Question

1. Which of the following statements about mechanical waves is true?

Answer 1

1) a. mechanical waves require a medium to travel through
In fact, mechanical waves cannot travel trough empty space, but they always require a medium for their propagation. (on the contrary, electromagnetic waves can also travel through empty space)

2) d. gamma rays
In fact, gamma rays are the radiations with shortest wavelength on the electromagnetic spectrum (range in the picometer). Correspondingly, this is the radiation with highest energy, because the energy of the radiation is inversely proportional to its wavelength, according to:
$E=h \frac{c}{\lambda}$
where h is the planck constant, c the speed of light and $\lambda$ the wavelength.

3) d. Infrared have lower frequency than x-rays
In fact, infrared radiation has frequency in the range of the THz (terahertz, $10^{12} Hz$), while x-rays have frequency in the range of hundreds of PHz (Petahertz, $10^{15}Hz$).

4) d. parallel
In longitudinal waves, the oscillations of the wave (such as the motion of the particles in a compression wave) occur in the direction parallel to the direction of propagation of the wave itself.

5) c. gases
Sound travels slowest in gases. In fact, the speed of sound is proportional to the density of the medium: the more dense the medium is, the faster is the sound in that medium, and vice-versa. Since gases are less dense than liquids and solids, sound travels slowest in gases. The reason is that sounds propagates through the oscillations of the molecules of the medium; if the medium is less dense (such as in gases), it takes more time to transmit the sound through the medium itself.