/sci/ - Science & Math

[Anonymous No.15187752]

Alright /sci/. I have a mechanics question. If you want to solve part a) for this problem, you use Newton’s second law to set up and solve a BVP with a second order differential equation. If you want to solve part b), you can use impulse-momentum or conservation of energy to solve for the velocity as the rope leaves the table.

My question is, in general, how the fuck do I learn how to approach a problem? I understand how to use energy conservation, NII, impulse momentum etc but I don’t get how to tell which to apply. Thanks love you

[Anonymous No.15187974]

>>15187752
>My question is, in general, how the fuck do I learn how to approach a problem?
Draw a free body diagram

[Anonymous No.15188517]

>>15187974
How would a free body diagram tell one when to use energy conservation vs Newtons second

[Anonymous No.15188604]

bump

[Anonymous No.15188677]

>>15187752
Just go through your tools and think if you can apply them or not.
Do enough problems and you will see at a glance what to apply to solve a particular problem.
Also, don't be afraid to ask your professor (not /sci/) about it if you are dead stuck on something.

[Anonymous No.15188757]

>>15187752
Nobody's going to tell you and even if they could it wouldn't do you any good since you'll immediatelly get stuck on the next problem. This is going to continue until you stop asking for hints and start figuring stuff out on your own.

Write down all the equations you know and just try everything you can. It's going to be time consuming and frustrating for the first 20 exercises but then next 500 are then going to be piss easy for you.

>be employee
>boss tells you to do something
>"but boss, how do I approach this problem?"
>"What? How the fuck should I know? I pay you to figure this shit out, otherwise I would have saved money and just done it myself. Figure it out or you're fired."

[Anonymous No.15188891]

>specifies approach for a)
>specifies approach for b)
>asks internet how to approach problem
are you brain damaged? your insecurity will force you to die a virgin.

[Anonymous No.15190407]

>>15188517
An FBD does not "tell" you anything. It's the methodical starting point for any mechanics question. It's alI you need for any mechanics question because every mechanics question is ultimately solvable by newtons 2nd. It may be very sloppy or it may be straight forward, but it will always eventually give you the right answer. You won't break physics by using it. If it starts looking really sloppy or you get stuck, try another method that might be a "shortcut" to the answer.
By asking "when" to use "newtons 2nd" over "energy conservation" you seem to be asking what is the most elegant/fastest solution to answer a textbook question that is contrived to force you to use one method over the other for learning. That is really just asking how to be a savant and see though a problem to the end before you even start. I can't really help you there. Refer to the current chapter headline in your textbook and use that method, because it's probably the method it wants you to use.

[Anonymous No.15190664]

>>15187752
the potential energy at [eqn]t=0[/eqn] is [eqn]U(0) = -mgL_0/2[/eqn]. after the rope has fallen [math]dy[/math] the potential energy has dropped by [math]dU = -mg dy/2[/math]. once the cable has left the table, potential energy has dropped by [eqn]\Delta U = \int_{0}^{L-L_0} -mg dy/2 = -mg(L - L_0)/2[/eqn] and so by conservation of energy we must have [eqn]\Delta T = mg(L-L_0)/2 = mgv^2/2[/eqn], which finally gives us the final speed of the rope immediately when the cable leaves the table: [eqn]v = \sqrt{g(L-L_0)}[/eqn]

[Anonymous No.15192241]

Thanks guys