Which physical law is in control of a spaceship which cruises through space at constant speed without using any fuel?

Newton’s first law Newton’s first law states that an object will remain at rest or in uniform motion unless acted upon by an external force. This means an object at rest will remain at rest and an object in motion will remain in motion with the same speed and the same direction unless it is acted on by an unbalanced force. This implies that a spaceship will continue through space at constant speed without using any fuel if it remains uninterrupted by an external force.

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