An incredibly powerful force, not just in physics but in human psychology as well.
Sales opportunities have a momentum and energy. Lose the momentum, lose the deal. Force needs to be applied to get around friction. Fail to tend to a deal for too long, and the opportunity fades.
Business has a momentum as well. When things are going well - when you're winning - everyone is your cheerleader and wants to pile on. When things are going poorly, well.
Be aware of these energy flows, the reality of momentum. Apply force thoughtfully, don't let things get too slow or the flywheel will come to a gradual stop.