Chapter 6: Materials

Question 1

What is meant by the extension and compression of a string?

Answer 1

Tensile forces act away from the centre of the spring in both directions and will stretch it out, causing the spring to extend
Forces acting towards the centre of the spring in both directions is called compression

Question 2

What happens to the spring when tensile/compressive forces are exerted?

Answer 2

Spring undergoes tensile deformation or compressive deformation

Question 3

What is Hooke’s Law?

Answer 3

force applied is directly proportional to the extension in length, up to the limit of proportionality
F = KE

Question 4

What is tensile stress?

Answer 4

Force per unit area

Question 5

What is tensile strain?

Answer 5

Measure of how a material stretches: extension divided by original length

Question 6

What is the difference between elastic and plastic deformation?

Answer 6

Elastic: force is removed and the object will return to its original shape
Plastic: after the force is removed, the object will not return to the original shape

Question 7

How is energy stored during elastic deformation?

Answer 7

Work done is transferred and stored as elastic potential energy

Question 8

Describe the energy changes that occur during plastic deformation

Answer 8

As material is stretched, energy from work done is used to break bonds between molecules, causes permanent deformation

Question 9

What is breaking stress?

Answer 9

the amount of stress a material can take without it breaking

Question 10

What does it mean when a material is brittle?

Answer 10

Does not extend much when a force is applied (tensile strain stays low), tends to break

Question 11

What is the elastic limit?

Answer 11

A point along a force extension graph where a material stops deforming elastically and started behaving plastically

Question 12

What does the area under a force-extension graph represent?

Answer 12

energy stored in a material

Question 13

Equation calculating elastic strain energy in terms on spring constant and extension

Answer 13

1 half x k x (delta x ) squared

Question 14

What is Young’s modulus?

Answer 14

tensile stress/ tensile strain

Question 15

How do you find the Young’s modulus from a stress-strain graph?

Answer 15

gradient

Question 16

What is the limit of proportionality?

Answer 16

The point along the force-extension graph where a material no longer obeys Hooke’s law as Force is no longer proportional to extension

Question 17

What is a spring constant (K)?

Answer 17

The force required to stretch/compress a string by 1 meter

Question 18

Describe a force-extension graph for a metal

Answer 18

The force increases proportionally and then starts to curve, and when unloading, we can extrapolate a straight line down, ensuring the gradient of the unloading curve is the same as the loading curve

Question 19

What is the force-extension graph for plastic

Answer 19

The loading and unloading curves form a leaf sort of shape, and they have different gradients, and the area between the graph is the same as the energy lost by heat

Question 20

What is a hysteresis loop?

Answer 20

Loop formed by unloading and loading

Question 21

Describe a force extension graph for polythene

Answer 21

Loading curve is an uneven curve while the unloading curve is a straight line, and the area between the curves is the energy that never returns

Question 22

Amorphous

Answer 22

Without a clearly defined shape or form

Question 23

Techniques and procedures used to determine the Young Modulus for a metal

Answer 23

1) Measure diameter of wire in 3 different places using a micrometer
2) Attach a meter ruler to a work bench so that the lower end is facing the G clamp and place a marker on the wire at 0cm on the ruler
3) Measure the length of wire from the blocks of wood to the marker on the wire when it is taut
4) Attach a mass to the wire and record the total mass attached to the end of the wire in kg. Record new position of the marker
5) Add another 100g and mark the new position and repeat at least 7 times

Question 24

Techniques and procedures used to investigate force-extension characteristics for arrangements which may include springs, rubber bands, polythene strips

Answer 24

1) Hang spring from a clamp and boss and secure it so that it will not fall off
2) Secure the meter rule vertically to the clamp, using the set square to ensure it is straight and place it adjacent to the spring
3) Record the meter rule reading at the bottom of the spring and place the fiducial marker on this point.
4) Hang mass hanger from the bottom of the spring
5) Add another mass. Record the new meter rule reading, number of masses and total extension
6) Repeat until spring has become permanently stretched
7) Repeat the experiment for the rubber band and polythene strip

Question 25

What is ultimate tensile strength

Answer 25

The maxima stress a material can withhold before it breaks