3.4 - Materials

What is extension and compression of a spring?

Tensile forces act away from the centre of the spring in both directions and will stretch it out causing it to extend.

Forces acting towards the centre of the spring in both directions compress.

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

The spring undergoes 'tensile deformation' (tension forces) or 'compressive deformation' (compressing forces)

What is Hooke's law?

The force applied is directly proportional to the extension in length (up to limit of proportionality)

F=kx k = spring constant, x = change in length

What is meant by tensile stress?

Force per unit area.

What is tensile strain?

A measure of how the material stretches: the extension divided by the original length. This value is a ratio, so it has no units.

How is energy stored during elastic deformation?

Elastic potential energy.

What is breaking stress?

The amount of stress a material can take without it breaking.

How to determine if a material is brittle?

To determine if a material is brittle based on its stress-strain graph, look for an almost purely linear plot that ends abruptly with very little strain

What is meant when a material is described as brittle?

It does not extend much when a force is applied (tensile strain stays low). Material tends to break rather than stretch under a large force.

What is the elastic limit?

The point after the which plastic deformation occurs. It is sometimes also referred to as the 'limit of proportionality'.

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

The energy stored in material

Give the equation that calculates elastic strain energy in terms of spring constant and extension.

E = 1/2 k L^2

What is Young's modulus?

Young's modulus = tensile stress / tensile strain

Describing a material?

- State what type of material it is (brittle, polymer etc)

- Elastic?

- Obeys Hooke's law?

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

Gradient

What measurements are required to determine Young's modulus through an experiment?

Initial length

Extension (initial and final lengths)

Weight (mg)

What equipment is needed to determine Young's modulus?

Micrometer or Vernier caliper (diameter of wire)

Ruler (initial length)

Scales (mass)

Newtonmeter (weights of masses)

How can you determine Young's modulus from measurements?

Stress = Force/cross-sectional area

Strain = Extension/original length

Young's modulus = Stress/strain, Determined by gradient of Stress/strain graph