Potential energy and energy conservation question

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Homework Help Overview

The discussion revolves around the conservation of energy, specifically the application of the energy conservation equation involving kinetic energy, gravitational potential energy, elastic potential energy, and work done by non-conservative forces. The original poster expresses confusion about the variables in the equation and how to determine which terms can be ignored or set to zero in a specific problem context.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the general energy conservation equation and its components, with some questioning how to apply it to a specific problem involving a block and a spring. There are inquiries about which energy terms can be canceled or ignored in the context of the problem.

Discussion Status

Participants are actively engaging with the original poster's questions, providing clarifications about the equation and encouraging the poster to present a specific problem for more tailored assistance. There is a focus on understanding the implications of different energy terms in the equation.

Contextual Notes

There is a mention of forum rules requiring the original poster to show an attempt at a solution, which influences the direction of the discussion. The original poster's absence during the introductory lesson is noted as a factor in their confusion about the material.

Tastosis
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Homework Statement


My teacher told me that this will be the general equation that we will be using. My question is how do I know what to cancel out given a problem? I don't even know what these variables mean.

I was absent during the intro of this lesson and I need a quick answer. Thanks!


Homework Equations


K1 + Uel1 + Ugrav1 + Wothers = K2 + Uel2 + Ugrav2
½MV1^2 + ½KX^2 + mgy1 + Wothers = ½MV2^2 + ½KX2^2 + mgy2
 
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Tastosis said:
K1 + Uel1 + Ugrav1 + Wothers = K2 + Uel2 + Ugrav2
½MV1^2 + ½KX^2 + mgy1 + Wothers = ½MV2^2 + ½KX2^2 + mgy2
This is a general equation that applies when you have kinetic energy, gravitational PE, elastic PE, and external work. Depending on the particular problem, one or more terms may be zero.

If you pose a specific problem perhaps we can be more helpful.
 
This is the general conservation of total energy equation that states that the initial energy of a system plus the work done on it by forces other than gravity and springs is equal to the final energy of the system, where K is kinetic energy, Uel is elastic spring potential energy, Ugrav is gravitational potential energy, Wothers is work done by non conservative forces (forces that are not gravitataional or spring forces), M is mass, V is speed, K is the spring constant, x is the spring displacement, g is the acceleration of gravity, y is the vertical position of the gravity (weight) force with respect to a reference elevation, and the subscripts 1 and 2 refer to initial and final, respectively. It is an extremely important and useful equation, and I suspect you will learn more about each of these terms and the application of such, as your course of study unfolds. Basically it states that energy cannot be created or destroyed ...just transformed into different forms of energy.

Edit: Doc Al is quick!
 
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nice. Thanks guys!

Here's a sample problem. A 2kg block is dropped from a height of 0.4m onto a spring whose force constant is 1960N/m. Find the maximum distance the spring will be compressed.

Based on the equation, what should and shouldn't be cancelled? And why??
 
Well, now you are getting specific, and you must show an attempt at a solution, per forum rules.
 
Tastosis said:
Based on the equation, what should and shouldn't be cancelled? And why??
Rather than think in terms of things 'cancelling', compare the initial and final positions and their associated energy terms.
 
Sorry it took me long to get back to you guys...

[PLAIN]http://img407.imageshack.us/img407/6095/73837542.jpg

Do I cancel U2g? =3
 
Last edited by a moderator:
Tastosis said:
Sorry it took me long to get back to you guys...

[PLAIN]http://img407.imageshack.us/img407/6095/73837542.jpg

Do I cancel U2g? =3
When you say 'cancel', what you mean is 'ignore because it equals zero'. (OK.) As far as U2g is concerned, whether it equals zero or not depends on where you measure it from. Hint: If you measure gravitational PE from the lowest point of the block's motion, then you can set U2g = 0. (But then what does U1g equal?)
 
Last edited by a moderator:

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