3.4 Materials

Question 1

What is a tensile force?

Answer 1

Equal and opposite forces acting on a material to stretch it.

Question 2

What is a compressive force?

Answer 2

Two or more forces together that reduce the length or volume of an object.

Question 3

What is Hooke’s law?

Answer 3

Extension of a spring is directly proportional to the force applied.

Question 4

What equation related force applied to a spring and its extension?

Answer 4

Force (N) = Force Constant (Nm-1) x Extension (m)

Question 5

What does the spring/force constant tell you about the spring?

Answer 5

It is a measure of the stiffness of a spring.

Question 6

What is an experiment used to verify Hooke’s law?

Answer 6

• Attach a spring at one end using a clamp stand setup and put a metre rule attached to the same clamp stand parallel next to it.
• Put slotted masses suspended from the other end of the spring, record the total mass and extension.
• Repeat this when unloading, for two results for each mass.
• Plot a graph of force against extension, and if there is a straight line of best fit through the origin, it obeys Hooke’s law.

Question 7

What does elastic limit mean?

Answer 7

The value of stress/force beyond which elastic deformation becomes plastic deformation, and the material/object will no longer return to its original shape or size when the load is removed.

Question 8

What does plastic limit mean?

Answer 8

An irreversible change in the shape of an object due to a compressive or tensile force, and the removal of stress or force produces permanent deformation.

Question 9

How can elastic potential energy be determined from a force-extension graph?

Answer 9

It is determined from the area under the graph.

Question 10

What are the two equations that can be used to calculate elastic potential energy?

Answer 10

E = 1/2Fx
E = 1/2kx²

F = force (N)
k = force constant (Nm-1)
x = extension (m)

Question 11

If you double the extension of the spring, what happens to the energy stored?

Answer 11

The energy stored will increase by a factor 4.

Question 12

What is a hysteresis loop?

Answer 12

A loop shaped plot obtained when loading and unloading a material produces different deformations.

Question 13

How can you determine the thermal energy lost to the surrounding from a force-extension graph?

Answer 13

The area between the two plotted graph lines/underneath the uppermost line.

Question 14

What is the molecular structure of a rubber band?

Answer 14

Rubber bands consist of squashed and tangled long chain molecules.

Question 15

What is the explanation of the shape of the force-extension curve for a rubber band in terms of molecular structure?

Answer 15

The long chain molecules can be easily untangled with small forces but require large forces to go further once fully straightened.

Question 16

How is tensile stress defined?

Answer 16

The force applied per unit cross sectional area of the material.

Question 17

What is the equation for tensile stress?

Answer 17

Stress (Pa) = Force (N)/Cross-sectional Area (m2)

Question 18

How is tensile strain defined?

Answer 18

The fractional change in the original length of the material.

Question 19

What is the equation for tensile strain?

Answer 19

Tensile Strain = Extension (m)/Original Length (m)

Question 20

What is meant by ultimate tensile strength?

Answer 20

The maximum stress that a material can withstand before it breaks.

Question 21

How is Young Modulus defined?

Answer 21

The ratio of tensile stress to tensile strain when these quantities are directly proportional to each other.

Question 22

What is the equation for Young Modulus?

Answer 22

Young Modulus (Pa or Nm-2) = Tensile Stress (Pa or Nm-2)/Tensile Strain

Question 23

What is an experiment used to determine the Young Modulus of a metal wire?

Answer 23

• Measure the wire’s diameter multiple times using a micrometer and calculate A using (pi x d²)/4.
• Add masses and each time measure the extension using the metre rule underneath the wire and the marker on the wire, and the force applied.
• Calculate stress and strain, and use these to plot a graph stress against strain, the gradient is the Young modulus.

Question 24

What is a polymeric material?

Answer 24

A material consisting of long chain molecules that show large strains.

Question 25

What is meant by the limit of proportionality?

Answer 25

The value of stress or force beyond which stress is no longer directly proportional to strain.

Question 26

What is a yield point on a stress-strain graph?

Answer 26

A point beyond which the deformation is not entirely elastic.

Question 27

What is the difference between ultimate tensile strength and breaking strength?

Answer 27

• UTS = the maximum strength a material can withstand.
• Breaking Strength = the value of strain experience at the point of the material breaking.

Question 28

Which quantity can be determined from the area under a stress-strain curve?

Answer 28

The area determines the strain energy.