FORCE AND MOTION. 31 



grams and moves with a velocity of 20 meters a second. The other 

 moves with a velocity of 500 meters a second. What is its weight ? 



9. A force of 1000 dynes acts on a certain mass for one second and 

 gives it a velocity of 20 cm. per second. What is the mass in 

 grams? Ans. 50. 



10. A constant force, acting on a mass of 12 g. for one second, 

 gives it a velocity of 6 cm. per second. Find the force in dynes. 



11. A force of 490 dynes acts on a mass of 70 g. for one second. 

 What velocity will be produced ? Ans. 7. 



12. Two bodies start from a condition of rest and move towards 

 each other under the influence of their mutual attraction ( 7 and 

 98). The first has a mass of 1 g. ; the second, a mass of 100 g. The 

 force of attraction is T ^ dyne. What will be the velocity acquired 

 by each during one second ? 



72. Laws of Motion. The following propositions, 

 known as Newton's Laws of Motion, are so important and 

 BO famous in the history of physical science that they 

 ought to be remembered by every student : 



(1.) Every ~body continues in its state of rest or 

 of uniform motion in a straight line 

 unless compelled to change that state by 

 an external force. 



(2.) Every motion or change of motion is in the 

 direction of the force impressed and is 

 proportionate to it. 



(3.) Action and reaction are equal and opposite 

 in direction. 



73. The First Law. The first law of motion re- 

 sults directly from inertia ( 38). It is impossible to 

 furnish perfect examples of this law because all things 

 within our reach or observation are acted upon by some 

 external force. A base-ball when once set in motion has 

 no power to stop itself ( 38, a). If it moved in obe- 



