# Elasticity - Long Question Answer | Physics Grade 11 Solution

### Long Question Solution

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#### Viva Voice Questions # Elasticity - Long Question Answer 1

Question: Define elastic limit and Young’s modulus of elasticity. How can you determine Young’s modulus for material in the laboratory? Explain.

Ans: Elastic limit: elastic limit is the upper limit of deforming force up to which if deforming force is removed, the body regains its original form completely and beyond which if deforming force is increased, the body loses its property of elasticity and gets permanently deformed.

Young’s modulus of elasticity (Y): It is defined as the ratio of normal stress to the longitudinal strain within the elastic limit.
i.e. γ = Normal Stress / Longitudinal Strain

Determination of young’s modulus (Y): Let us consider two vertical wires A and B in which wire A represents reference wire whose upper end is fixed with rigid support and its lower end is connected with the main scale and hence finally with a scale pane containing a certain weight and another wire be represent experimental wire whose value of young’s modulus has to be determined. For it let its upper end also connects (fixed) with a vernier scale and hence finally with a scale pane for containing deferent higher weights as shown in the figure above.

At first, let initial length and initial diameter of experimental wire ‘B’ be measured represented by L and d. Then let a fixed weight like 1kg be placed into a scale pane of wire ‘B’. After this let the initial reading of a vernier calliper on the main scale be noted with empty scale pane than some weight is placed into this pane so that final reading of a vernier calliper on the main scale be also an initial reading of a vernier calliper represents the extension of wire ‘B’. Let this process be repeated by taking different higher weight into scale pane of wire ‘B’ and also their corresponding extension be also noted. After this we plot a graph between weight and extension as shown in fig below: In this figure, let a straight line be obtained passing through an origin whose slope (θ) determines the value of w/∆l
Which can be written as,
Slope = tanθ = Stress/Strain
Since young’s modulus of elasticity can be written as  stress/strain = FL/A∆l = LW/A∆l
Thus, by knowing the value of L, d and W/∆l from above observation value of young’s modulus of experimental wire ‘B’ can be calculated experimentally.

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