January, 1011. 



KNOWLEDGE. 



the plane is exacth' halancfd b\- the scale pan and 

 weights. Now if the string is reinowd the trolley 

 will accelerate under a force which is equal and 

 opposite to that which has been removed, viz., the 

 weight of the scale pan and its contents. The 

 acceleration can be measured as before. If the 



bullet embeds itself. After the shot the trolleys 

 retire with equal momenta. 



Figure 6 shows an inertia bar, which is set in rotation 

 In" the impact of a trolley. Vibrating springs are 

 employed as before to measure the momentum 

 of the trolle\', and the angular momentum of the bar. 



Hmm. 



Figure 7. 



A vibrator used for measiirin.i,' the anguldr 

 velocity of a fly wheel. 



Figure S. 



The vibrator adapted to an Atwood's machine in which a paper ribbon 



replaces the string. 



observations are now repeated with a different angle 

 of slope the two accelerations will be found to be in 

 the same ratio as the two accelerating forces.* An 

 approximate verification of Newton's Second Law- 

 may seem unsatisfactory ; but the method has the 

 merit of making the notion of acceleration nuich 

 more eas}' for the ordinar\' mind to grasp. 



Figure 5 shows how. by means of two of these 

 trolleys, laws of momentum and impact may be 

 verified. In one form of the apparatus, employed by 

 Mr. Ashford at Dartmouth Naval College, one trolle\- 

 carries a pistol and the other a target in which the 



Figure 7 shows the vibrator employed in measuring 

 the angular velocity of a fly wheel. The notion of 

 kinetic energ\- becomes clearer after a series of 

 measurements taken with an apparatus of this kind. 



Mr. G. Cussons, of Manchester, to whom we are 

 indebted for the photographs used in illustrating 

 this article, has adapted the vibrator to a form of 

 Atwood's machine in which a paper ribbon replaces 

 the usual string. The apparatus is shown in 

 Figure 8. Excellent results are obtained from this 

 instrument, and Figure 4 is a reproduction of one of 

 the curves obtained from it. 



■■ See Egg.ar's Mechanics, E. Arnold. 



