[Mechanical Properties of Matter]

Question 1

9. Work Done

Answer 1

W. Force x distance moved in the direction of the force. Equals energy transferred. Equal to the area under an F-x graph. It is a scalar quantity. Measured in joules, J. W = 1⁄2 F x = 1⁄2 k x2.

Question 2

10. Elastic Potential Energy

Answer 2

E EPE. Energy stored in an object because of it deformation. Equal to the area under an F-x graph. Measuredinjoules,J.EEPE =W=1⁄2Fx=1⁄2kx2.

Question 3

11. (Tensile) Stress

Answer 3

σ. The force per unit cross sectional area. Measured in Pascals, Pa. σ = F / A.

Question 4

12. (Tensile) Strain

Answer 4

ε. The extension per unit length. A dimensionless quantity, sometimes written as a percentage, %. ε = x / L.

Question 5

13. Ultimate Tensile Strength

Answer 5

Maximum stress that a material can withstand when being stretched before it breaks.

Question 6

14. Breaking Strength

Answer 6

Stress value at the point of breaking

Question 7

15. Necking

Answer 7

Beyond the ‘ultimate tensile strength’, the material may become longer and thinner at its weakest point before it breaks.

Question 8

14. Young Modulus

Answer 8

E. Within the limit of proportionality, stress is directly proportional to strain. Ratio of tensile stress to tensile strain. Stiffness. Gradient of the straight part of a stress–strain graph. Measured in Pascals, Pa or Nm-2. Often in mega (M) or giga (G) Pascals. E = σ / ε.

Question 9

15. Ductile

Answer 9

Property of a material. Can be drawn into wires or hammered into sheets. Has a large plastic region on a stress–strain graph.

Question 10

16. Brittle

Answer 10

Property of a material. Does not show plastic deformation and deforms very little (if at all) under high stress.

Question 11

17. Polymeric

Answer 11

Property of a material. Materials that consist of long molecular chains, e.g. rubber. They behave differently depending on their molecular structure and temperature. For example, both rubber and polythene show large strains but rubber shows elastic behaviour and polythene shows plastic behaviour.

Question 12

18. Elastic Deformation

Answer 12

A reversible change in the shape of an object due to compressive or tensile force – removal of stress or force will return the object to its original shape and size. No permanent strain or deformation.

Question 13

19. Plastic Deformation

Answer 13

An irreversible change in the shape of an object due to compressive or tensile force – removal of stress or force will not return the object to its original shape and size. Produces permanent strain or deformation.