Physics 101 Homework Solutions
First Assignment
8.3 Early in its history, the Earth was in a completely molten state. It
had been heated during its formation because of the kinetic energy
delivered to it during the gravitational accretion process. Denser
materials (mostly metals) sank to the core and less dense materials
(mostly silicate rocks) floated on the surface. Note that this does not
have to do with weight or mass. It has to do with density of a specific
material compared to the average density of the Earth. It is very much
analogous to floating or sinking in water. A cork sphere will float, and
a glass marble will sink, even though they may be the same weight.
The cork sphere would have to be very much larger in volume to have
the same mass as a glass marble – same mass distributed through a
larger object means lower density.
8.4 Density is determined by mass and volume. Specifically, it is the
amount of mass in one unit of volume. To measure density, then, we
need to measure both the mass of the Earth and its size. The size
measurement some of you did in lab. The mass can be measured by
astronomical data, either from the strength of the Earth’s
gravitational field or from the orbital motion of satellites. These
measurements give an average density of 5500 kg/m3. In other words,
a 1 m3 sample of the average material of the Earth would have a mass
of 5500 kg. However, the density of material near the Earth’s surface
can be measured directly since we can collect and measure them in
the lab. The material within the first mile or so of the surface of the
Earth has an average density of about 2.4 kg/m3. If the density of the
outer part of the Earth is lower than the average, then the density of
the inner part must be higher. After all, the average density has to be
in the middle of the two figures.
8.11 The motion of the Earth’s plates allows lava to erupt in rifts that
(mostly) are in the oceans. These eruptions produce new ocean crust
as the plates drift apart. That means that the ocean floor must be the
youngest part of the crust since it is being created right now. The
continents, however, have been floating around on plates since they
first formed during the differentiation of the crust. This means that
the continents must be the oldest part of the crust.
9.6 All craters (with the exception of those perched on the tops of
volcanoes) were formed by high-speed meteorite impacts which
resulted in explosions. There are several lines of evidence to support
this fact. First, such explosions would loft a substantial amount of
material which would then fall back down to the surface. We see
evidence of this in the rays that radiate from some craters, as well as
