# 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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### Where did I commit a mistake in calculating rotation? [closed]

I did something wrong in my calculation, can somebody tell me what?
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### How does $p_x$ commute with $p_y$, i.e. $[p_x,p_y]=0$? [closed]

I know it's a simple and basic question but would someone show me how to evaluate $[\hat{p}_x,\hat{p}_y]$?
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### Rubber bands and Hooke's law [closed]

Why does increasing the width of a rubber band increase its elastic constant($k$)? And why are the two variables directly proportional? Explain it in terms of the structure of the band, if that is ...
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### Linear speed of falling pencil [closed]

Find the final linear speed of pencil. Pencil would move because of gravity. Let's assume that it is balancing on the tip. We do not take friction into consideration. – Given $$d=0.15m$$ Similar ...
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### The sign of the normal force.

The notes state that $N=mg$. However, if I have a $4\text{kg}$ box on a table, taking upward as positive $$N=-mg=-4*-9.81$$ Is this correct? If it is correct, why do the notes state that $N=mg$. ...
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### There is no electricity at home,I need to light a 15W CFL Bulb.Can I Do it with the help of a hamster? [closed]

We know that i)avg speed of A Hamster is 30km/hr. ii)Avg mass of hamster is 1.5 kg. From the above info: Kinetic energy=1/2 X mass X velocity^2 So, K.E=1/2 x 1.5 x 30 x 30 ...
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### A Rolling Quarter [closed]

A U.S quarter is rolling on the floor without slipping in such a way that it describes a circular path of radius $R=4 \text{cm}$. The plane of the coin is tilted at an angle of $\theta=45^{∘}$ with ...
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### Center of a mass of a hemisphere [closed]

How can I show that position vector of the center of a mass of a hemisphere is $(0,0,\frac{3a}{8})$ where $a$ is radius of a hemisphere, $x$ and $y$ axis are laying on the base and $z$-axis is ...
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### Costant volume gas thermometer [closed]

I have two constant volume gas thermometers, one with an ideal gas $pv=RT$ and the other following the Van der Waals equation $(p+\frac{a}{v^2})(v-b)=RT$. They are calibrated using the freezing point ...
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### Wavefunction operators and the observable [closed]

So I got this from the exam I had yesterday. I couldn't really answer it other and it played on my mind through the night Show that if a wave function $\psi$ , is an eigenfunction of an operator [Q], ...
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### Lorentz transformations and a time dilation don't return the same result [closed]

Lets deal with a problem like this: Lets say we have a twin A in coordinate system $x'y'$ who does a round trip to a star which is $12$ light years away and he is travelling with a speed ...
I have read on web that: Proper time $\tau$ is a time interval measured by a clock which at rest relative to the observer. But lets consider this problem: The plane is flying with a speed ...