...there is an external force causing the moon not to continue in a straight line.
Explanation: According to Newton's first law of motion, every object will continue in its state of rest or uniform motion along a straight line unless acted upon by an external force.
The external force causing the moon not to continue in a straight line is the force of gravitational attraction between the earth and the moon. This is in accordance to Newton's universal law of gravitation which states that every object attracts another object with a force that is directly proportional to the product of the masses of the objects and inversely proportional the the square of the distance between them. Also in this case, the gravitational pull between the moon and the earth is equivalent to the centripetal force that keeps the moon towards the earth, the earth being the center of motion for the moon. Recall that if the moon is orbiting the earth, then certainly the earth becomes the center of its motion.
Therefore the moon will only continue in a straight line in absence of the earth's gravitational pull.
