Order of magnitude problem involving the force of repulsion between two persons

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Homework Help Overview

The discussion revolves around an order of magnitude problem concerning the force of repulsion between two individuals, utilizing Coulomb's law to calculate the force based on their respective charges. The context involves comparing this force to gravitational forces, particularly that of an object with the mass of Earth.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss calculations involving the charges of electrons and protons, questioning the accuracy of their results and the implications of roundoff errors. There is also inquiry into how to determine the number of protons involved in the calculations.

Discussion Status

The discussion is ongoing, with participants sharing their calculations and questioning the results. Some have suggested alternative estimates for the force, indicating a variety of interpretations and approaches being explored without a clear consensus on the final answer.

Contextual Notes

Participants mention potential roundoff errors in their calculations and the need for precision in intermediate results. There is also a reference to a similar thread, suggesting that this problem may have been previously discussed.

StrawHat
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Homework Statement



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Homework Equations



\vec{F} = k\stackrel{q<sub>1</sub>q<sub>2</sub>}{r^{2}}

The Attempt at a Solution



6.022e23*(0.505) = 3.041e23C <-- electrons
6.022e23*(0.495) = 2.981e23C <-- protons
3.041e23 - 2.981e23 = 6e21C <-- the difference between the two charges
\vec{F}_{e} = (9e9Nm^{2}/C^{2})(6e21C)^{2} / 2.25m^{2}
\vec{F}_{e} = 1.44e53 N
 
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CAF123 said:
Given that the force of repulsion is similar to the gravitational force exerted on an object with a mass the size of earth, you should expect F ~ 1025N.

There is an identical thread here:
https://www.physicsforums.com/showthread.php?t=431698

The answer gives me this error: "Your response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error."
 


What is the answer that you put in the box?
 


nasu said:
What is the answer that you put in the box?

I put in F~10 25N.
 


How do you get the number of protons?
 


StrawHat said:
I put in F~10 25N.

I would consider trying to enter 26 in the box. Depending on how you estimate, you may get 10^26. And the "weight" of the Earth is also of the order 10^26 (6x10^25).
 

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