73
Gravity thus also shapes life on
Earth by limiting the size of animals.
The largest land animals ever were
dinosaurs weighing up to 40 tons.
The largest animals of all, whales, are
found in the oceans, where the water
supports their weight. Gravity is also
responsible for the tides, which are
produced because water bulges
toward the sun and moon on the
side of Earth nearer to them, and
also bulges away from them on the
other side where their gravitational
pull is weaker. When the sun and
moon are aligned, there is a high
spring tide; when they are at right
angles, there is a low neap tide.
Escape velocity
Gravity profoundly affects human
mobility. The height a person
can jump is determined by the
gravitational field at ground level.
Newton realized that the strength
of gravity would affect the ease of
travel beyond the atmosphere. To
break free from Earth’s gravitational
pull, it is necessary to travel at
25,020 mph (40,270 km/h). It is much
easier to get away from less massive
bodies such as the moon and Mars.
Turning the problem around, this
escape velocity is also the minimum
velocity that an incoming asteroid
or comet can have when it hits
Earth’s surface, and this affects
the size of the resulting crater.
Today, gravity is held to be
most accurately described by
the general theory of relativity
proposed by Albert Einstein
in 1915. This does not describe
gravity as a force, but instead as
a consequence of the curvature of
the continuum of spacetime due
to the uneven distribution of mass
inside it. This said, Newton’s
concept of a gravitational forceTHE TELESCOPE REVOLUTION
is an excellent approximation in
the vast majority of cases. General
relativity only needs to be invoked
in cases requiring extreme
precision or where the gravitational
field is very strong, such as close
to the sun or in the vicinity of a
massive black hole. Massive bodies
that are accelerating can produce
waves in spacetime, and these
propagate out at the speed of light.
The first detection of one of these
gravitational waves was announced
in February 2016 (pp.326–29). ■Newton illustrated escape velocity with a thought experiment of a
cannon firing horizontally from a high mountain. At velocities less than
orbital velocity at that altitude, the cannonball will fall to earth (A and
B). At exactly orbital velocity, it will enter a circular orbit (C). At greater
than orbital velocity but less than escape velocity, it will enter an
elliptical orbit (D). Only at escape velocity will it fly off into space (E).A
B E
C
D
The motions of the comets
are exceedingly regular,
and they observe the
same laws as the
motions of the planets.
Isaac Newton