**Problem 1.** The mean life of a meson is 2 x 10^{- 6} s. Calculate the mean life of a meson moving with a velocity 0.9c?

**Problem 2.** Calculate the rest energy of electron and proton in eV. Given m = 9.11 x 10-31 kg e. and m _{P} = 1.673 x 1()-21 kg.

**Hint: ** E = m c^{2}

**Problem 3.** At what speed will be mass of a body is 2.25 times its rest mass?

**Hint: **

**Problem 4.** The binding energy of hydrogen is 13.6 eV. Find the loss of mass in the formation of one atom of hydrogen.

Hint: E =M C^{2}

**Problem 5.** The rest mass of an electron is 9.1 x w-·31 kg. What will be its mass if it was moving with 4/51/z the speed of light?

**Problem 6.** A rocket is chasing enemy’s space-ship. An observer on the earth observes the speed of rocket to be 2.5 x 108m/sec and that of space-ship 2 x l0 ^{8} m/sec. Calculate (i) the velocity of enemy’s ship as seen by rocket (ii) the velocity of rocket as seen by enemy.

**Problem 7**. The rest mass of an electron is 9.1 x 10-31 kg. What will be its mass if it was moving with 3/511′ the speed of light.

**Hint:** M= 9.1 x 10^{-31} / √ 1-(0.6)^{2} =11.38 x 10 ^{-31} kg