EFFECTS OF FORCES ACTING AT THE SAME TIME 55 



to the under side of the ball. Pull slowly on the bottom string, 

 Which string breaks? Why? Pull very suddenly on the 

 lower string, and explain results. 



c. Bend a piece of clock spring about eight inches long into 

 the form of a V. Put it between two blocks, of unequal size, 

 compress and release. Note the different distances which 

 the blocks moved. Repeat this several times, and give an 

 explanation. 



38. EFFECTS OF Two OR MORE FORCES ACTING AT THE 



SAME TIME 



If two or more forces are acting on a body, each force acts 

 independently of all the other forces. That is, the final posi- 

 tion of the body will be -the same as it would have been if one 

 force had acted alone, stopped, and then each of the other 

 forces had acted successively. 



This fact is stated in Newton's Second Law of Motion. 

 Examples are everywhere present if we do but notice them. 

 For instance, when a person crosses a moving car, the final 

 position he will occupy will be the same as it would have been 

 if he had crossed the car after it stopped. The graphical 

 representation of two or more forces acting together is given 

 in Section 97, Resolution of Forces. 



References : 

 Any reference in Section 37. 



EXPERIMENT FOR THE TEACHER 

 Newton's Second Law of Motion 



Use the standard machine to illustrate, and repeat at least 

 three times, trying the different positions of the trigger. Be 

 sure that the machine is horizontal. 



