Quick Question - Explain Work Energy Concept

AI Thread Summary
The discussion focuses on using the work-kinetic energy theorem to determine the force needed to accelerate an electron from rest to a specified speed over a given distance. Participants clarify that work is defined as the change in kinetic energy, which can be expressed through the equation W = Fx. The confusion arises from understanding why W must equal Fx when the kinetic energy formula K = 1/2 mv^2 provides the necessary information. Ultimately, the key takeaway is that while the kinetic energy can be calculated, the force is derived from the work-energy relationship. Understanding these concepts is crucial for solving the problem effectively.
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[Solved] Quick Question - Explain Work Energy Concept

Homework Statement



Use the work - kinetic energy theorem to find the force required to accelerate an electron (m = 9.11 x 10^-31 kg) from rest to a speed of 1.50 x 10^7 m/s in a distance of .0125 m.


Homework Equations


W = Fx cos(theta), K = 1/2 mv^2


The Attempt at a Solution



I know that I would set Fx cos(theta) = 1/2 mv^2 and then solve for F but I don't understand why I would do this.
 
Last edited:
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Does anyone know why I have have to solve it like this?
 
work = the change in kinetic energy, in other words the final kinetic energy - the initial kinetic energy. The electron starts from rest and has no initial kinetic energy. That pretty much IS the work-energy theorem

Make sure you understand what work is, it's how much energy you're going to impart to the electron, and you know what final kinetic energy you would like the electron to have
 
So with K final, all you have is mass and velocity. With W = Fx, you have force and distance. Why do you divide mass and velocity by distance?
Edit: I just realized that you divide because you're trying to solve for F.

What I still don't understand is that why you have all the information for K = 1/2 mv^2 which is W = K but that doesn't work. I don't get why you have to set it equal to W = Fx.
 
Last edited:
What I still don't understand is that why you have all the information for K = 1/2 mv^2 which is W = K but that doesn't work. I don't get why you have to set it equal to W = Fx.

Well with W=K and knowing K, you would've found work. Were you looking for work? No. Can you use the definition of work to find force? Yes. And that's what you did
 
blochwave said:
Well with W=K and knowing K, you would've found work. Were you looking for work? No. Can you use the definition of work to find force? Yes. And that's what you did

Wow I'm so dumb. I can't believe I wasn't paying attention to W and F.
 

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