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.

Newton's 2nd Law

Sometimes to understand physics subject, we need to use some common sense. Let me give u one example, assuming that a coconut and a tennis ball  is falling from the same height onto your head..which one will hurt u the most?? I can bet that a 13 year school boy who didn't learn physics can tell us  that the coconut will exert greater force. All he needs is to use some common sense.

Now lets look at  physics point of view. We learned that gravity will pull both coconut & the ball at the same acceleration if the air resistance is negligible. Then curiosity is telling us there must be other factor that causes a painful force produced by the coconut. Finally we can conclude that the different mass explained different forces produce by the ball & coconut. This situation describes how the concept of Newton's 2nd law can be applied.

According to this law,the force of a moving object is equal to product of its mass & acceleration. These two factors i.e the mass & acceleration will determine how big the force is produced. In other word, Newton 2nd law is concerned in relating acceleration, mass & the net force.

Fnet= ma

If u notice, i use the term net force instead of force only. This is because, in SPM most of the question we deal with a case that involves more than one force. The net force means that we need to add or minus all the forces involved. For example,

A car with a mass of 100 kg moves with a driving force of 500N. Each tyre of the car causes a frictional force of 5N. Determine the car's aceleration??

Before u solve this problem, u need to identify all the information given. There are two type of forces involved i.e driving force & frictional force. Both forces are in different direction...seems each tyre causes friction, thus 4 tyres causes (4 x 5N) = 20 N. This 20 N  frictional force will oppose motion of the car. 

Thus the NET FORCE will be (500 - 20)N. The final calculation should look like this;

      Fnet = m x a

500 - 20 = 100 x a

           a = 4.8 ms-2

To understand better about Newton's 2nd law of motion, u should watch the following videos.

Newton's 1st Law

Isaac Newton a 17th century scientist who is a very prominent figure in the world of physics. His theory & laws had been widely used till now and  it even helped  us to enjoy the rapid development of Formula 1 racing technology.
We are now living in a post IT society which is full of entertainment. We are able to do almost everything through online i.e playing games, facebook, shopping, watching movies etc. Those days in 17 th century, they don't have many entertainment as what we're having now. I would say the best entertainment for them are to seek more knowledge as possible. They don't have youtube, facebook, PS3, or among the worst Datuk T's pornographic movie. In other words they were not distracted with all these time wasting stuff.


Sometimes i 've been thinking that being a knowledgeable person at those days could be like a Hollywood artist. People will respect & adore them. In fact, this was the century where gifted persons like Isaac Newton, Galilieo Galilei, Blaise Pascal managed to revolutionalize their society towards seeking knowledge.That's why once they 've founded a principle or laws, they would place their names for us to appreciate until the end of time.

As for today, i want to discuss one of his laws that we will learn in SPM. It is the Newtons 1st Law of motion. Usually when we refer to this law,SPM students are more familiar with the term inertia. So what do this law tells us?

Newton's 1st law of motion often stated as:
An object at rest has tendency to stay at rest and object in motion stays in motion with the same speed and in the same direction UNLESS there is unbalance force acting on it.


Allow me to make it easier for u to understand. Its like this, all objects tend to '"keep on doing what they're doing" unless there is 'unwanted Force' yang kacau. For example, imagine it's a school holiday and your mom try to wake you up solat subuh. In this case, u have a 'big tendency' to remain on your bed, unless your mom pour a bucket of water on your face :) ..So the bucket of water will be the 'unwanted force'


Let me give u another classic analogy. If u are a noble, honest & never watch pornography in your life, then u should maintain your 'inertia' i.e your dignity eventhough there is unwanted forces like 'Mr T' who tempted u to watch sex ,lies & video tape (:


To understand better on concept of inertia or Newton's 1st law of motion,u should watch all these videos.