The Study of Astronomy, adapted to the capacities of youth / In twelve familiar dialogues, between a tutor and his pupil: explaining the general phænomena of the heavenly bodies, the theory of the tides, &c.

by teacher of astronomy John Stedman

Pupil. Then as the planetary motion is circular, it cannot be simple?

Tutor. No. It is a compound of the two forces I have been mentioning: the one is called the attractive or centripetal force; the other, the projectile or centrifugal force.

Pupil. The former I clearly comprehend, but not the latter. I can conceive, that if two bodies approach each other by attraction they must move in a right line.

Tutor. If you shoot a marble on a smooth piece of ice, in what direction will it run?

Pupil. Strait forward.

Tutor. This is a projectile force.—Could you, do you think, shoot it in any other direction?

Pupil. No, Sir.

Tutor. Then is not this motion also rectilineal?

Pupil. It is.

Tutor. When you strike a ball with your cricket-bat, or throw a stone with your hand, is it not projected or thrown forward by the force of the bat or hand?