However, some applications involve changing masses (for example, chains being rolled up or being unrolled) and in those cases both terms m ˙ ˙ i v i and m i v ˙ ˙ i have to remain present, giving ∑ ∑ i ( F i − − m i a i − − m ˙ ˙ i v i ) ⋅ ⋅ δ δ r i = 0.