Newton's Laws of Motion =77. Inertia, First Law of Motion.=--One often observes when riding in a train that if the train moves forward suddenly the passengers do not get into motion as soon as the train, and apparently are jerked backward. While if the train is stopped suddenly, the passengers tend to keep in motion. This tendency of matter to keep moving when in motion and to remain at rest when at rest is often referred to as the property of _inertia_. _Newton's first law of motion_, often called the _law of inertia_, describes this property of matter as follows: _Every body continues in a state of rest or of uniform motion in a straight line unless it is compelled to change that state by some external force._ This means that if an object like a book is lying on a table it will remain there until removed by some outside force. No inanimate object can move itself or stop itself. If a ball is thrown into the air it would move on forever if it were not for the _force_ of attraction of the earth and the resistance of the air. It takes time to put a mass into motion, a heavy object requiring more time for a change than a light object. As an example of this, note the movements of passengers in a street car when it starts or stops suddenly. Another illustration of the law of inertia is the so-called "penny and card" experiment. Balance a card on the end of a finger. Place on it a coin directly over the finger, snap the card quickly so as to drive the card from beneath the coin. The coin will remain on the finger. (See Fig. 59.) [Illustration: FIG. 59.--The ball remains when the card is driven away.] According to Newton's first law of motion a moving body which could be entirely freed from the action of all external forces would have uniform motion, and would describe a perfectly straight course. The curved path taken by a baseball when thrown shows that it is acted upon by an outside force. This force, the attraction of the earth, is called _gravity_. [Illustration: SIR ISAAC NEWTON "By Permission of the Berlin Photographic Co., New York." Sir Isaac Newton (1642-1727) Professor of mathematics at Cambridge university; discovered gravitation; invented calculus; announced the laws of motion; wrote the Principia; made many discoveries in light.] [Illustration: GALILEO GALILEI "By Permission of the Berlin Photographic Co., New York." Galileo Galilei (1564-1642). Italian. "Founder of experimental science"; "Originator of modern physics"; made the first thermometer; discovered the laws of falling bodies and the laws of the pendulum; invented Galilean telescope.] [Illustration: FIG. 60.--Cross-section of the DeLaval cream separator.]