We are standing in a stationary bus. When bus suddenly start why we are thrown in backward direction ?

Explanation

When we are standing in stationary bus our feet are in touch with the floor.
Frictional force is between feet and floor of the bus.
When bus suddenly start due to friction our feet start moving in direction of bus whereas rest of the body try to retain position due to inertia.
hence we are thrown in backward direction in opposite direction of of bus whereas rest of the body try to retain position due to inertia
hence we are thrown in backward direction in oppsite direction of acceleration of bus.