A hydrogen atom (m = 1.674 × 10^?27 kg) is moving with

a velocity of 1.1250 × 10⁷ m/s. It collides elastically with

a helium atom (m = 6.646 × 10^?27 kg) at rest. After the

collision, the hydrogen atom is found to be moving with

a velocity of ?6.724 × 10⁶ m/s (in a direction opposite to

its original motion). Find the velocity of the helium atom

after the collision in two different ways: (a) by applying

conservation of momentum; (b) by applying conservation of energy.