# Find missing force for accelerating electron

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1. Jan 29, 2015

### Trina

• Moved from general physics forum
[Mentor's note: no template because this thread was moved into the homework forum after it had already attracted several useful replies]
[Mentor's note: Thread title changed to reflect question content]

The question is:
An electron is a subatomic particle (m = 9.11 10-31 kg) that is subject to electric forces. An electron moving in the +x direction accelerates from an initial velocity of +5.71 105 m/s to a final velocity of +2.03 106 m/s while traveling a distance of 0.037 m. The electron's acceleration is due to two electric forces parallel to the x axis: vector F 1 = +8.08 10-17 N, and vector F 2, which points in the -x direction. Find the magnitudes of the net force acting on the electron and the electric force vector F 2.

Vf= final velocity Vi = initial velocity
Fn= net force d= distance
F1= forces parallel to x-axis

to get acceleration I did: (Vf)^2- (Vi)^2 / 2d and I got 5.128 m/s^2
I then calculated Fn= ma = 4.671 N
To get F2 I did F1- Fn and got -4.671N

but Webassign told me these are wrong, and I've already done other things and got other (wrong) answers... what am I doing wrong? This seems fairly straightforward.

Thanks!

Last edited by a moderator: Jan 30, 2015
2. Jan 29, 2015

### Stephen Tashi

Is there a factor with a power of ten associated with that result?

3. Jan 29, 2015

### Trina

Oh yes, sorry. 5.128 X 10 ^13

4. Jan 29, 2015

### Stephen Tashi

Why do think that calculation computes acceleration? Have you studied such a formula?

5. Jan 29, 2015

### Trina

After some searching on the web, someone had used that to find it. I'm guessing that is wrong? Should I do F1/ m to get it? I just wasn't sure if I should use that formula since its only the force in the +x direction and not net force.

Or well, I guess using F1/m won't work, cause then the find Fn it'd be Fn=ma, which would get me F1 , and then F2 would be 0...

6. Jan 29, 2015

### Stephen Tashi

There is an equation that relates force and distance to a change in kinetic energy, but kinetic energy is $\frac{mv^2}{2}$ not $\frac{v^2}{2}$. Perhaps you saw a problem where $m$ happened to be 1 .

7. Jan 29, 2015

### Trina

Hmm okay, so then should I be using (mv^2)/ 2 to find the acceleration? ... does that v stand for initial or final velocity?

Sorry for all these questions, I probably seem so clueless... I've just never had a physics class before so I'm slightly overwhelmed. Thanks for you help, though.

8. Jan 29, 2015

### Stephen Tashi

Yes

Almost all physics formulas have several interpretations. One interpretation will be the "change in" interpretation.

For example the formula: Work = (Force)(distance) can be applied without reference to a "change in"
However, when some initial amount of Work has already been done.
Total Work = (Work already done) + (change in work)
= (Work already done) + (Force)(change in distance) [ assuming Force is constant with respect to distance.]

The formula (Work done) = kinetic energy likewise has a "change in" interpretation.
In your problem the mass has an initial velocity so it has an initial kinetic energy. The Force in the problem acts to create a change in the kinetic energy. So you need the "change in" interpretation and that would be (change in work done) = (change in kinetic energy). The change in kinetic energy involves the initial and final kinetic energies, so it's like the formula that you used, except you need the $m$ term. Notice the 0.037 meters is a "change in distance", so you don't worry about finding the difference beween an initial and final distance.

9. Jan 29, 2015

### Trina

Okay, I think I'm beginning to get this... my original equation had the divisor of 2*d... are you saying it should just be everything divided by 2, or should it be divided by 2*d ?

Thanks!

10. Jan 29, 2015

### Stephen Tashi

Solve for $F$ in the equation $(F)(d) = \frac{m v^2}{2}$ using the "change in" interpretation. $F =$ your original equation but with an $m$ in the numerator of the fraction.

11. Jan 29, 2015

### Staff: Mentor

Which specific answers did it say were wrong? All of them? Or only some?

12. Jan 30, 2015

### Staff: Mentor

It is fine. It follows from the standard kinematics equation: ${v_f}^2 - {v_i}^2 = 2 a d$.