Elasticity

CONTENT

• Definition of elasticity
• Hook’s law
• Tensile stress, tensile strain and young modulus

Elasticity is the tendency of a material to regain its original size or shape after deformation or after it has been compressed or extended.

Hook’s law states that provided the elastic limit is not exceeded, the extension in an elastic material (wire) is proportional to the force applied i.e Fαe

F = Ke…………………………………………1

Where K is force constant, stiffness or elastic constant

Force constant is the amount of force that causes a unit extension. It is the ratio of force to extension of an elastic material.

K → force constant, stiffness or elastic constant

Hook’s law applies up to the elastic limit. For load beyond Lthe wire (material) stretch permanently. The point where small ↑ in load produces large extension is known as yield point.

Breaking point is the point where the wire cannot withstand any further increase in load.

Yield point: it is the minimum stress/load acting on an elastic material beyond which plastic deformation sets in.

Elastic limit is the maximum load (force) which a body can experience and still regain its original size.

EVALUATION

1. Define (a) elastic limit (b) elastic constant (c) yield point (d) breaking point
   1. State Hooke;s law of elasticity

Tensile stress is the force acting on a unit CSA of a wire/rod or force per unit CSA of a wire or rod.

Tensile stress = Force/Area …………………….2

Tensile strain is the extension per unit length

Tensile strain = extension/Original length ……………………..3

Young modulus can be defined as the ratio of tensile stress to tensile strain

Young modules = tensile stress/ tensile strain…………………..4

EVALUATION

1. Define young modulus of elasticity
   1. A spiral spring extends from a length of 10.01cm to 10.10cm when a force of 20N is applied on it.Calculate the force constant of the spring

READING ASSIGNMENT

Read more on elasticity – New school physics (pg 93-96)

GENERAL EVALUATION

1. Explain the anomalous behaviour of water.
   1. State three ways of producing static electric charges

WEEKEND ASSIGNMENT

1. The SI unit of tensile stress is ….. (a) N/m (b) Nm (c) N/m² (d) m²
2. The SI unit of tensile strain is ….. (a) N/m (b) Nm (c) N/m² (d) none of the above
3. Young modulus of elasticity is the  atio of tensile  stress to tensile strain  provide the load does not exceed the (a) breaking point (b) elastic limit (c) yield  point (d)  stress limit
4. A piece of rubber 10cm long stretches 6mm when a load of 100N is hung from it. What is the strain? (a) 60 (b) 0.6 (c) 6×10-² (d) 6×10-³
5. Hooke’s law states that (a) F α A (b)F α e  (c) E α F (d) E α A

THEORY

1. A wire is gradually stretched by loading it until it snaps (a) sketch a load- extension graph for the wire
2. indicate on the graph the elastic limit (E), yield point (Y) and breaking point (B)