Question

A body continues to be in the state of motion in a straight line unless & until an external unbalanced force is applied on it. But how the spacecraft or rocket change their direction in space where no atmosphere is there?

Answer 1

In space there is no medium to travelll
But rocket change their direction by applying force Rockets used to go in straight line path but thrusters attached to it to apply force in direction where we want to turn the rocket by applying force.

Answer 2

This is based on Newton's 3rd law of motion ie every action has equal and opposite reaction.
A spacecraft exhaust high pressure gas out in a particular direction and gets equal amount of reaction in return. And like this it can change its director in space without atmosphere. If astronauts also want to move in space, they throw a obj with some force in a direction as action and get back the equal amount of force to move forward.

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