Why Delta PE is Negative Work: Understanding the Relationship and Derivation

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In summary, the conversation discusses the concept of potential energy and the relationship between potential energy and work. The participants discuss the idea of proving the change in potential energy is negative work and whether this can be applied in all cases. The proof is ultimately derived from the work energy theorem, which states that the work done on an object is equal to its change in kinetic energy. This can be seen in the equation ΔU=-W, where ΔU is the change in potential energy and W is the work done on the object. The conversation also touches on the derivation of the work energy theorem, which involves integrating the force acting on an object over a distance.
  • #1
CrazyNeutrino
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Can someone prove that the change in potential energy is negative work.
I have a very basic understanding of the concept. I do not understand where it is derived from.
 
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  • #3
Unfortunately you can't prove that in the general case. For gravity it's easy. What you can prove is that work done=1/2*m*v^2= kinetic energy, and from conservation of energy (dE/dt=0) you can derive the remaining stuff.
 
  • #4
Thanks that helps.!
 
  • #5
So the proof would be...

mgy(b)+KE(b)=mgy(a)+KE(a)

That is: U(b)+K(b)=U(a)+K(a)
So U(b)-U(a)=K(a)-K(b)=-(K(b)-K(a)
So U(b)-U(a)=-W ( By work energy theorem )
Therefore:

ΔU=-W

Is this proof correct?
 
  • #6
CrazyNeutrino said:
So the proof would be...

mgy(b)+KE(b)=mgy(a)+KE(a)

That is: U(b)+K(b)=U(a)+K(a)
So U(b)-U(a)=K(a)-K(b)=-(K(b)-K(a)
So U(b)-U(a)=-W ( By work energy theorem )
Therefore:

ΔU=-W

Is this proof correct?

Yeah it's ok. But another important question is wheather you know where work energy theorem comes from?
 
  • #7
Yeah.
W= ∫from a to b of Fdx
=∫from a to b of madx
=∫from va to vb of mdv/dt vdt. (dx/dt =v so dx =vdt)
=∫from va to vb of mvdv
=1/2mv^2 evaluated at va and vb
= 1/2mvb^2-1/2mva^2
=KEb-KEa
Therefore
W=KEb-KEa
 
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FAQ: Why Delta PE is Negative Work: Understanding the Relationship and Derivation

Why is delta PE considered negative work?

Delta PE is considered negative work because it represents the change in potential energy of a system as it moves from a higher potential energy state to a lower potential energy state. This change in potential energy is always negative because the system loses energy as it moves.

What is the relationship between delta PE and work?

The relationship between delta PE and work is that delta PE is equal to the negative of the work done on the system. This means that the change in potential energy of a system is equal to the negative of the work done on the system.

How is the negative work of delta PE derived?

The negative work of delta PE is derived using the formula W = F * d * cos(theta), where W represents work, F represents force, d represents displacement, and theta represents the angle between the force and displacement vectors. By substituting the formula for potential energy, PE = mgh, and rearranging the equation, we can derive the negative work of delta PE.

Can delta PE ever be positive?

No, delta PE cannot be positive. This is because potential energy is always considered to be a measure of the system's energy relative to a reference point. Therefore, as the system moves from a higher potential energy state to a lower potential energy state, the change in potential energy will always be negative.

How does understanding the relationship and derivation of delta PE help in scientific research?

Understanding the relationship and derivation of delta PE can help in scientific research by providing a better understanding of how energy is transferred and transformed in various systems. This knowledge can be applied in fields such as physics, engineering, and environmental science to analyze and predict the behavior of different systems.

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