When to use Hookes?

[kenhung123]

I am really unsure when do I actually use hookes law because sometimes when something is tied to an elastic band and launched at a height, I assumed mgh=1/kx^2 provided extension, mass and height I solved for k. However sometimes that is wrong and you are actually supposed to use Hooke's law to solve for F=-kx where F=mg. So my main concern is when do I know I should use Hookes, and why can't I use conservation of energy?

[TrueTears]

Hooke's law should be applied when the elasticity of the object is linear.

Conservation of energy is used only if the system is an isolated system.

[kamil9876]

this may be useful http://vcenotes.com/forum/index.php/topic,14137.0.html

[kenhung123]

Hmm i get what your trying to say but its still quite tricky in terms of identifying which method to use in a question

[kamil9876]

Give examples of specific questions then.

[kenhung123]

John was driving too fast in his bike and crashed into a tree. The front of his car compressed by 35cm. (There was a characteristic graph which can e used to identify 'k') Then it asked whats the maximum force experience by the bike when crashing into the tree. I took the approach of finding Ug using 1/2kx^2 then make it equal fx (work) then divided the work by 0.35 to get force..

[kamil9876]

what's a characteristic graph?

Also: your approach assumes that the force was constant, Work=Force x Distance only when the force is constant.

[kenhung123]

Just a F vs extension used to find the k constant

[kamil9876]

Wouldn't that just be a matter of finding the maximum directly by looking at the graph?

[googoo]

Conservation of energy is used only if the system is an isolated system.

When we use con. of energy in the case of roller-coaster ride, what is the isolated system?

[m@tty]

The roller coaster. There is no friction, so there is no leakage of energy. Hence the energy at any point on the track must sum to the energy at the beginning.

[googoo]

But what is the isolated system?

[kenhung123]

Wouldn't that just be a matter of finding the maximum directly by looking at the graph?

True but I don't get why we need to use F=kx

[m@tty]

You are looking for force, not energy, so F=kx is a simpler method, especially if you have been given a graph.

[TrueTears]

Conservation of energy is used only if the system is an isolated system.

When we use con. of energy in the case of roller-coaster ride, what is the isolated system?

are you talking about the roller coaster car (the thingy u ride in) or the entire structure of the roller coaster?

if the latter then the isolated system is the earth and the roller coaster.