Why does a person in a bus tends to fall forward when it stops suddenly?

A person in a bus tends to fall backward, when it starts suddenly?

Why does the horse rider fall forward when a running horse suddenly stops ?

There are different type of forces in nature.We learnt their effects and applications.Give the exact reasons for the following. Why are passengers thrown forward when a speeding bus stops suddenly?

A man sitting in a bus gets a jerk forwards when the bus suddenly stops, Why?