Motions and Forces
SECTION 3
A. Second law of Motion
An object acted on by an
unbalanced force will
accelerate in the direction
of the force.
1.
 Acceleration equals force divided
by mass, or a=force/mass
2.
Force is equal to the
combination of all forces, or
the total force that acts on an
object.
B.
Second law can also be
used to find force if mass
and acceleration are
known.
1.
Near Earth’s surface, the
force of gravity causes all
objects to fall with the same
acceleration – 9.8m/s2
A.
For any object that is falling, the
force of gravity equals mass
times acceleration due to
gravity, or F=m X (9.8 m/s2)
2.
Weight and Mass are
not the same thing.
A.
Weight changes when
the acceleration due to
gravity changes.
B.
Mass remains the
same no matter
what weight is.
C. Friction
A force that resists motion
and is always present
between two moving
surfaces.
1. Static FrictionKeeps a stationary object
from moving on a surface
when a force is applied to
the object.
2. Sliding Friction
Occurs when two surfaces
slide past each other;
slows down the moving
object.
3. Rolling Friction
Friction between a surface
and a wheel when the
wheel rolls over the
surface.
4. Air resistance-
Typical action of air
molecules on a forwardmoving object, slowing its
motion.
Section 4: Newton’s Third Law
A. Third law of Motion-
Forces always act in equal
but opposite pairs.
1. When a force is exerted
on an object, the object
exerts the same amount of
force.
2. Third law of motion
applies whether forces
are contact or Longrange.
B. Things move
because action and
reaction forces work on
different objects.
1. Friction is a factor
in the third law.
2. Using the second law
equation, the object with
the larger mass has the
smaller acceleration if
the same force is applied.
3. All objects in the universe
exert a force on all others;
however, differences in mass
may make these forces
unnoticeable.
C. The three laws of
motion described how
any object moves when
forces act on it.