# Find missing force for accelerating electron

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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!

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Stephen Tashi
to get acceleration I did: (Vf)^2- (Vi)^2 / 2d and I got 5.128 m/s^2

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

Oh yes, sorry. 5.128 X 10 ^13

Stephen Tashi
to get acceleration I did: (Vf)^2- (Vi)^2 / 2d

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

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...

Stephen Tashi
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.

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 .

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.

Stephen Tashi
should I be using (mv^2)/ 2 to find the acceleration?
Yes

... does that v stand for initial or final velocity?
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.

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!

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.

PeterDonis
Mentor
Webassign told me these are wrong

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

gneill
Mentor
to get acceleration I did: (Vf)^2- (Vi)^2 / 2d and I got 5.128 m/s^2

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

After some searching on the web, someone had used that to find it. I'm guessing that is wrong?

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