. 



CHAPTER IV 

 FORCE AND MOTION; PHYSICAL STATES OF MATTER 



55. Force. A moving object attracts attention, and it 

 is natural to ask, "What started it?" A single word is 

 the answer: force. To the question, "What stops it?" the 

 answer is the same: force. Force is that which produces 

 or tends to produce motion, or change of motion. Force 

 does not always produce motion. A man uses force when 

 he pushes against a wall, but the wall may not move. A 

 force is sometimes described as a push or a pull. 



56. Gravity. The force which causes an unsupported 

 body to fall toward the earth is called gravity. The fall is 

 explained by saying that the earth attracts or pulls the body 

 toward itself. It is gravity which causes the air to remain 

 near the earth and to move with it, instead of being left 

 behind as the earth moves around the sun with the velocity 

 of a cannon ball. 



If a body does not fall toward the earth, it is because it 

 is kept from moving by some opposing force. If the body 

 is held in the hand, gravity is opposed by muscular force. 

 If it rests on a table, gravity is opposed by cohesion, which 

 holds the fibers of wood together. If it is suspended by a 

 chain, gravity is opposed by the tenacity of iron. 



57. Action of Two Forces. The force of gravity acts 

 vertically downward. Any force which opposes it must act 

 vertically upward. If the opposing force is equal to gravity, 

 the body remains at rest; if it is less than gravity, the body 

 moves downward; if greater, the body moves upward. 



The effect of two or more forces acting at the same point 

 is called the resultant. For example, a body weighing 30 



56 



