Chapter 11 - Materials

Question 1

How do you calculate the density of an alloy, composed of 2 metals?

Answer 1

Fins the mass of each metal in the alloy by multiplying density by its volume. Then add masses and divide by overall volume of the object.

Question 2

Define density

Answer 2

The mass of an object per unit volume - units kg m-3

Question 3

State Hooke’s Law

Answer 3

Force needed to stretch a material is directly proportional to,the extension of the spring.

Question 4

Give the units used for spring constant.

Answer 4

Nm-1

Question 5

Define spring constant

Answer 5

The force needed to stretch a material one metre.

Question 6

What is the point called on a force extension graph when the gradient is no longer linear?

Answer 6

The elastic limit or limit of proportionality.

Question 7

What does the area under the line of a force extension graph tell you?

Answer 7

The amount of elastic potential energy stored in the spring. Energy transferred = work done = force x distance.

Question 8

Two springs in parallel each with spring constant 120 Nm-1. What is the spring constant of the combination?

Answer 8

Double one spring = 240 Nm-1

Question 9

Two springs in series, each with a spring constant of 120. What is the spring constant of the combination?

Answer 9

Half of one spring = 60 Nm-1.

The 1/k rule applies.

Question 10

What type of energy is stored in a stretched spring?

Answer 10

Elastic potential energy.

Question 11

Define plasticity.

Answer 11

Permanent deformation or extension of the material. It will not return to its original length once load has been removed.

Question 12

Define elasticity

Answer 12

The ability of the material to return to its original shape once the load has been removed.

Question 13

Define tensile stress

Answer 13

Tension divided by cross sectional area of the material. Units Nm-2

Question 14

Define tensile strain

Answer 14

Extension of the material divided by original length. No units as it is a ratio!

Question 15

What is Young Modulus a measurement of?

Answer 15

How stiff a material is.

Question 16

How do you determine Young Modulus from a stress strain graph?

Answer 16

The gradient. The steeper the line the stiffer the material.

Question 17

Define brittle.

Answer 17

A material which snaps without any plastic deformation, no noticeable yield.

Question 18

Define strength of a material.

Answer 18

The maximum stress (ultimate tensile strength) the material can take.

Question 19

Define ductile.

Answer 19

Material can be drawn out into a wire. This is seen by an extended curved section after the straight line on a stress strain graph.

Question 20

What is the unit for Young Modulus?

Answer 20

Pa. the values are usually so high MPa or GPa is often used. Mega = 10^6 Giga = 10^9

Question 21

What does the area under a stress strain graph represent?

Answer 21

Work done per unit volume to stretch the material.

The work done to stretch a certain mass of wire can be calculated by taking the area under the line and multiplying by volume.

Question 22

Is the energy stored in a parallel arrangement of springs the same as, less than or greater than the arrangement of the same springs in series?

Answer 22

The arrangement in series is greater because the extension is more and the force applied is the same. Energy is proportional to extension (energy = 1/2Fe)

Question 23

Compared the work done by a spring when the load is removed after exceeding its limit of proportionality with that done by the load in producing the extension.

Answer 23

The work done by the spring in unloading will be less than the work done by the load in extending the spring initially. Energy lost as heat in the spring.

Question 24

Which 2 features of a force extension graph will show a material obeys Hooke’s law?

Answer 24

A straight lined graph going through the origin.

Question 25

Define elasticity.

Answer 25

Material returns to its original length/shape when the load stretching it has been removed.

Question 26

What is meant by the limit of proportionality of a spring?

Answer 26

The point at which force is no longer proportional to extension.
No longer obeys Hooke’s law
Force extension graph is no longer linear.