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Section 2.1 Outline
Forces Change Motion
I. A Force is a Push or a Pull
1) FORCE: a push or a pull
2) Example: picking up your backpack
A. Types of forces
1) Contact force: when an object pushes or
an object by touching it
Example: skater applies CONTACT FORCE to the
ground when skating
2) Gravity: force of attraction between two
Example: Earth’s GRAVITY pulls skater to the
3) Friction: a force that resists motion
between two surfaces that are pressed
Example: ground’s surface resists the skater’s
B. Size and Direction of Forces
1) like velocity, force is a vector which
means the force has both size and
2) example: shooting a basketball (needs
both direction and amount of force)
C. Balanced and Unbalanced
1) NET FORCE: overall force acting on an
object when all forces are combined
2) If the net force on an object is zero, the
forces are balanced; balanced forces have
the same effect as no force at all
3) Only an unbalanced force can change an
object’s motion; it does not matter whether
the object started at rest or was already
D. Forces on Moving Objects
1) An object with forces acting on it can be
moving at a constant velocity as long as
those forces are balanced
2) Balanced forces CANNOT change an
object’s speed or direction
3) An unbalanced force is needed to change
an object’s motion
II. Newton’s First Law Relates to
and Motion
1) Ancient Greeks concluded that it was
necessary to apply a constant force to
keep an object in motion
2) Greeks reasoned that an object stops
moving because you stop pushing it
A. Galileo’s Thought Experiment
1) Galileo concluded that it does not take a
force to keep an object moving; it takes a
force (friction) to stop an object that is
already moving
2) Objects at rest and objects in motion both
resist changes in motion
3) Objects at rest tend to stay at rest and
objects in motion tend to continue moving
unless a force acts on them
B. Newton’s First Law
1) NEWTON’S FIRST LAW: states that
objects at rest remain at rest, objects in
motion remain in motion, unless acted
upon by an unbalanced force
2) example: a soccer ball will continue
moving until stopped by a goalie or net
C. Inertia
1) INERTIA: the resistance of an object to
change in the speed or direction of its
2) Inertia is closely related to mass; when
measuring the mass of an object, you
also measure its inertia
3) It is harder to change the motion of an
object (inertia) if it has more mass