Question about the energy of 2 blocks with a spring between them

[hello478]

Homework Statement

image below

Relevant Equations

energy equations

My attempt:
my answer was b
correct answer is d

 

[BvU]

Can you explain why you picked b ?
Do you know the expression for kinetic energy?

 

[hello478]

Can you explain why you picked b ?
Do you know the expression for kinetic energy?

because before the release they are stationary
and it is elastic collision, maybe...
so the kinetic energies would add upto 0
so that the total KE is conserved...

 

[kuruman]

so that the total KE is conserved...

"Conserved" means "it is the same" throughout whatever is going on. Before release the sum of kinetic energies is zero because neither block is moving. If, as you claim, this sum is conserved, then neither block should be moving after release. Do you think that is the case?

 

[Orodruin]

because before the release they are stationary
and it is elastic collision, maybe...
so the kinetic energies would add upto 0
so that the total KE is conserved...

Why would kinetic energy be conserved? This would mean the blocks just remain stationary as kinetic energy is non-negative.

 

[BvU]

Do you know the expression for kinetic energy?

 

[hello478]

1/2 mv²

 

[hello478]

oh ok so, the velocities direction would not be considered.... thats the only thing i understand uptill now

 

[Orodruin]

oh ok so, the velocities direction would not be considered.... thats the only thing i understand uptill now

It sounds to me that based upon your many posts with seemingly little thought behind your attempts that what you really need to do before attempting these problems is to sit down with your book and try to understand the theory behind the problems first. Otherwise you will just be shooting wildly with your answers.

 

[hello478]

It sounds to me that based upon your many posts with seemingly little thought behind your attempts that what you really need to do before attempting these problems is to sit down with your book and try to understand the theory behind the problems first. Otherwise you will just be shooting wildly with your answers.

ill try to do that, though i sort of know what the book says... physics is just not my thing so its difficult for me to grasp
when i read the book, everything is so easy until the questions blow my mind...

 

[Orodruin]

Sort of know typically won’t do. You need to actually know and most of all understand what it says. Work through any derivations so that you understand where things come from, do the same with any examples. Go equation by equation and make sure you understamd how and why it came about.

 

[BvU]

ill try to do that, though i sort of know what the book says... physics is just not my thing so its difficult for me to grasp
when i read the book, everything is so easy until the questions blow my mind...

Tip (that worked for me): while reading, invent tough exercises you could test your classmates with.

Re 'not my thing': seems to me math isn't either. But systematic reasoning and logical thinking are indispensable in all fields! (what's yours?)

$\$

 

[hello478]

Tip (that worked for me): while reading, invent tough exercises you could test your classmates with.

ok ill try to do that too, but i dont have much time till my exams...
and im a private student
so... i dont have classmates

(what's yours?)

and im currently doing physics AS levels and biology A levels