Second Law: The force
acting on a body is directly proportional to, and in the
same direction as, its acceleration
]
Newton’s first law of motion is, essentially, the definition of
inertia
—
the tendency of
an object to resist a change in motion. The more mass an
object has, the
greater its
inertia. An object with greater inertia is more difficult to start
or stop.
Newton’s second law tells us that the force (F) on an object
is directly proportional
to it’s acceleration, and that is dependent on its mass. So,
the second law can
be
summed up as F=ma. Or, acceleration depends on the
objects mass and on the net
force acting on the object:
Force
Acceleration =

Mass
Orbital Motion: Why do Earth and the moon remain in their
orbits? Newton
concluded that two factors: inertia and gravity combine to
keep the Earth in orbit
around the sun and
the moon in orbit around the earth. What exactly, is the force
of gravity? 9.8 m/s
2
. Where have we seen that unit of measurement before, and
what does it describe? [
it’s the acceleration
]
The Earth’s gravity keeps pulling the moon toward it,
preventing th
e moon from
moving in a straight line. At the same time, the moon keeps
moving forward
because of inertia. If not for Earth’s gravity, the moon would
shoot off into space in
a straight line, and if not for the moon’s inertia, the moon
would collide with Earth
due
to the pull of Earth’s gravity
Image created by the author. License: Public Domain
Actual Orbit
Moon
Earth
Gravitational
Force of Earth
Moon’s motion
without Gravity
(due to inertia)
Newton’s Laws of Motion (cont’d)
Newton proposed that whenever one object exerts a force
on a second object, the
second object exerts a force back on the first. The force
exerted by the second object
is equal in strength and opposite i
n force to the first force. If we think of one force
as the action and the other force as the reaction, then
Newton’s third law states
that for every action, there is an equal and opposite reaction.
Can anyone think of an example of an action

reaction pa
ir? [
jumping, rowing...]
In those examples, there was always a motion as a result of
the forces acting
against each other, right? But, can we always detect a
motion when paired forces
are in action? The answer is no. For example, if I drop my
pen, we see
gravity pull
the pen towards the ground. At the same time, we know from
Newton’s third law,
that the pen must be pulling Earth upward with an equal and
opposite reaction
force. But we don’t feel a giant jolt of Earth moving in that
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 Fall '10
 noris
 Isaac Newton