# Tagged Questions

Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework questions.

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### Parallel and series circuit problem

When two resistors, each of resistance 4.0 ohms, are connected in parallel with a battery, the current leaving the battery is 3.0 A. When the same two resistors are connected in series with the ...
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### How to derive the scale factor for special conformal transformation? [closed]

By definition a conformal transformation of the coordinates is an invertible mapping $x\rightarrow x'$ which leaves the metric invariant upto a scale factor: g_{\mu\nu}'(x') = ...
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### 2 simple Generators = Im lost (intensity)

What happens? we have 2 Generators and 2 intensities, when I2 and I3 come together (they have opposite sides ) what happens? I tried writing down the voltage relations but I'm stuck because I don't ...
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### The variation of the Lagrangian density under an infinitesimal Lorentz transformation

I'm trying to introduce myself to QFT following these lectures by David Tong. I've started with lecture 1 (Classical Field Theory) and I'm trying to prove that under an infinitesimal Lorentz ...
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### What's the relation between output voltage and time to boil water given the same kettle?

An electric kettle rated 220V, 2000W needed 10 minutes to boil water when it is half filled with water in Singapore where the output voltage is 220V. Estimate the amount of time needed to do the ...
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### Finding resultant and direction of resultant

In this question- A motorboat is racing towards north at 25km/h and the water current in that region is 10km/hr in the direction of 60 degree east of south. Find the resultant velocity of the ...
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### Non-stationary capacitor

A common exercise in non-stationary electromagnetism is to find the electric and the magnetic field generated by a capacitor with round plates, if the potential difference between the plates varies in ...
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### Change in momentum [duplicate]

A cube has a side length of 20 cm. An atom in the gas moves around the cube as shown. It continually bounces off the four lateral walls of the cube. The atom has a mass of 6.6×10−27 kg. Because of the ...
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### Pressure and Buoyancy [closed]

It is well known that you can trap water in a drinking straw by placing the tip of your finger over the top while the straw is in the water, and then lifting it out. The inner diameter of the straw is ...
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### How fast must a torus spin to have a felt gravity? [closed]

Given a rotating torus (like a gyroscope or a rotary) with a radius of 1km, how fast must it rotate to have someone inside the torus feel normal gravity of 1G?
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### Conservative Vector Fields

I was always told that to find whether or not a vector field is conservative, see if the curl is zero. I have now been told that just because the curl is zero does not necessarily mean it is ...
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### Question about torque and center of mass [duplicate]

If a yo-yo's string is not attached to anything and the yo-yo is dropped, it is obvious it will fall at $g$. In this scenario, Tension of string = 0. If a yo-yo's string is attached to a cieling and ...
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### Is this scene from the A Team explainable by Physics?

This scene (youtube link) from the movie The A-team, the four members are in the tank and its falling from the air, they fire the canon and it slows the tank from falling for a moment before falling ...
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### Navigating a Time Machine

Notes: The background for this question is working out details of a sci-fi story. Answers to the effect of "time travel isn't possible" or "FTL isn't possible" are therefore not helpful. I'm looking ...
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### Time Dilation Problem [closed]

I'm having some trouble using the time dilation formula. Say an astronaut leaves Earth for 10 years, at 0.85c. How much time has passed according to an observer on Earth? I tried using the ...
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### Calculating coefficient of friction

Consider a body attached to a horizontal spring and resting on a surface, inclined at an angle $\theta$ from the ground. The spring constant is $k$. Initially the spring was kept in its ...
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### The vector r points from $P'(x',y',z')$ to $P(x,y,z)$ [closed]

For some reason this question is giving me a hard time :( The vector $r$ points from $P'(x',y',z')$ to $P(x,y,z)$. (a) Show that if $P$ is fixed and $P'$ is allowed to move, then ...
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### Projectiles Launched at an Angle with unspecified Initial Velocity [closed]

I'm attempting to do my Physics homework, and I did the first one right, but that problem gave me initial velocity. This problem gives me only the angle relative to horizontal and the distance it ...
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