Solving Three Charges: Find Force on Q2

[jmatthews1991]

Homework Statement

Three charges, Q1, Q2, and Q3 are located in a straight line. The position of Q2 is 0.323 m to the right of Q1. Q3 is located 0.125 m to the right of Q2
In the above problem, Q1 = 1.37 μC, Q2 = -2.53 μC, and Q3 = 3.43 μC. Calculate the total force on Q2. Give with the plus sign for a force directed to the right.

Homework Equations

F=kq1q2 / r^2

The Attempt at a Solution

I used the above formula to find the force for charge 1-2, and charge 2-3, and then added them together. I got the wrong answer.
My answers were: 0.29867 N + 4.99 N = 5.29 N
What am i doing wrong !?

 

[berkeman]

Homework Statement

Three charges, Q1, Q2, and Q3 are located in a straight line. The position of Q2 is 0.323 m to the right of Q1. Q3 is located 0.125 m to the right of Q2
In the above problem, Q1 = 1.37 μC, Q2 = -2.53 μC, and Q3 = 3.43 μC. Calculate the total force on Q2. Give with the plus sign for a force directed to the right.

Homework Equations

F=kq1q2 / r^2

The Attempt at a Solution

I used the above formula to find the force for charge 1-2, and charge 2-3, and then added them together. I got the wrong answer.
My answers were: 0.29867 N + 4.99 N = 5.29 N
What am i doing wrong !?

Q1 and Q3 are both positive, but on opposite sides. Would their forces add?

 

[jmatthews1991]

Q1 and Q3 are both positive, but on opposite sides. Would their forces add?

I added them

 

[berkeman]

I added them

And got the wrong answer...

 

[jmatthews1991]

And got the wrong answer...

Yeah I got the wrong answer.. What did I do wrong? What should I do ?

 

[berkeman]

Yeah I got the wrong answer.. What did I do wrong? What should I do ?

The force equation that you wrote in your original post (OP) is actually a vector equation. The forces have both magnitude and direction. You need to take that into account when you sum the forces to get the resultant.