What % of PEg must be converted in KE in order to double v?

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Homework Help Overview

The discussion revolves around determining the percentage of gravitational potential energy (PEg) that must be converted into kinetic energy (KE) to double the speed of a pendulum. The original poster has provided specific values from their experiment, including initial kinetic energy and potential energy, but their findings do not align with the expected answer provided by their teacher.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to calculate the required percentage of PEg based on their experimental data. They question the validity of the 25% figure suggested by their teacher, citing their own calculations that yield a different result. Other participants inquire about the initial conditions of the pendulum and seek clarification on the experimental setup.

Discussion Status

The discussion is ongoing, with participants asking for more context and details about the experiment. There is a focus on understanding the initial conditions and the nature of the pendulum's motion. Some participants express confusion about the original post and suggest that the question may be misinterpreted.

Contextual Notes

Participants are discussing a pendulum that swings from a certain angle or distance away from the vertical position. There is a need for clarity on the experimental setup and initial conditions to better analyze the problem.

ness8
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Homework Statement


What % of PEg must be converted in KE in order to double the speed of a pendulum? So I have spent a couple of days trying to figure out this problem (it's a lab), and I can't seem to figure it out. I have asked my physics teacher about it, and he said the answer to the question is 25% of gravitational potential energy, which does not match my data.

Homework Equations


Not sure if I need to use any equations here, but if so, PEg = KE (mgh = 1/2mv^2).

The Attempt at a Solution


So the initial KE that I'm working with is 0.014 J, the PEg for that value is 2.836 J, and the velocity for that value is 0.236 m/s. If I double the velocity = 2*0.236 m/s = around 0.478 m/s (I'm using 0.478 m/s because it is the next value of my data, so it would be easier to use.) With this new velocity, the KE is 0.057 J. Therefore, I subtract 0.057 J - 0.014 J = 0.043 J. To find the percentage of 2.836 J, I divide 0.043 J/2.836 J = 0.015*100 = 1.5%, NOT 25%. 25% of 2.836 J is 0.709 J, which does not make sense with my data. I know for a fact that what I did was incorrect because if I try 1.5% with other values in my data, it does not get me to the correct values.
 
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Perhaps someone else might understand this problem, but I can't see what you experiment involves.

Is the pendulum moving initially?
 
PeroK said:
Perhaps someone else might understand this problem, but I can't see what you experiment involves.

Is the pendulum moving initially?
My first value for velocity is 0.236 m/s.
 
ness8 said:
My first value for velocity is 0.236 m/s.

I think you should reread your original post and decide whether anyone not completely familiar with your experiment could figure out what you are trying to do.
 
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Could you provide a more complete context for your question and the question exactly as it was given to you? It seems to me you're misinterpreting the question that was asked.
 
vela said:
Could you provide a more complete context for your question and the question exactly as it was given to you? It seems to me you're misinterpreting the question that was asked.
This is the exact question: What % of PEg must be converted in KE in order to double the speed of the pendulum?
 
PeroK said:
I think you should reread your original post and decide whether anyone not completely familiar with your experiment could figure out what you are trying to do.
This is the exact question: What % of PEg must be converted in KE in order to double the speed of the pendulum?
 
Let us try some Q + A .

What type of pendulum ?

A continuous swinging one like in a clock or a single swing type like used in a Charpy impact test ?
 
300px-Simple_gravity_pendulum.svg.png
A pendulum like this, but with only one swing from left to right.
 
  • #10
So it's more like a Charpy pendulum . For the purpose of the question we have to examine a single swing starting with pendulum pulled a certain angle or distance away from vertical position . We progress .

Now - how have you tried to analyse this problem yourself ? Please show us your workings .
 
  • #11
Nidum said:
So it's more like a Charpy pendulum . For the purpose of the question we have examine a single swing starting with pendulum pulled a certain angle or distance away from vertical position . We progress .

Now - how have you tried to analyse this problem yourself ? Please show us your workings .
So the initial KE that I'm working with is 0.014 J, the PEg for that value is 2.836 J, and the velocity for that value is 0.236 m/s. If I double the velocity = 2*0.236 m/s = around 0.478 m/s (I'm using 0.478 m/s because it is the next value of my data, so it would be easier to use.) With this new velocity, the KE is 0.057 J. Therefore, I subtract 0.057 J - 0.014 J = 0.043 J. To find the percentage of 2.836 J, I divide 0.043 J/2.836 J = 0.015*100 = 1.5%, NOT 25%. 25% of 2.836 J is 0.709 J, which does not make sense with my data. I know for a fact that what I did was incorrect because if I try 1.5% with other values in my data, it does not get me to the correct values.
 
  • #12
Sorry but I don't understand any of that .

Let's try some more Q + A :

What exactly are the initial conditions for the pendulum at the start of the swing ?
 
  • #13
Nidum said:
Sorry but I don't understand any of that .

Let's try some more Q + A :

What exactly are the initial conditions for the pendulum at the start of the swing ?
V = 0.236 m/s
KE = 0.014 J
PEg = 2.836 J
Y-pos = 0.579 m
 
  • #14
Please explain clearly what your experimental set up is and show your workings in some understandable form .

Otherwise I shall ask the mentors to close this thread .
 

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