Materials

Question 1

How is density defined?

Answer 1

The mass per unit volume. [kgm^-3]

Question 2

How do you measure the density of an irregular solid?

Answer 2

1. Read off the volume from the beaker or measuring cylinder without and with the object submerged in water
2. The difference in volumes is the volume of the solid
3. Measure the mass using a balance

Calculate density using ρ=M/V

Question 3

How do you calculate the average density of an alloy?

(Eg 200cm³ 5kg rod of 60% copper (8960kgm^-3) and 40% aluminum (2700kgm^-3) by volume?)

Answer 3

1. Work out the mass of each and the volume of each
2. Add together to get the total mass and volume
3. Then do the density calculation

Question 4

Define Hooke’s Law

Answer 4

When a material is stretched, its extension is proportional to the force applied, up until the limit of proportionality

F=kx

Question 5

Define the limit of proportionality

Answer 5

The point at which the material stops obeying Hooke’s law.

The graph is no longer a straight line.

Question 6

Define the elastic limit

Answer 6

The point at which when stretched further the material no longer returns to its original length (there is a permanent extension)

Question 7

What do the gradient and area of a force extension graph tell you for a spring.

Answer 7

Gradient → The spring constant (must be taken before limit of proportionality)

Area under line → The strain energy stored loading the spring or energy released unloading the spring

Question 8

What equation calculates energy stored when a material is stretched?

Answer 8

E=½Fx

Question 9

What is the difference between the elastic limit and the limit of proportionality?

Answer 9

Limit of proportionality is the point at which a stretched spring (or wire) stops obeying Hooke’s law.

The elastic limit is the point at which it doesn’t return to its original length when unloaded.

Question 10

What is a ductile material?

Answer 10

A material with a large plastic region.

Question 11

What is a brittle material?

Answer 11

A material with a small plastic region.

Question 12

What is the fracture point of a material?

Answer 12

The point at which a material breaks

Question 13

What is the formula for Young’s Modulus that you need to remember?

Answer 13

Question 14

What does the gradient and area under a stress-strain graph give?

Answer 14

Gradient → Young’s modulus (before the limit of proportionality)

Area → strain energy per unit volume