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5:40 AM
Hello @JohnRennie sir
John Rennie
John Rennie
Hi :-)
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Why minus sign in change in potential energy?
I searched a lot also worked on equations but didn't find any satisfactory answer.
John Rennie
John Rennie
What minus sign? What equation do you mean?
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W = - ∆U
F = - ∆U/∆x
What is logical answer put minus sign in above equations?
John Rennie
John Rennie
Suppose a system starts with an internal energy U and it does work W.
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5:45 AM
Yes
John Rennie
John Rennie
That work has to come from the system's internal energy, so U has to decrease by W i.e. U + W stays constant.
Since U decreases the change ΔU is a negative number, but the work done W is a positive number. So -ΔU is the same as W.
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Sir pls don't use thermodynamics , pls use mechanics example. Thanks
John Rennie
John Rennie
That's how we get the first equation.
What I've said above applies to mechanical systems e.g. the system could be a spring and U is the energy stored in the spring.
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What about example object moving upward and gravitational force downward.
If I take care of direction of displacement and force in opposite and put cos180, I get accurate result with proper sign. But if I don't put cos180 then I found sign problem.
In example of gravitational potential energy with upward motion.
∆KE = ∆PE Pls use this equation
 
1 hour later…
Binky McSquigglebottom
Binky McSquigglebottom
7:10 AM
@JohnRennie Hi !
John Rennie
John Rennie
Hi :-)
 
2 hours later…
Pizza
Pizza
9:28 AM
@JohnRennie Hi :)

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