EXPERIMENT 3: NEWTON'S SECOND LAW Introduction: In this lab, we will minimize friction on a moving cart by using carts having small wheels with nearly frictionless bearings. You will use a computer together with an ultrasound motion detector to measure the position, as a function of time, of such a cart moving along a level track. You will test Newton's Second Law by analyzing the cart motions for a series of experiments; the accelerated mass in each experiment will be the same, but the net force causing the acceleration will vary. Newton's Second Law gives the relationship between the total mass M of a system of objects, the amount of total force Fnet acting on the system, and the amount of acceleration a of the system. The Second Law is usually written Fnet = M a If we arrange, in a series of experiments, to keep the mass M of the system of objects always the same, then a graph of Fnet versus a for those experiments should be a straight line with a slope equal to the mass M . To understand this statement, recall (from high-school algebra) that the equation for a straight line on an x-y graph is y = mx + b, where m is the slope and b is the y-intercept. In our case (if there is truly no friction), the y-intercept is zero (because there is no acceleration if there is no net force), so the equation of a line becomes y = mx. Comparing y = mx with Fnet = M a, it is easy to see that the mass in our experiment will be the slope of the graph of Fnet versus a (with Fnet on the y-axis and a on the x-axis). Procedure A sketch of the apparatus is shown below. The computer is not shown. The positions of the 10.0 g masses in the sketch are appropriate for the first of the six experiments described in the steps following the figure. motion detector hanger 10 g mass cart reflector screen 10 g masses track 1