Kinetic energy = (1/2) x (mass) x (speed)²
8 joules = (1/2 x 1 kg) x (speed)²
Divide each side by 1/2kg : 8 joules / 0.5 kg = speed²
Take the square root of each side:
speed = √[ (8 kg-m²/sec²) / (0.5 kg) ]
= √ (8 m²/sec²)
= √8 m/s = 2.83 m/s (rounded)