When a body is at rest or in uniform motion,no force acts on it. Is this statement true or false?

What is dynamics? What is mechanics?

$A$ person driving a car suddenly applies the brakes on seeing a child on the road ahead. If he is not wearing a seat belt,he falls forward and hits his head against the steering wheel. Why?

$A$ person is sitting in a travelling train and facing the engine. He tosses up a coin and the coin falls behind him. It can be concluded that the train is

Passengers standing in a bus are thrown outwards when the bus takes a sudden turn. This happens because of

An astronaut accidentally gets separated from his small spaceship accelerating in interstellar space at a constant rate of $100 \; m/s^2$. What is the acceleration of the astronaut the instant after he is outside the spaceship (in $; m/s^2$)? (Assume that there are no nearby stars to exert gravitational force on him.)