FORCES AND MOTIONS IN DAILY LIFE 



147 



centripetal force a force in a rotating body which pulls 



toward the center of rotation. 

 acceleration increasing speed of a moving body. 

 if^nji^SBte- decreasing speed of a moving body. 

 density the weight of a unit volume of any substance. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 104. What causes friction and how 



may it be reduced? 



Equip a chalk box with four wheels which turn easily 

 and which can be removed. Weigh the box and wheels to- 

 gether and then place a two-pound weight and the wheels 



FIG. 228 



in the box as a load. Attach a string and spring balance 

 to the box as shown in the illustration and determine the 

 average effort needed to keep the box moving uniformly 

 over the surface of the table top. Keep the balance parallel 

 with the table top. Sprinkle sand on the table top and repeat 

 your measurements. Attach the wheels to the box as shown 

 at B in Figure 228. With the table top clean determine the 

 force that is necessary to keep the box and load moving 

 uniformly. Again sprinkle sand on the table top and repeat 

 the measurement. 



The effort force is when sand is placed on the table 



top because sand . 



The effort force is when wheels are placed under the 



car. 



For each of the four trials, divide the effort force by 

 the total load and express the quotient as a decimal. 



Experiment 105. Do equal volumes of different sub- 

 stances have the same weight? 



Secure regular solids of such substances as aluminum, 

 brass, wood, iron, lead, glass, and any other that may be 



1 See workbook, p. 52. 



available, and carefully determine their volumes in cubic 

 centimeters. Record these in a neat table. If regular solids of 

 these or other substances are not available, devise a method 

 of determining the volume of irregular solids of the same 

 substances by using a graduate. Record these volumes in a 

 neat table. Carefully determine the weight of each solid, in 

 grams, and record it beside the volume as determined above. 

 By calculation find the weight of a unit volume of each of 

 the substances. (Divide the weight by the volume.) Record 

 in the table. Answer the following questions about the experi- 

 ment. Do equal volumes of all substances have the same 

 weight? Can you suggest a reason why this is true? 



Experiment 106. What is inertia? 



Secure a marble, a pint milk bottle, and a piece of thin 

 cardboard about two inches square. Place the cardboard 

 on the top of the milk bottle and the marble on the card- 

 board. With the thumb and first finger, snap the cardboard 



FIG. 230 



on one of its corners and observe what happens. Record 

 your results. 



Place a smooth sheet of paper on the table and stand a 

 glass of water on it. Give the paper a sudden pull and observe 

 what happens. Record your results. 



Cut a notch from a stick of wood as shown in Figure 

 230. Build a sample frame similar to the one shown and 

 attach the notched stick to it by means of rubber bands or 

 strips of rubber cut from an old automobile tire inner tube. 

 Twist the bands and then place a marble in the notch of the 

 stick as it is drawn back. Let the stick go. What happens 

 when the stick is 'stopped by the frame. Repeat this several 

 times with different materials as weights and observe the 

 results carefully. Make a record of your findings. 



As you try to reach a conclusion in answer to the title 

 question of this experiment, be guided by these questions. 

 Did the marble take up the motion of the card? Was the 

 glass of water pulled off the table when the sheet of paper 

 was set in motion ? What happened to the motion of the 

 marble used in the last part of the experiment when the 

 notched stick was suddenly stopped by the frame? What 

 would you have had to do to set the marble in motion? 





