How Do I Calculate Net Force on Charges in AP Physics B 2005 Form B Question 3?

[mitiejoe]

I have been working on the 2005 AP Physics B Free-Response Questions from Form B. Question 3 is giving me a lot of difficulty, and I am not sure if i am going in the right direction. The question can be found here:

http://www.collegeboard.com/prod_downloads/ap/students/physics/ap05_frq_physics_b_form_b.pdf

For part a, the force on the -Q charge is (2KQ^2)/5a^2, from each of the -2Q charges. To find the net force, do I have break that into X and Y components and add them together as vectors?

Thanks for your help,
-joe

 

[whozum]

Yes, the net force is in the positive x direction, the y forces cancel.

 

[thepatientmental]

Hello Joe,

I'm glad to hear that you are working on the 2005 AP Physics B Free-Response Questions from Form B. Question 3 can definitely be tricky, so it's great that you are seeking clarification.

For part a, you are correct in your calculation of the force on the -Q charge. This force comes from the electric field created by the two -2Q charges, and can be found using Coulomb's Law.

To find the net force, you will indeed need to break the force into its X and Y components and then add them together as vectors. This is because the two -2Q charges are not aligned with the -Q charge, so their forces will have both X and Y components. You can use trigonometry to find the components of each force, and then add them together to find the net force.

Remember to pay attention to the directions of the forces and their components when adding them together. This will ensure that your final answer is accurate.

I hope this helps and good luck with the rest of the questions!