Question

When a runing bus suddenly stop the passengers are thrown forward. why ?

Answer 1

because bus suddenly giving a force which can cause to passenger thrown forward

Answer 2

Due to Inertia
Inertia is the property of matter by which it continues to be in the existing state, i.e, either moving or in rest.

So when the bus is moving our body is moving along the bus. But, when the bus stops the lower part of the body comes to rest except the upper part of the body. Upper part of body, due to inertia tries to remain in the moving state, so, it continues to move so we move forward whereas the our lower part is in rest. Therefore we fall forward.