The value of acceleration due to gravity at Earth's surface is \( 9.8 \, \text{m/s}^2 \). The altitude above its surface at which the acceleration due to gravity decreases to \( 4.9 \, \text{m/s}^2 \) is close to: (Radius of Earth \( R = 6.4 \times 10^6 \, \text{m} \))