Tutor profile: Moaz A.

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

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.

When KVL is applicable?

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.

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 $25^\circ \mathrm{C}$.

\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.