MATERIALS AS

Question 1

Define density

Answer 1

Mass per unit volume

Question 2

A block of wood is cut in half. Explain why the density of the two pieces do not change.

Answer 2

• volume is halved, but mass is also halved. density = mass/volume, so density does not change.

Question 3

State Hooke’s Law in words.

Answer 3

• The change in length of an object
• is directly proportional to the tensile force applied
• up to the limit of proportionality.

Question 4

Describe elastic deformation.

Answer 4

Elastic deformation is deformation in which the object returns back to it’s original shape and size when the force is removed.

Question 5

Describe plastic deformation.

Answer 5

Plastic deformation is deformation in which the object does not return back to it’s original shape and size when the tensile forces are removed.
Permanent deformation.

Question 6

Define the elastic limit.

Answer 6

The point on a load-extension or stress-strain curve beyond which an object undergoes plastic deformation.

Question 7

Different materials have different load-extension curves. Sketch loading and unloading curves for:

A metal wire
Rubber
Polythene strip (e.g a carrier bag)

And describe their key features.

Answer 7

Key features:
Metal wire: extension is d.p to the load up to limit of proportionality, beyond elastic limit, plastic deformation occurs, permanent extension shown by two parallel lines.

Rubber:
Initially high energy required to untangle elastic fibres, curves out, then steepens again
Unloading curve lower than loading curve

Polythene strip:
Similar to rubber, however, plastic deformation occurs as the polymers cannot tangle together again

Question 8

Define stress in words and give the equation.

Answer 8

• the tensile force per unit cross-sectional area

Question 9

Define strain in words and give the equation.

Answer 9

• the extension per unit length

Question 10

Give the equation for the young’s modulus.

What is it a measure of?

If I wanted to measure the young’s modulus of a wire, how could I use graph paper to do so?

Answer 10

tensile stress over tensile strain

Young’s modulus is a measure of the stiffness of a material.

Plot a graph of stress against strain
Gradient of straight line section = Young’s modulus

Question 11

On a stress-strain graph, what does the area under the straight-line section represent?

Answer 11

-Energy stored per unit volume

Question 12

Define breaking stress.

Answer 12

• The stress at which a material fractures.