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# Tutor profile: Moaz A.

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Moaz A.
Teaching Assistant at ZC
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## Questions

### Subject:Physics (Newtonian Mechanics)

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Question:

Can you derive Newton's third law out of the second law? What does it really means?

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Moaz A.

The answer is yes, the third law could be derived if you imagined coplanar forces acting on an imaginary circle with mass $m=0$. Newton’s second law states that the sum of the external forces is equal to $m \frac{dp}{dt}$, that means $\Sum F-{ext} = 0$ in our case as $m=0$. Now, take a specific example, assume only two forces acting on the system, when $\Sum F_{ext} =0$ that means $F_1+F_2 = 0$ which is know as Newton's third law. This could be written as well as $dp_1/dt+dp_2/dt$ (Newton’s second law again applied to each of them) this is the statement of conservation of momentum which is another way to think about the third law.

### Subject:Physics (Electricity and Magnetism)

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Question:

When KVL is applicable?

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Moaz A.

KVL or kirchoff Voltage loop is applicable as long as there is a conservative field in the system e.g. electric field. Where the sum of the potential along a closed loop is equal to zero.

### Subject:Physics

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Question:

If light source has a wavelength of 171 $\mathrm{nm}$ and intensity of $359$ Watts, and the work function of this metal is $12\times 10^{-19}\:\mathrm{J}$, get the maximum KE of the emitted electrons? Room temperature is $25^\circ \mathrm{C}$.

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Moaz A.

\begin{equation*} KE_{max} = \frac{hc}{\lambda} - W = 7.2 eV - 7.5 eV = -0.3 eV \end{equation*} Negative KE means that no electrons are emitted.

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