I dont understand some part of work energy theorem

In summary, when an elevator is being pulled by a force equal to its weight and moving at a constant velocity, the net force is equal to zero. However, this does not mean that there is no change in potential energy. The total mechanical energy should only include the work done by non-conservative forces, such as the tension in the cable. Depending on the specific question, the elevator may have zero power, but this does not necessarily mean it has zero work being done on it.
  • #1
madah12
326
1

Homework Statement



[tex]\Sigma[/tex]W = [tex]\Delta[/tex]E
so if we have an elevator getting pulled by force equal to it's weight making it move with a constant velocity then the net force is equal to 0 so the sigma W is equal to zero but isn't there non zero change in potential energy ?

Homework Equations





The Attempt at a Solution

 
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  • #2
If E is the total mechanical energy, you only need to account for the work done by the non-conservative forces. In this case, you have two forces on the elevator: the force of gravity, which is a conservative force, and the tension in the cable, which is non-conservative. The only work that you include on the lefthand side is the work done by the tension. The work done by gravity is already accounted for via the gravitational potential energy.
 
  • #3
oh I see, would that said elevator have any power? because P=F*V=0*V=0
so then would it have 0 power but then it will have zero work or should we not take the net force?
 
  • #4
It depends what exactly the question is. If you're asking "What power is required to lift the elevator at a speed V?" you'd only consider the force doing the lifting, that is, the tension, and multiply it by the speed to find the power needed.
 
  • #5


The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy. In this case, the elevator is moving at a constant velocity, so there is no change in its kinetic energy. However, there is a change in its potential energy as it moves up or down in the elevator shaft. This change in potential energy is accounted for in the change in total energy (E) of the elevator, which is represented by the ΔE term in the equation. So even though the net work (ΣW) may be zero, there is still a change in energy due to the change in potential energy. This is consistent with the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred from one form to another. In this case, the work done by the force pulling the elevator is transferred into potential energy, resulting in a non-zero change in total energy. I hope this helps clarify any confusion you may have had about the work-energy theorem.
 

1. What is the work-energy theorem?

The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. In other words, the net work done on an object is equal to the change in its velocity.

2. How is work related to energy?

Work is a measure of the transfer of energy from one object to another. When work is done on an object, its energy changes.

3. Can you provide an example of the work-energy theorem?

Imagine a ball rolling down a hill. As it rolls, the force of gravity does work on the ball, increasing its kinetic energy. This is an example of the work-energy theorem in action.

4. Is the work-energy theorem always true?

Yes, the work-energy theorem is a fundamental law of physics and is always true. It is based on the principle of conservation of energy.

5. How is the work-energy theorem applied in real life?

The work-energy theorem has many real-life applications, such as calculating the amount of work needed to lift an object, determining the speed of a roller coaster at different points on its track, and understanding the energy transformations in a car engine.

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