Chapter 2: Motion
 What could you do to change the
velocity of a soccer ball?
 Explain how motion and force are
related.
 Describe what inertia is and how it is
related to Newton’s first law of motion.
 Identify the forces and motion that are
present during a car crash.
 Force: A push
or a pull
 Sometimes it is obvious that a force is
being applied.
 Kicking a soccer ball
 Sometimes it is not obvious
 Force of the atmosphere pushing against you
 Forces can cause the motion of an
object to change.
 Example: hitting a baseball
 Measure force in Newtons (N)
 Force doesn’t always change
velocity.
 Net Force: the sum of the forces
acting upon an object.
 Balanced Forces: forces on an object
that are equal in size but opposite in
direction.
 Net force = zero
 Object does not move
 Unbalanced Forces: unequal forces acting
on the same object.
 Object moves in the direction of the larger
force
10 N
4N
10 N
8N
4N
4N
10 N
4N
 Inertia: the tendency of an object to
resist any change in its motion.
 If an object is moving, it will keep moving
at the same speed and in the same
direction unless an unbalanced force acts
on it.
 If an object is at rest, it tends to remain
at rest unless an unbalanced force acts
on it.
 Which has
greater inertia, a
truck or a hot
wheels car?
 A greater force is needed to move the
truck than the hot wheels car.
 The inertia of an object is related to its
mass.
 The greater the mass of an object, the
greater its inertia.
 Newton’s First Law of Motion: an
object moving at a constant velocity
keeps moving at that velocity unless
an unbalanced net force acts on it.
 Sometimes called the law of inertia.

When a car traveling
about 50 km/h collides
head-on with something
solid, the car crumples,
slows down, and stops
within about 0.1 s.
 What happens to any passenger not
wearing a safety belt?
 They continue to move forward at the
same speed the car was traveling.
 What is the net force on a
refrigerator if you push on the
refrigerator and it doesn’t move?