An ultra-high-energy cosmic ray particle has an energy of 1.25×1020 eV1.25 \times 10^{20}\text{ eV}1.25×1020 eV. In a hypothetical cosmic collision, this particle transfers all of its energy to a small dust grain of mass 4.0 g initially at rest.
Assuming all of the transferred energy is converted into kinetic energy of the dust grain, what is the final speed of the dust grain?
3.2 m s−13.2\text{ m s}^{-1}3.2 m s−1
50 m s−150\text{ m s}^{-1}50 m s−1
71 m s−171\text{ m s}^{-1}71 m s−1
100 m s−1100\text{ m s}^{-1}100 m s−1
