Chapter 10 - Deformation

Question 0

Define tensile strain

Answer 0

Strain is the extension per unit original length

Question 1

Define tensile stress

Answer 1

Stress is force per unit cross-sectional area

Question 2

Define the Young modulus

Answer 2

Young modulus is the ratio of stress to strain

Question 3

Explain what is meant by extend elastically

Answer 3

The object returns to its original shape when load is removed

Question 4

Describe the meaning elastic limit

Answer 4

It is the point beyond which the spring will not return to its original length when load is removed

Question 5

State what is meant by elastic deformation

Answer 5

1. It is the change of shape of an object

2. When the force applied is removed, it returns to its original shape

Question 6

State Hooke’s law

Answer 6

Force is proportions to its extension, provided the proportional limit is not exceeded
OR
Extension is proportional to force

Question 7

Equation for strain energy

Answer 7

E(s) = 1/2 kx^2

Question 8

Equation for stress

Answer 8

Stress = Force / Area

= F / A

Question 9

Equation for strain

Answer 9

Strain = extension / original length

= ΔL / L

Question 10

Equation for Young modulus

Answer 10

Young modulus = stress / strain

E = FL / Ae ; e = ΔL or extension

Question 11

Equation for resistance of wire

Answer 11

R = ρL/A ; ρ = resistivity

Question 12

Equation for Young modulus, relating to resistivity ρ and resistance R

Answer 12

E = FR / eρ

Question 13

Equation for extension of spring

Answer 13

F = kx

Question 14

Equation for work done, E

Answer 14

Work done, E = 1/2 kx^2

Question 15

Springs in parallel & series

Answer 15

Parallel: k = k1 + k2
Series: 1/k = 1/k1 + 1/k2

Question 16

Suggest why a thin rod can bend more than a thick rod without breaking

Answer 16

1. When bent, the top and bottom edges have different extensions
2. With thick rod, the difference between the extension lengths is greater than with a thin rod
3. So, it breaks with less bending

Question 17

Why the walls of tyres become warm, in accordance to the stress-strain graph of rubber?

Answer 17

1. The area between the lines represents the lost energy when rubber is stretched and then released
2. This energy is seen as thermal energy

Question 18

Suggest how you could check that the elastic limit of the wire is not exceeded when the extra load is added

Answer 18

Remove the extra load and see if the wire returns to its original length

Question 19

Describe apparatus for a Young modulus experiment

Answer 19

1. Weights are used to straighten wire
2. The horizontal wire is fixed to a clamped wooden
   block over a pulley bearing the weights

Question 20

Describe the method used to require the measurements

Answer 20

1. Measure the original length of wire to the marker
   with a metre rule
2. Measure diameter of wire with a micrometer
   screw gauge to calculate the cross-sectional
   area
3. Measure the diameter in several places and take
   the average
4. Measure the initial and final reading for extension
   of wire with a metre rule
5. Record the weights used for extension
6. Take several readings with different load

Question 21

Describe how the measurements taken can be used to determine the Young modulus

Answer 21

1. Calculate the area from πd^2 / 4
2. Determine extension from final and initial
   readings of extension
3. Plot a graph of force against extension
4. Calculate E from E = Fl / Ae

Question 22

Why Young modulus is not used to describe liquids or gases?

Answer 22

Fluids have no fixed shape