Examkrackers 1001 Physics # 783

[inegomontoya]

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Hey guys I don't want to seem stupid but this question threw me off because it seems easy and I am most likely being stupid!!

From what I understand the force that charge 2 feels as a result of the source charge 1 is F = k*q1*q2/ r^2

In this question it says that q2 is moved from its position at 2r to a distance r from q1.

Why then does the force experienced increase by a factor of 4?

 

[inegomontoya]

Never mind, I just answered my own question after I posted that!

Must have been a long day...

For some reason I had to think what would happen to the force if the charge had been moved to a distance 2r. It would have obviously decreased my a factor of 4.

Thus, if you move it in to a distance of r, it must have increased by a factor of 4...easy! I just didn't picture that when I was using the formula.

The more important question is how do you keep from making mistakes like that on the test 😕

 

[movax]

Thus, if you move it in to a distance of r, it must have increased by a factor of 4...easy! I just didn't picture that when I was using the formula.

The more important question is how do you keep from making mistakes like that on the test 😕

It is a common tactic from the test writers, checking to see if you can ID inverse-square laws. If you're good at algebra, you can just play with the variables. If you're not, just assign some "dumb" and easy to play with numbers, and you'll end up at the same conclusion.

Inverse-square laws will show up here for charges, as well as gravitation. You may also see questions that asks how kinetic energy changes when speed is doubled; remember it is .5mv^2.