**P. 4751.** What can be observed if the handle of a resonating tuning fork

\(\displaystyle a)\) touches the tabletop, or

\(\displaystyle b)\) is emerged into water?

Explain the observations.

(4 points)

**P. 4757.** The work function of the cathode of a photocell in a circuit is \(\displaystyle 6\cdot 10^{-19}\) J. Ultraviolet wave of wavelength 250 nm hits the cathode perpendicularly.

\(\displaystyle a)\) What is the stopping potential at which the photo-current becomes zero?

\(\displaystyle b)\) What is the linear momentum given to the cathode, during one elementary process, if the emitted electron is ejected oppositely to the direction of the radiation?

\(\displaystyle c)\) Let us change the frequency of the radiation. At most by what factor can the linear momentum of the emitted electron be greater than that of the entering photon?

(5 points)