A car accelerates from rest to \( u \, \text{m /s} \). The energy spent in this process is \( E \, \text{J} \). The energy required to accelerate the car from \( u \, \text{m /s} \) to \( 2u \, \text{m /s}\) is in \( E \, \text{J} \). The value of \( n \) is __________.
