Newton's second law of motion relates external forces ( F i → → ) to the change in linear momentum of the whole system (including rocket and exhaust) as follows: ∑ ∑ i F i → → = lim Δ Δ t → → 0 P 2 → → − − P 1 → → Δ Δ t where P 1 → → is the momentum of the rocket at time t = 0 : P 1 → → = m V → → and P 2 → → is the momentum of the rocket and exhausted mass at time t = Δ Δ t : P 2 → → = ( m − − Δ Δ m ) ( V → → + Δ Δ V → → ) + Δ Δ m V e → → and where, with respect to the observer: