An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. It is also known as law of inertia.
If an object such as a rocket, is at rest then the forces on it are balanced. If the object is moving, it takes an unbalanced force and to stop it, we should change its direction from a straight line path or increase or decrease its speed. In rocket flight, forces become balanced and unbalanced almost all the time. So, it's very important to apply the law of inertia of newton's 1st law of motion into a count while working with rockets, or making one.
