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I believe you’ve already been told that everyone on AN is a volunteer, so not everyone will be available at all times. Please avoid using red bold font or people may feel less inclined to answer. In addition, the red bold font is even less likely to get you a faster response. (The quote below applies to the whole of AN and not just one board.)
This was a fair comment from AngelWings; I was really not inclined to answer because it felt like more of a pushy demand as opposed to a legitimate question. However, I will go ahead and answer this anyway; but be wary that I or other people may not answer your other questions in the future if this continues.
Basically we have a 'paradox' between the observations of A and B. As B is an outside observer watching the barn doors close simultaneously and watching the ladder move at a relativistic speed relative to the barn, B will see the ladder contract to a length of about 8.72m (using the length contraction formula - the question asks for calcs), thus seeing the ladder fit completely inside the barn with the doors shut the instant the button is pressed. The paradox arises when we then consider the observations of A; instead, the barn moves towards the ladder A is carrying at 0.9c, and shrinks to a size of about 6.54m (again using the length contraction formula) as observed by A. Clearly, the barn is far too small to even fit the ladder, and thus a paradox arises at the instant the doors close; how can we possibly have the ladder simultaneously fitting in the barn and becoming a part of the doors in the same instant of time?
However, the answer to the question is in fact that the ladder will never touch the doors; because events observed to be simultaneous by A will not be simultaneous for B. B will observe the ladder entering the barn, then being enclosed for an instant, then exiting, while A will observe the ladder entering, the front door shutting and opening, then during that instant, A will have already moved a fraction and will then observe the back door opening and shutting, and then the ladder will exit.
Hopefully this makes sense
Can someone please help out with this multiple choice question?
Thank you.
Basically we have the formulae \(F=\frac{1}{4\pi \epsilon_0} \frac{q_1q_2}{r^2}\) (for the force between two point charges) and similarly \(E=\frac{1}{4\pi \epsilon_0} \frac{q_1}{r^2}\) (for the magnitude of the electric field produced by a point charge). The first one is on the reference sheet, while the second one can be derived reasonably easily.
I'm honestly not sure why the answers are like that; from what I've calculated (just subbing in values into the first formula), none of them match. Also, the units are wrong, as they're asking for a force, not a field strength, which is odd. I'd think this question is a dud actually
Hope this helps