11.4 More about stress and strain

Question 1

How is the tension in a strip of material increased

Answer 1

By increasing the weight it supports

Question 2

For a metal wire, Its loading and unloading curves are the same provided that…

Answer 2

Its elastic limit is not exceeded.

This means it returns to its original length when unloaded.

Question 3

What happens to the unloading line beyond the elastic limit

Answer 3

It is parallel to the loading line

Question 4

If the wire is slightly longer when unloaded, it has a ______ extension

Answer 4

Permanent

Question 5

For a rubber band, the change of length during unloading is…

Answer 5

…greater than during loading for a given change in tension.

Question 6

If the rubber band returns to the same unstretched length but the unloading curve is below the loading curve, the band remains elastic but has a …

Answer 6

…low limit of proportionality

Question 7

Is the extension during unloading greater or less than the extension during loading

Answer 7

Greater
But the strip does not return to the same initial length when it is completely unloaded.
Has a low limit of proportionality AND suffers plastic deformation

Question 8

What is strain energy

Answer 8

The work done to deform an object

Question 9

What is the area under the line of a force extension graph

Answer 9

The work done to stretch the wire

Question 10

What type of material is polyethene

Answer 10

A polymer

Question 11

What does it mean if a material is a polymer

Answer 11

Its molecules are long chains of atoms

Question 12

Before a polythene strip is stretched:

Answer 12

The molecules are tangled together. Weak bonds or cross links form between the molecules.

Question 13

When polythene is under tension:

Answer 13

It easily stretches as the weak cross-links break. In this stretched state, new weak cross-links form and when the tension is removed, the strip stays stretched

Question 14

What type of material is rubber

Answer 14

A polymer

Question 15

Unstretched rubber:

Answer 15

Its molecules are curled and tangled together when it is unstretched

Question 16

Rubber under tension

Answer 16

Its molecules are straightened out but these curl up again when the tension is removed. The rubber regains its initial length

Question 17

The elastic energy stored in a stretched wire =

Answer 17

1/2 TΔL

Question 18

What is the work done to stretch a rubber band represented by

Answer 18

The area under the loading curve.

Question 19

What is the work done by the rubber band represented by

Answer 19

The area under the unloading curve

Question 20

What does the area between the loading and unloading curves therefore represent for a rubber band

Answer 20

The difference between energy stored in the rubber band when it is stretched and the useful energy recovered from it when it is unstretched

Question 21

Why is there a difference between energy stored in the rubber band when it is stretched and the useful energy recovered from it when it is unstretched

Answer 21

Because some of the energy stored in the rubber band becomes the internal energy of the molecules when the rubber band unstretches

Question 22

What does the area between the loading and unloading curves represent for polythene

Answer 22

As it does not regain its initial length, the area between the loading and unloading curves represents work done to deform the material permanently as well as internal energy retained by the polythene when it unstretches