Help w/ conservation of mech. energy

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    Conservation Energy
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Homework Help Overview

The discussion revolves around the concept of conservation of mechanical energy, particularly in the context of physics problems involving potential and kinetic energy. Participants are exploring scenarios involving objects in motion and the calculations related to their energy states.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants express difficulty in visualizing mechanical energy and seek clarification on its definition and application. Specific problems are presented, such as calculating the height of an object based on its kinetic energy and understanding energy conservation in different scenarios. Questions arise about starting points for calculations and the relationships between kinetic and potential energy.

Discussion Status

Some participants have provided guidance on how to approach specific problems, emphasizing the importance of understanding initial mechanical energy and the conservation principle. Multiple interpretations of the problems are being explored, with participants questioning their assumptions and the details of the scenarios presented.

Contextual Notes

Participants are working under the constraints of homework assignments, which may limit the information available for problem-solving. There is an emphasis on not using non-conservative forces and ensuring proper unit conversions in calculations.

Femme06Fatale
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Alright - i have a huge physics test on tuesday and certainly need TONS of help w/ this stuff - i have a really hard time visualizing what it is that we are calculating. So now, we were just introduced to the conservation of mechanical energy - so i was wondering if there's anyone out there that would care to help me w/ it .. like trying to understand any of it -- PLEASE :) It'd be greatly appreciated! Thanks much :)
 
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You're going to have to be more specific. Mechanical energy is the sum of the total kinetic and potential energy of a system or object.

[tex]E_{\mbox{mechanical}}=E_{\mbox{kinetic}}+E_{\mbox{potential}}[/tex]

Mechanical energy isn't really anything you can "understand" or visualize; just think of it as a useful term for defining the above sum.

Ask questions about specific problems for more help.
 
The largest watermelon ever grown had a mass of 118 kh. SUppose this watermelon is exhibited on a platform 5.00 m above the ground. After the exhibition, the watermelon is allowed to slide to the ground along a smooth ramp. How high above the ground is the watermelon at the moment its kinetic energy is 4.61 kJ? -- Where do I even start?
 
We assume an isolated system (no non-conservative forces like friction), so we can say that mechanical energy is conserved. Therefore, at any point during its fall the watermelon has only as much energy as it had before it was allowed to slide to the ground. You are given kinetic energy and you have information about the mechanical energy.

Use [tex]E_{\mbox{potential gravitational}}=mgh[/tex].
 
Femme06Fatale said:
The largest watermelon ever grown had a mass of 118 kh. SUppose this watermelon is exhibited on a platform 5.00 m above the ground. After the exhibition, the watermelon is allowed to slide to the ground along a smooth ramp. How high above the ground is the watermelon at the moment its kinetic energy is 4.61 kJ? -- Where do I even start?

Start by finding the initial mechanical energy. At the top of the platform, if the watermelon in motionless, its total mechanical energy is the gravitational potential energy, since there is no kinetic energy. To solve the problem, remember that the total mechanical energy (sum of Ep and
Ek) always remains constant.
 
The deepest mine ever drilled has a depth of 12.3km. Suppose you drop a rock w/ a mass of 120.g down the shaft of this mine. What would the rock's kinetic energy be after falling 3.2km? What would the potential energy associated w/ the rock be at that same moment? Assume no air resistance & a constant free-fall acceleration.

So I know that the equation for ke=1/2mv^2 and I know that pe=mgh
So do I just plug in what i know for pe ... making it 120(9.81)12.3 for pe and then for ke, i have to find velocity first --- can anybody tell me if I'm thinking correctly?
 
Femme06Fatale said:
So I know that the equation for ke=1/2mv^2 and I know that pe=mgh
So do I just plug in what i know for pe ... making it 120(9.81)12.3 for pe and then for ke, i have to find velocity first --- can anybody tell me if I'm thinking correctly?
(1) Energy is conserved. That means: [itex]\Delta {KE} = - \Delta {PE}[/itex].
(2) What's the change in height? (It's not 12.3 km.)
(3) The mass is 120 g. Convert to kg.
 

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