Answer the following in brief:
A spacecraft is moving with constant velocity in a region where gravitational force is negligible. How can the spacecraft change its velocity? Give a scientific reason for your answer.
A spacecraft moving in a region where the gravitational force is negligible will continue to move with in a straight line according to . Since there is acting on it, its velocity will remain unchanged. To change its speed or direction, the spacecraft must experience an . This is achieved by , which produce by ejecting gases backward. According to , the reaction force (thrust) pushes the spacecraft forward, thereby changing its velocity.