# Quick Question - Explain Work Energy Concept

1. Jan 9, 2008

### physicsmarkb

[Solved] Quick Question - Explain Work Energy Concept

1. The problem statement, all variables and given/known data

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.

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

3. 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: Jan 9, 2008
2. Jan 9, 2008

### physicsmarkb

Does anyone know why I have have to solve it like this?

3. Jan 9, 2008

### blochwave

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 gonna impart to the electron, and you know what final kinetic energy you would like the electron to have

4. Jan 9, 2008

### physicsmarkb

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: Jan 9, 2008
5. Jan 9, 2008

### blochwave

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

6. Jan 9, 2008

### physicsmarkb

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