Isn't Momentum always conserved?

[Redpancreas]

2012 TBR; Pg. 260...


Considering the momentum and total energy of the two masses and spring, after the oscillations begin:

a.) momentum and total energy are both conservative.

b.) momentum is not conserved, but total energy is conserved.

c.) momentum is conserved, but total energy is not conserved.

d.) neither momentum nor total energy is conserved.


Apparently the answer is B? Can someone explain. I stopped reading the explanation in TBR after it said momentum isn't conserved because that goes against almost all of the stuff I've been thought in physics. 😡

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[milski]

Makes no sense to me either. The only time moment won't be preserved is when you're not accounting for the whole system and missing part of it. Spring with two masses is not that case though. Unless gravity is involved? As the masses bouncing up and down? What's the TBR's explanation?

 

[milski]

Ok, I looked it up in BR. It is as expected - they are asking you to consider only part of the system. Since the second mass is attached with the spring to the wall and some of the momentum is transferred to/from the wall. To be able to say that the momentum is preserved, you have to include the wall (and anything connected to it) in the system under consideration.

I agree - the explanation of the answer could be simpler and more clear.

 

[theCLE]

Reading this tomorrow. There's no doubt I would've been stumped on it. Thanks for the heads up.

 

[milski]

Might be a good idea to put TBR in the thread title - it's a good question to try without knowing the answer.

 

[Redpancreas]

Ok, I looked it up in BR. It is as expected - they are asking you to consider only part of the system. Since the second mass is attached with the spring to the wall and some of the momentum is transferred to/from the wall. To be able to say that the momentum is preserved, you have to include the wall (and anything connected to it) in the system under consideration.

I agree - the explanation of the answer could be simpler and more clear.

Thanks! This helps alot!