Lesson 6, Topic 2
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# Verification of Hooke’s Law

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Aim: Verification of Hooke’s Law.

Apparatus: Stand and clamp, meter rule, spring, pan, 5 equal masses, Plasticine and optical pin.

The experiment is set up as shown. A pointer P is attached to the spring with plasticine and the hanger suspended on the spring. The pointer level is read from the clamped metre rule.

Read and record the initial reading of the pointer when no mass is added as L0. Add a mass or weight (W1) to the scale pan. Read and record the new position (reading) of the pointer as L1.

Find the extension e1 as e1 = L1 – L0.

As weights are added gradually, there is an extension produced for each weight added which is recorded.

After taking five (5) readings as different weights are added, a graph of load against extension is plotted. A straight line is obtained through the origin.

The slope of the graph is $$\frac{F}{e}$$, which according to Hooke’s law represents the elastic constant of the material, k.

When further load is added to the spring, that the elastic limit is exceeded to the point that the spring breaks, the graph obtained is shown.

From the graph,

From point O to point P, Hooke’s law is obeyed. The extension is proportional to the force applied. Point P is the proportionality limit of the material.

Point E is the elastic limit of the material. Up to this point, the elasticity of the material still holds. The object will return to its original length if the force applied is removed. Beyond the elastic limit, the material loses its elasticity.

Elastic limit is the limit of force beyond which the stretched wire does not return to its original length when the stretching force is removed.

OE is called the elastic region. That is the material is still elastic in its behaviour. Beyond E, the wire extends rapidly when more force is applied. In this region, the material does not return to its original length.

Point Y is called the yield point, any additional load will produce a large extension, which defies Hooke’s law. At this point, the material loses all its elasticity permanently and becomes plastic. This change from elastic to plastic by the material is noticed as a sudden and rapid extension when a small force is applied.

Yield point is the point beyond the elastic limit in which the elastic material has yielded all the elasticity permanently and has become plastic.

Point B is called the breaking point. Maximum extension is reached at this point. Any further addition of force to extend the material beyond this point will cause the spring to break.

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