8 



FIRST YEAR SCIENCE 



acted upon by some force, and when in motion of con- 

 tinuing to move in a straight line with the same speed, 

 unless acted upon by an outside force, 

 is called inertia. Sir Isaac Newton 

 first stated this fact, and so it is some- 

 times called Newton's First Law. It 

 is the force of inertia which throws 

 one out of an automobile if it is sud- 

 denly stopped. 



Experiment 4. Suspend a heavy ball by a 

 string not much too strong to hold it. (Place 

 a pad beneath it to catch it if it drops.) 

 Attach a similar string / to the bottom of the 

 ball. Attempt to lift it suddenly by the upper 

 string. What happens? Suspend it again 

 and pull down gradually on the lower string. 

 What happens? Suspend it again and pull 

 down suddenly on the lower string. What 

 Fig. 3. happens? 



When we tried suddenly to lift the suspended ball the 

 force of inertia was so great that it broke the string. 

 When the string was attachejd to the bottom of the ball 

 and the pull gradually exerted, the upper string broke, 

 since it had both the weight of the ball and the pull of 

 the string to withstand ; but when the pull was suddenly 

 exerted, the inertia of the ball was sufficient to withstand 

 the pull, and the lower string broke. 



It is the inertia of the water which enables the small, 

 rapidly revolving propeller to move the big ship. The 

 same is true of both the propelling and supporting of fly- 

 ing machines. The resistance which the particles of air 

 offer to being suddenly thrown into motion, their inertia, 

 enables the propeller to push the aeroplane along and 

 keeps it from falling to the ground as long as it is moving 

 rapidly. It is inertia which keeps the heavenly bodies 



