According to a well-known legend, Sir Isaac Newton was struck in the head by an apple that fell from a tree which inspired him to develop the theory of gravity.
Newton came to the conclusion that the apple is being pulled to the ground by an enigmatic force. He identified it as a gravitational force.
He contrasted the cannonball's curved trajectory with the apple's straight course.
He began speculating about what may occur if the cannonball continued to accelerate.
The cannonball would eventually follow a curved course around the earth, he realised, and never touch the ground.
The moon's orbit around the Earth is described in this way.
Newton saw that if there was no gravitational pull towards the Earth, the moon would fly out in a straight path and away from it.