“I can make a body move by various means,” answered Tom.
“But they may all be reduced to one,” said Mr. Seymour; “viz. some exertion which is called Force; thus the force of my hand put your ball in motion; while gravitation was the force which made it fall to the earth; and I must, moreover, inform you, that a body always moves in the direction of the force which impels it, and with a velocity, or rate of motion, which is proportional to its degree, or strength; and, were there no other forces in action but that which originally produced the motion, the body would proceed onwards in a right line, and with a uniform velocity for ever.”
“For ever!” exclaimed Louisa.
“Ay, my dear, for ever: but we will discuss that question presently; you must first tell me whether you understand what is meant by uniform velocity.”
“I suppose that uniform velocity is that which is regular, and of an equal rate throughout.”
“Philosophers,” replied her father, “call the motion of a body uniform, when it passes over equal spaces in equal times.--Now, Tom, it is your turn to answer a question. Can you describe the meaning of the terms Accelerated and Retarded motion?”
“I conclude that motion is said to be accelerated when it moves every moment quicker and quicker; and to be retarded when it moves slower and slower.”
“You are perfectly right; and gravity may either act in occasioning the one or the other; our experiment at the well this morning afforded you an example of gravity producing a regularly accelerated motion. I did not fully explain the fact at the time, because I was desirous of avoiding too many new ideas at once; we must win our way slowly and cautiously through the mazes of philosophy: I will, however, now endeavour to give you as clear an explanation as the subject will allow.--It is, I think, evident, that if, at the moment you dropped the stone from your hand, the force of gravity could have been suspended, it would have descended to the bottom of the well with a uniform velocity; because there could have been nothing either to accelerate or retard its motion. But this was not the case, for the power of gravity was in constant operation; and, if you keep this fact in mind, you will readily understand how the velocity became accelerated: for, suppose the impulse given by gravity to the stone, during the first instant of its descent, be equal to one, the next instant we shall find that an additional impulse gives the stone an additional velocity equal to one, so that the accumulated velocity is now equal to two; the following instant, again, increases the velocity to three, and so on till the stone reaches the bottom.”
Mr. Twaddleton observed, the fact might be shortly expressed by saying, that “the effects of preceding impulses must be added to subsequent velocities.”
Mr. Seymour then remarked that the same explanation would apply to retarded velocity. “If,” said he, “you throw a stone perpendicularly upwards, the velocity will be as much retarded, as it was in the other case accelerated, by gravity; the consequence of which is, that it will be exactly the same length of time ascending that it was descending.”