Springs and Materials

Question 1

How do you determine the spring constant of a spring?

Answer 1

1. Record extension of the spring with varying forces (force is the weight of the mass)
2. Plot force against extension graph
3. Gradient of the graph is the spring constant

Question 2

What are the features of a force extension graph?

Answer 2

Straight line through the origin up until elastic limit, at this point the string has been permanent deformed and the graph begins to level out.

Question 3

What is Hooke’s law and when do springs obey it?

Answer 3

Hooke’s law states that the extension of a spring is directly proportional to the force.
It is obeyed when the elastic limit has not been exceeded.

Question 4

What is strain and its units?

Answer 4

The extension produced per unit length, no units.

Question 5

What is the equation for strain?

Answer 5

extension/original length or x/L

Question 6

What is stress and its units?

Answer 6

Force acting per unit cross-sectional area, Pascal (Pa)

Question 7

What is the equation for stress?

Answer 7

Force/X sectional area or F/A

Question 8

What is Young’s Modulus?

Answer 8

Stiffness of a material being stressed.
Stress/Strain or FL/Ax
Gradient of a stress/strain graph

Question 9

How do you determine young modulus?

Answer 9

• Two long wires of same material, length and diameter hung from common support
• Q is wire under test, P is comparison wire stop errors due to: expansion because of change in temperature, sagging of support.
• Measure length of Q
• Measure X-Sectional area of Q (micrometer, multiple readings)
• Measure extension of Q using vernier scale
• Change load on Q and measure extension
• Plot force against extension
• Gradient = f/x
• E = gradient * L/A

Question 10

What is elastic deformation?

Answer 10

If the wire/spring returns to original length when applied deforming force is removed.

All materials show elastic behaviour up to elastic limit.

Question 11

What is plastic deformation?

Answer 11

Occurs when loaded beyond elastic limit.

It is permanent deformed when deforming force is removed.

Question 12

What does strength mean?

Answer 12

Stress at which the material breaks.

Question 13

What does ultimate tensile strength/breaking stress mean?

Answer 13

Stress at which the material breaks.

Question 14

Describe what brittle means.

Answer 14

Elastic behaviour up to the point of fracture

Sudden application of large stress will cause a brittle material to fracture.

Question 15

Describe what ductile means.

Answer 15

Behaves elastically up to elastic limit, beyond this it stretches more and more and will not return to original length (plastic)

Can be shaped by stretching, rolling, squashing and hammering

Question 16

What is responsible for the behaviour of polymeric materials?

Answer 16

Temperature and molecule structure.

Question 17

What is elastic potential energy?

Answer 17

Energy stored when a solid is stretched or compressed.

Question 18

When can elastic potential energy be recovered?

Answer 18

When the material has been strained elastically.

If the material has been strained plastically some of the work done has gone into moving atoms past each other and the energy is non-recoverable.

Question 19

How do find work done from a force extension graph?

Answer 19

Area under the graph