Conservation of energy, object at rest

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Discussion Overview

The discussion revolves around the application of the conservation of energy principle to an object released from rest, exploring concepts of kinetic and potential energy in various contexts, including planetary motions and collisions. Participants seek clarification on the definitions and relationships between these forms of energy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant proposes that when using conservation of energy, the kinetic energy would have an initial velocity, leading to the equation K1 + U1 = K2 + U2, assuming the object eventually comes to rest.
  • Another participant suggests that this approach could be applied to determine the height a ball thrown vertically will reach based on its initial speed.
  • A different participant extends the discussion to planetary motions, questioning if the same conservation principles apply when two masses are released from rest at a distance apart.
  • One participant challenges the initial claim, stating that velocity is not a form of energy and that K1 does not equal U1, emphasizing the need for clarity on the definitions of K and U.
  • Another participant points out the vagueness of the original question, asking for clarification on what K1, U1, K2, and U2 represent, and notes that potential energy does not have to be zero when the object is at rest.
  • There is a discussion about the conservation of energy in the context of collisions, with one participant noting that energy can be transformed into other forms, such as heat or chemical bonds, complicating the conservation discussion post-collision.
  • One participant clarifies that energy itself does not have velocity, but objects do, which results in kinetic energy.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and relationships between kinetic and potential energy, and whether the conservation of energy applies uniformly across different scenarios. The discussion remains unresolved with multiple competing interpretations of the concepts involved.

Contextual Notes

There are limitations in the clarity of definitions and assumptions regarding the initial conditions of the system, as well as the specific contexts being discussed (e.g., planetary motions versus simple collisions). The discussion also highlights the complexity of energy transformations in different physical scenarios.

oldspice1212
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Hey,

Say we have an object released at rest, separated by a certain distance..blah, blah, blah. When we use conservation of energy here, the kinetic energy would have a initial velocity right?
So, K1+U1=K2+U2 assuming the object eventually comes to a rest...so U1 = 0, K2 = 0, so we would have K1 = U2 right, unfortunately I am unable to find some problems similar to such and would like to know if my thinking is correct, thanks :)!
 
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oldspice1212 said:
am unable to find some problems similar to such and would like to know if my thinking is correct, thanks :)!
Sounds like you want to determine how high a ball thrown vertically upwards will fly, based on its initial speed. You can use your approach to derive a general formula.
 
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Yes, sort of, I was thinking more so for planetary motions, so let's say: If two masses are separated by a radius...and released from rest, I would say the same thing applies right?
 
oldspice1212 said:
Yes, sort of, I was thinking more so for planetary motions, so let's say: If two masses are separated by a radius...and released from rest, I would say the same thing applies right?
Yes, ignoring air resistance, the ball will come back down with the same speed, as it was thrown up.
 
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oldspice1212 said:
So, K1+U1=K2+U2 assuming the object eventually comes to a rest...so U1 = 0, K2 = 0, so we would have K1 = U2 right
Nope. Velocity(U1) is not a form of energy. Energy associated with motion is kinetic energy. Therefore K1 ≠ U1.

For calculating the speeds of objects in various contexts, you can use the following kinematic formulas:
1) at = v-u
2) S = 0.5at² + ut
3) 2aS = v²-u²
 
Last edited:
oldspice1212 said:
Hey,

Say we have an object released at rest, separated by a certain distance..blah, blah, blah. When we use conservation of energy here, the kinetic energy would have a initial velocity right?
This is pretty much all "blah, blah, blah". You have an object, "separated by a certain distance"- from what? Did you mean two objects? Are they attracted to one another by gravity or an electric force? And kinetic energy is NOT an object so does NOT have a velocity!

So, K1+U1=K2+U2 assuming the object eventually comes to a rest...so U1 = 0, K2 = 0, so we would have K1 = U2 right, unfortunately I am unable to find some problems similar to such and would like to know if my thinking is correct, thanks :)!
It would help if you told us what "K1", "U1", "K2", and "U2" mean. I might guess that "K1" and "K2" are kinetic energy but what are "U1" and "U2"? Because of "conservation of energy" I would think "potential energy" but then there is no reason for potential energy to be 0 when the object is at rest. If you mean U1 and U2 to be speeds, you cannot add energy and speed.

I think you need to review basic definitions and formulas.
 
oldspice1212 said:
Hey,

Say we have an object released at rest, separated by a certain distance..blah, blah, blah. When we use conservation of energy here, the kinetic energy would have a initial velocity right?

KE doesn't "have a velocity".

You said the object was released at rest so it's velocity is zero and it's KE is zero.

So, K1+U1=K2+U2 assuming the object eventually comes to a rest...

You haven't defined what K and U are but never mind.

Just before they collide the sum of their potential and kinetic energy will be the same as they started with.

It's more difficult to include what happens after the collision. Some of the energy in the system might be turned into heat or chemical bonds. All you can say is that the sum of all energy in the system is conserved.

Edit: If you look up inelastic collisions you will see statements that say energy is not conserved. That's because they are only considering the conservation of KE or PE not other forms of energy such as thermal or chemical. If you draw the right boundary around your system and include all forms of energy then energy is always conserved.
 
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Hey CW, sorry this question was very vague I agree as I look at it now.

But, you said KE doesn't have a velocity? Are you just saying that because initially the system was at rest (so KE = 0)? Because KE = 1/2mv^2.
 
All I meant is that in general Energy doesn't have velocity. It's objects that have velocity and as a result the object has kinetic energy.
 
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