Volume 1
A course of lectures on natural philosophy and the mechanical arts / by Thomas Young.
- Thomas Young
- Date:
- 1845
Licence: Public Domain Mark
Credit: A course of lectures on natural philosophy and the mechanical arts / by Thomas Young. Source: Wellcome Collection.
Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.
52/670 page 18
![In all these cases it is of importance to attend to the composition of motion, or the joint effect of more than one motion existing at the same time. The existence of two or more motions at the same time, in the same body, is not at first comprehended without some difficulty. It is in fact only a combination or separation of relations that is con- sidered ; in the same manner as by combining the relation of son to father, and brother to brother, we obtain the relation of nephew to uncle, so, by combining the motion of a man walking in a ship, with the motion of the ship, we determine the relative velocity of the man with respect to the earth’s surface. It is, however, necessary, for ascerfaining these relations, to consider the affections of a space or surface in motion, and to examine how it may move in the most simple manner with respect to another space. If any number of points move in parallel lines, describing equal spaces in equal times, they are at rest with respect to each other; for it may easily be demonstrated that the rectilinear distance of each, from each of the rest, remains unchanged; and if aU the points of a plane move in this manner on another plane, either plane may be said to be in rectilinear motion with respect to the other. This is easily exemjilified by causing one plane to move on another, so that two or more of its points shall always remain in a given right line in the second plane : as when a [car- penter’s] square is made to shde along the straight edge of a board, the surface of the square is in rectilinear motion with respect to the board. (Plate I. Fig. 4.) If, besides this general motion of the plane, any point be supposed to have a particular motion in it, the point will have two motions with re- spect to the other plane : the one in common with its plane, and the other pecuhar to itself; and the joint effect of these motions, with respect to the second plane, is called the result of the two motions. Thus, when a car- riage moves on a perfectly [straight and] level road, all its points describe parallel lines, and it is in rectilinear motion with respect to the road : its wheels parfake of this motion, but have also a rotatory motion of their own ; and the result of the two motions of each point of the wheels is the cycloid, or trochoid, that it describes in a quiescent vertical plane. (Plate I. Fig. 5.) When an arm is made to slide upon a bar, and a thread, fixed to the bar, is made to pass over a pulley at the end of the arm next the bar, to a slider which is moveable along the aim, the slider moves on the aim with the same velocity as the arm on the bar; but if the thread, instead of being fixed to the slider, be passed again over a pulley which is attached to it, and then brought back to be fixed to the arm, the motion of the slider will be only half that of the arm; and this will be true in what- ever position the arm be fixed. Here we have two motions in the slider, one in common with the arm, and the other peculiar to itself, which may be either equal or unequal to the first; and by tracing a line on a fixed plane, with a point attached to the slider, we may easily examine the joint result of both the motions. (Plate I. Fig. 6.) The joint result of any two motions is the diagonal of the parallelogram of which the sides would lie described, in the same time, by the separate mo-](https://iiif.wellcomecollection.org/image/b21301840_0001_0052.jp2/full/800%2C/0/default.jpg)
