Simple Nature - Light and Matter

i / The highjumper’s body passesover the bar, but his center ofmass passes under it.(DuniaYoung)unknown #3: final velocity of the mystery particleThe number of unknowns is greater than the number of equations,so there is no unique solution. But by creating collisions withnuclei of another element, nitrogen, he gained two more equationsat the expense of only one more unknown:equation #3: conservation of momentum in the new collisionequation #4: no loss of kinetic energy in the new collisionunknown #4: final velocity of the mystery particle in the newcollisionHe was thus able to solve for all the unknowns, including themass of the mystery particle, which was indeed within 1% of themass of a proton. He named the new particle the neutron, since itis electrically neutral.Discussion QuestionsA Good pool players learn to make the cue ball spin, which can causeit not to stop dead in a head-on collision with a stationary ball. If this doesnot violate the laws of physics, what hidden assumption was there in theexample in the text where it was proved that the cue ball must stop?3.1.5 The center of massFigures i and k show two examples where a motion that appearscomplicated actually has a very simple feature. In both cases, thereis a particular point, called the center of mass, whose motion issurprisingly simple. The highjumper flexes his body as he passesover the bar, so his motion is intrinsically very complicated, andyet his center of mass’s motion is a simple parabola, just like theparabolic arc of a pointlike particle. The wrench’s center of masstravels in a straight line as seen from above, which is what we’dexpect for a pointlike particle flying through the air.The highjumper and the wrench are both complicated systems,k / In this multiple-flash photograph, we see the wrench from aboveas it flies through the air, rotating as it goes. Its center of mass, markedwith the black cross, travels along a straight line, unlike the other pointson the wrench, which execute loops. (PSSC Physics)j / Two pool balls collide.each consisting of zillions of subatomic particles. To understandwhat’s going on, let’s instead look at a nice simple system, two poolballs colliding. We assume the balls are a closed system (i.e., their140 Chapter 3 Conservation of Momentum