AAMC Self-Assessment Physics #36

[DSimone]

The question refers to a two spring system that models the motion of atoms oscillating in a solid. Basically, the mass of the atom is held between two springs with the same k. It asks, "In the oscillatory motion of an atom described by the model, what quantity is conserved?"

A - Total energy
B - Potential energy
C - Linear momentum
D - Angular momentum

I understand that total energy will be conserved (is there any situation in which total energy will not be conserved?), but I also feel like linear momentum is conserved (is there any situation in which linear momentum is not conserved?) Could someone clarity this for me? I thought both momentum and total energy were ALWAYS conserved...there must be something I am missing here . Thanks!

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[Dapik]

To give you a smartass answer, this is an instance when it is not conserved

Energy is conserved because when the PE increases, the KE will correspondingly increase (and vice-versa) to make up for the individual gains/losses.

Linear Momentum is not conserved because the velocity of the ball changes (maximum at the center, 0 at the two end points, and also factor in change of direction).

Most of the time you will see conservation of linear momentum is when there are two objects in a system hitting each other with no outside intervening forces. My suggestion would be not to try and memorize general rules, or if you do and get stuck with them, think about what the concepts actually mean!

 

[Temperature101]

To give you a smartass answer, this is an instance when it is not conserved

Energy is conserved because when the PE increases, the KE will correspondingly increase (and vice-versa) to make up for the individual gains/losses.

Linear Momentum is not conserved because the velocity of the ball changes (maximum at the center, 0 at the two end points, and also factor in change of direction).

Most of the time you will see conservation of linear momentum is when there are two objects in a system hitting each other with no outside intervening forces. My suggestion would be not to try and memorize general rules, or if you do and get stuck with them, think about what the concepts actually mean!

decrease

 

[DSimone]

Thank you, thank you, thank you! So, total energy is conserved because it does not change throughout the movement, whereas individually KE and PE each change (so neither is conserved). And, linear momentum is not conserved because velocity changes in the course of the motion. Got it!

 

[sazerac]

Momentum is only conserved in a system if no external forces are applied to the system..If external forces are applied to a system, then the momentum changes (net force x time). I know external forces are being applied to this system of ball and springs... who da hell is holding dem springs apart?

If you put this entire system into a cardboard box and threw it into the air, then the system of ball + springs + box would have its momentum conserved. The box would also jiggle left-to-right every time the ball jiggled right-to-left and vice versa.