15085 Hooke's Law Apparatus - info

INTRODUCION

This aparatus is carefully designed to provide componets which have the right properties to give suitable experimental results for use in class. With the Hooke’s Law Apparatus it is possible to verify Hooke’s Law, i.e. that the extension of an elastic body is proportional to the applied force.

CONTENTS

Qty                       DESCRIPTION                                         

1                      tripod base                                        

1                      metric rod                                         

1                      clamp ø 6 mm                                    

1                      rod with hook                                    

4                      springs with different diameters                   

1                      mass holder with index                               

5                      10 g disc mass                                   

5                      50 g disc mass                                   

1                      instruction manual

EXPERIMENT

(N.B. the instructions have diagram of component parts and complete apparatus) 

In the case of a spiral spring, the extension ∆L corresponds to the difference between the length of the spring L before and L + ∆L after the application of the deforming force F.

Practically it is enough to take note of the starting length over the graduated rod and then apply to the mass holder some increasing masses, noting the extension for each mass. (The force applied is the mass x the acceleration due to gravity “g”, i.e. F = mg)

It is useful to tabulate the results in three columns F, ∆L and F/∆L, and also to plot a graph of ∆L against F in Newtons (N) or m in grams (g).

It is possible to verify that within the limits of the experimental error the ratio between the extension and the force is a constant.

and K        = F/∆L             

We suggest you repeat the experiment with different springs as supplied, and verify that the stiffer the spring, the larger is K.

What kinds of materials obey Hooke’s law?

What happens if you substitute a rubber band for the metal spring?

Does it apply to a strip of plastic?

To compare the elasticities of different materials it is usual to express the elastic constant in the form of Young’s Modulus, E. This expresses the fractional extension of a solid material of unit length and unit cross-section area for unit force.

Young’s Modulus, E = applied stress/resulting strain = (F/A) /(∆L/L)

Where A = cross-section area. As the fractional extension has no units, The Young’s modulus has units of stress (or pressure), Pascal (Pa).

As the Pa is a very small unit (a pressure of 1 atmosphere,1 bar = 0.1MPa) and extensions for solid metals are very small, typical values of E can be expressed in GPa (gigaPascal), for example, approximately:

Nylon, E = 3 GPa; Lead, E = 16; Aluminium,  E = 70 GPa; Brass,  E = 100; GPa , E= 200 GPa; Tungsten, E= 400