i/The highjumper’s body passes
over the bar, but his center of
mass passes under it.(Dunia
Young)
j/Two pool balls collide.
equation #2: no loss of kinetic energy
unknown #1: mass of the mystery particle
unknown #2: initial velocity of the mystery particle
unknown #3: final velocity of the mystery particle
The number of unknowns is greater than the number of equa-
tions, so there is no unique solution. But by creating collisions with
nuclei of another element, nitrogen, he gained two more equations
at the expense of only one more unknown:
equation #3: conservation of momentum in the new collision
equation #4: no loss of kinetic energy in the new collision
unknown #4: final velocity of the mystery particle in the new
collision
He was thus able to solve for all the unknowns, including the
mass of the mystery particle, which was indeed within 1% of the
mass of a proton. He named the new particle the neutron, since it
is electrically neutral.
Discussion Questions
A Good pool players learn to make the cue ball spin, which can cause
it not to stop dead in a head-on collision with a stationary ball. If this does
not violate the laws of physics, what hidden assumption was there in the
example in the text where it was proved that the cue ball must stop?3.1.5 The center of mass
Figures i and k show two examples where a motion that appears
complicated actually has a very simple feature. In both cases, there
is a particular point, called the center of mass, whose motion is
surprisingly simple. The highjumper flexes his body as he passes
over the bar, so his motion is intrinsically very complicated, and
yet his center of mass’s motion is a simple parabola, just like the
parabolic arc of a pointlike particle. The wrench’s center of mass
travels in a straight line as seen from above, which is what we’d
expect for a pointlike particle flying through the air.k/In this multiple-flash photograph, we see the wrench from above
as it flies through the air, rotating as it goes. Its center of mass, marked
with the black cross, travels along a straight line, unlike the other points
on the wrench, which execute loops.(PSSC Physics)142 Chapter 3 Conservation of Momentum