Why do I always fall over on the bus?

Newton's 1st Law

Whether we’re on the bus, the tram, or the tube, we all dread being thrown backwards into the lap of an unsuspecting stranger when the thing starts moving before you’ve found a seat. But why do we get thrown backwards anyway? Aren’t we moving forwards? If you’ve ever found yourself meekly apologising to a fellow passenger and thinking “Why did THIS have to happen!?” then read on, because it can all be explained by Newton’s 1st Law.