2

What is upthrust?

Answer 2

Upthrust is the upward force acting on an object due to different pressures exerted on surface of object

Question 3

What is Archimedes’ principle?

Answer 3

Upthrust is equal to the weight of object.

Question 4

What is viscous drag?

Answer 4

Viscous drag is the
- frictional force between an object and fluid
- which opposes the motion between the object and the fluid.

Question 5

What is Stoke’s Law formula?

Answer 5

F = 6πηrv

Question 6

What are the conditions for Stoke’s Law formula to be used?

Answer 6

Object has to be:
- Small and spheric
- Low speed
- Laminar flow
- Terminal velocity

Question 7

What is laminar flow?

Answer 7

Laminar flow is when the particles in a fluid
- Adjacent layers of fluid are parallel
- No abrupt changes in speed/direction
- Velocity at point is constant

Question 8

What is turbulent flow?

Answer 8

Turbulent flow is when
- Layers of fluid cross into each other
- Edgy currents formed
- Streamlines no longer continuous

Question 9

What is viscosity?

Answer 9

Viscosity is a measure of how easily a fluid flows.
High viscosity means fluid is thick. And resistant to flow.

Question 10

How does viscosity change with temperature?

Answer 10

Liquids:
As temperature increases, viscosity of a liquid decreases.

Gases:
As temperature increases, the viscosity of a gas increases.

Question 11

What is Hooke’s Law?

Answer 11

Hooke’s law states that extension of spring is directly proportional to the force applied.

ΔF = kΔx

Question 12

What is stress and what are the 2 types?

Answer 12

Stress - Force applied per unit cross sectional area

Tensile force - pull on an object and extend it
Compressive forces - push onto an object to compress

σ = F/A

Question 13

What is strain?

Answer 13

Strain is the extension per unit length

Question 14

What is Young Modulus?

Answer 14

Young Modulus is measure of ability of a material to withstand changes in length with an added load

Young modulus is a value which describes stiffness of a material.

Question 15

How can Hooke’s Law be illustrated?

Answer 15

Hooke’s law can be illustrated on a:
- Force-extension graphs
- Straight line when Hooke’s Law obeyed

Question 16

Key features of Force-extension graph

Answer 16

Limit of proportionality
- Point beyond which Hooke’s Law is no longer obeyed; extension no longer proportional to applied force

Elastic limit
- After Elastic limit, object undergoes plastic deformation; will not return to original form

Yield Point
- Where material continues to stretch with minimal force applied to it.

Question 17

What is elastic deformation?

Answer 17

Elastic deformation is when a material returns to its original shape once force applied is removed.

Question 18

What is plastic deformation?

Answer 18

Plastic deformation is when material’s shape is changed permanently.

Question 19

What do stress-strain graphs help indicate?

Answer 19

• Upto what stress/strain they obey Hooke’s Law
• Whether they exhibit elastic/plastic behaviour
• Value of Young Modulus
• Value of breaking stress

Question 20

What is breaking stress?

Answer 20

Breaking stress is the value of stress which material will break apart.

Question 21

What can the shape of a stress-strain graph show?

Answer 21

Brittle
- Material undergoes little to no plastic deformation before fracturing
Ductile
- can undergo large amount of plastic deformation before fracturing
Plastic
- Material will experience large amount of extension as the load is increased.

Question 22

What is elastic strain energy?

Answer 22

Elastic strain energy is the work done on a material to stretch/compress it.

Question 23

How can you calculate elastic strain energy?

Answer 23

Area under force extension graph

E = 1/2 FΔx