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

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1. May 7, 2017

### ness8

1. The problem statement, all variables and given/known data
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.

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

3. 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.

2. May 7, 2017

### PeroK

Perhaps someone else might understand this problem, but I can't see what you experiment involves.

Is the pendulum moving initially?

3. May 7, 2017

### ness8

My first value for velocity is 0.236 m/s.

4. May 7, 2017

### PeroK

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.

5. May 7, 2017

### vela

Staff Emeritus
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.

6. May 7, 2017

### ness8

This is the exact question: What % of PEg must be converted in KE in order to double the speed of the pendulum?

7. May 7, 2017

### ness8

This is the exact question: What % of PEg must be converted in KE in order to double the speed of the pendulum?

8. May 7, 2017

### Nidum

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 ?

9. May 7, 2017

### ness8

A pendulum like this, but with only one swing from left to right.

10. May 7, 2017

### Nidum

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. May 7, 2017

### ness8

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. May 7, 2017

### Nidum

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. May 7, 2017

### ness8

V = 0.236 m/s
KE = 0.014 J
PEg = 2.836 J
Y-pos = 0.579 m

14. May 8, 2017

### Nidum

Please explain clearly what your experimental set up is and show your workings in some understandable form .