Questions on conservation of energy

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Homework Help Overview

The discussion revolves around a physics problem involving the conservation of energy, specifically related to an airplane dropping a package from a height of 250 meters. Participants are exploring the kinetic and potential energy of the package at different velocities and questioning the implications of the height in relation to energy calculations.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are attempting to calculate kinetic energy at specified velocities and are questioning how to determine the remaining energy. There is discussion about the relevance of the height and whether the initial speed of the package is zero. Some participants are exploring the relationship between total energy, kinetic energy, and potential energy.

Discussion Status

There is an ongoing exploration of the concepts of total energy, kinetic energy, and potential energy. Some participants have provided guidance on using conservation of energy principles, while others are still seeking clarity on how to apply these concepts to find the correct height and remaining energy.

Contextual Notes

Participants are working with a specific set of equations provided by the original poster and are navigating uncertainties regarding the initial conditions and the interpretation of energy values. There is a lack of consensus on the correct approach to finding total energy and the relationship between kinetic and potential energy in this context.

Kalix
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Homework Statement


Question: An airplane is 250m from the zero heigh and drops a package:

a. What is the objects kinetic energy if the velocity is moving at 10m/s. How much energy is left? What height should the object be at?

b. What is the objects kinetic energy if the velocity is 30m/s? How much energy is left? What height should the object be at?


Homework Equations


KE=.5mv^2
PE=mgh
mgh(initial)+mv(initial)^2=mgh(final)+.5mv(final)^2 (I am not sure if this equation is relevant but it might be so I thought I put it down)

The Attempt at a Solution


a. KE=.5(75)(10^2)=3750 J
I have no idea how to find how much energy is left. Is there an equation for this/
h=3750/75x9.81=5.1m

b. KE=.5(75)(30^2)=33750 J
Again I don't know how to find the amount of energy left over.
h=33750/75x9.81=5.1m

Also I don't understand what the 250m is there for but I guess it may have something to do with the amount of energy left.
 
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Kalix said:
Also I don't understand what the 250m is there for but I guess it may have something to do with the amount of energy left.
Are we to assume that the package (and the plane that drops it) has an initial speed of zero?

If so, what's the total energy? You have the formula.
 
Yes we can assume that the initial speed is zero when the package is dropped from the plane. I actually don't know how to find total energy. I don't see the equation on the equation sheet that my teacher gave us. It seems likely that TE might equal KE + PE but I really don't know for sure. Also in this particular problem are PE and KE equal to each other?
 
Kalix said:
It seems likely that TE might equal KE + PE but I really don't know for sure.
Yes, that's conservation of energy--what you need to solve this problem.

Also in this particular problem are PE and KE equal to each other?
No. Why would you think that?

As the object falls, one form of energy increases while the other decreases.
 
We did a similar problem to this in class and KE was equal to PE so I thought it might be the same case.

So this means that the height I found in parts a and b wasn't correct because I used KE as the PE value in the equation PE=mgh?
 
Kalix said:
So this means that the height I found in parts a and b wasn't correct because I used KE as the PE value in the equation PE=mgh?
Yes, that was not correct. Instead, use the third equation you listed.
 
So when I use the conservation of ME equation I am solving for the final height correct? But what equation do I use to find the energy left over. If I have KE what do I subtract from that to see what is left?
 
You need to find the potential energy and subtract the kinetic energy to get what is left over
 
Kalix said:
So when I use the conservation of ME equation I am solving for the final height correct? But what equation do I use to find the energy left over. If I have KE what do I subtract from that to see what is left?
You have the initial and final KEs and the initial PE. Solve for the final PE.
 
  • #10
Doc Al said:
You have the initial and final KEs and the initial PE. Solve for the final PE.

Oh yeah I see what you're saying, OP -you should do this.
 

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