Hooke's Law & Elasticity

Last weekend i gave physics tuition for form 5 student at a program organised by Masjid Negara. We managed to complete revision on chapter Forces & Motion which is one of the most feared chapter among student. The last topic we discussed was about Elasticity.

Masjid Negara tuition program ...unlike the boys the girls looked serious ha :)  

After the class, i can conclude that most student having difficulties to define physics concept because they don't fully understand each word or term used to describe it. For instance, in physics we do learn the concept of elasticity. Elasticity comes from the word elastic. Actually one doesnt need to learn physics to understand the term elastic, all he needs is a dictionary plus his experiences after a bungee jump :).
Ok..If u take out an Oxford english dictionary, elastic means
able to resume its normal shape spontaneously after being stretched or compressed.


 In physics , the term ELASTICITY is almost similar to the one defined in dictionary, i.e...
A property of object that enable it to return into its original length & shape after an applied external force is removed
However, we science stream student should be able to understand the term elasticity beyond what have been understood by the lament. Again, lets look in depth the term elasticity in physics point of view.


In SPM, the concept of elasticity leads us to appreciate another  famous physics law called a Hooke's Law.
This is the law that relates the force applied and the extension or compression of the elastic object such as spring and rubber band.

Hooke's law states that;
The extension of elastic material such as spring is directly proportional to the force applied provided the elastic limit is not exceeded.


In simple word it means, the larger the force applied, the longer the extension of the spring as long as it doesnt exceed the elastic limit. Please take note that extension means increase in length.. not the total length of the spring when the force is applied.

If we look at mathematics point of view, we have a relation between F( force) and x (extension), thus we arrived to a variation formula of

F= kx

where k is the 'spring constant or force constant'

The problem with our syllabus, the maths we learn in form 4 does not tally with the one that we learned in physics. The concept of variation or proportionality will only be learned in form 5 maths. That is part of the reason why some form 4 students are having difficulties to understand the term 'k' i.e force constant. 

However if u r form 4, at the moment just appreciate that same spring with same material has fixed value of 'k'

If we pull a spring with a force F, the spring length is displaced, i.e it is extended.This product of Force(F) & displacement (x) tells us that work is being done. (Recall Work=Force x displacement) Thus we can say taht there is energy stored in the spring. This energy is known as elastic potential energy. It can be determined using the following formula,

Elastic Potential Energy = 1/2 (F)(x)

                                        

Usually every physics's law has graphical explanation. Seems it is a directly proportional relation between F & x, so we'll have a straight line passes through origin.

From the graph, u must acknowledge that..

          Gradient = force constant (k)

where, the steeper the gradient, the bigger the spring constant,i.e  the stiffer the spring

Meanwhile, from the graph  its area represents potential energy, 

Area under the graph = Elastic potential energy

to understand further on this law, please watch this video.