Net force on a charge from two other charges using Coulomb's law

[riseofphoenix]

My brain has been shocked by this problem dealing with charges and Coulombs law :(

This is what I did...

F = k*q1*q2/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
F = 0.000000264 / 0.087025
F = 0.0000003034
F = 3.034 * 10^-7 N

F = k*q1*q3/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.295)^2
F = 0.000000132 / 0.087025
F = 0.0000001519
F = 1.519 * 10^-7 N

Resultant: √ (3.034 * 10^-7)^2 + (1.519 * 10^-7)^2 =√ ( 1.1488262 * 10^-13) = 3.39 * 10^-7

I got it wrong though :(

Please help

 

[VantagePoint72]

This looks like an online quiz. What is the class policy on seeking help vs. working independently for unsupervised quizzes?

 

[ap123]

Check your value for r in the second calculation.

 

[riseofphoenix]

This looks like an online quiz. What is the class policy on seeking help vs. working independently for unsupervised quizzes?

This isn't an online quiz. It's an online homework assignment that's been given to my class through Webassign.

 

[riseofphoenix]

Check your value for r in the second calculation.

Oh!
Ok give me a sec. Imma try to change r in the second equation to 0.100...

 

[riseofphoenix]

F = k*q1*q2/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
F = 0.000000264 / 0.087025
F = 0.0000003034
F = 3.034 * 10^-7 N

F = k*q1*q3/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.100)^2
F = 0.000000132 / 0.01
F = 0.0000132
F = 1.32 * 10^-5 N

Resultant: √ (3.034 * 10^-7)^2 + (1.32 * 10^-5)^2 =√ ( 1.743 * 10^-10) = 1.32 * 10^-5 (?)"INCORRECT. Your incorrect answer may have resulted from roundoff error. Make sure you keep extra significant figures in intermediate steps of your calculation."

WHAT?

 

[riseofphoenix]

EDIT

F = k*q1*q2/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
F = 0.000000264 / 0.087025
F = 0.000003037 (there was one extra 0 that i took off here)
F = 3.037 * 10^-6 N

F = k*q1*q3/r^2
F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.100)^2
F = 0.000000132 / 0.01
F = 0.0000132
F = 1.32 * 10^-5 N

Resultant: √ (3.037 * 10^-6)^2 + (1.32 * 10^-5)^2 =√ ( 1.834 * 10^-10) = 1.35 * 10^-5

Is that the answer?

 

[riseofphoenix]

What about direction?

This is what I did...

arctan(0.0000132/0.000003037) = 77.04

I submitted that answer and I got this -.-

"INCORRECT. Examine the diagram and determine which quadrant contains the angle of the net force."

The answer isn't -77 either. :(

 

[gneill]

Your force value for the first charge is off by a power of ten. You'd probably be better off sticking with exponential notation than going through fixed point notation; it's too easy to miscount decimal places.

The problem statement says it wants the direction angle measured counterclockwise from the positive x-axis. So a negative value is NOT okay. Sketch the force vectors and the resultant vector and calculate accordingly.

 

[riseofphoenix]

Your force value for the first charge is off by a power of ten. You'd probably be better off sticking with exponential notation than going through fixed point notation; it's too easy to miscount decimal places.

The problem statement says it wants the direction angle measured counterclockwise from the positive x-axis. So a negative value is NOT okay. Sketch the force vectors and the resultant vector and calculate accordingly.

Oh wait...the answer is -101 according to webassign
Idk how they got that

 

[Naty1]

Rise...
you can look at the charges and figure out which quadrant the resultant is in,right??