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Chapter 6
Section 1 Gravity and Motion
Gravity and Falling Objects
• Gravity and Acceleration Objects fall to the
ground at the same rate because the acceleration
due to gravity is the same for all objects.
• Acceleration Due to Gravity For every second
that an object falls, the object’s downward velocity
increases by 9.8 m/s.
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Chapter 6
Section 1 Gravity and Motion
Gravity and Falling Objects, continued
• Velocity of Falling Objects You can calculate the
change in velocity with the following equation:
∆v  g  t
• If an object starts at rest, this equation yields the
velocity of the object after a certain time period.
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Chapter 6
Section 1 Gravity and Motion
Air Resistance and Falling Objects
• Air resistance is the force that opposes the motion of
objects through air.
• The amount of air resistance acting on an object
depends on the size, shape, and speed of the object.
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Chapter 6
Section 1 Gravity and Motion
Air Resistance and Falling Objects, continued
• Acceleration Stops at the Terminal Velocity As
the speed of a falling object increases, air resistance
increases.
• The upward force of air resistance continues to
increase until it is equal to the downward force of
gravity. The object then falls at a constant velocity
called the terminal velocity.
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Chapter 6
Section 1 Gravity and Motion
Air Resistance and Falling Objects, continued
• Free Fall Occurs When There Is No Air
Resistance An object is in free fall only if gravity is
pulling it down and no other forces are acting on it.
• A vacuum is a place in which there is no matter.
Objects falling in a vacuum are in free fall because
there is no air resistance.
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Chapter 6
Section 1 Gravity and Motion
Orbiting Objects Are in Free Fall
• Astronauts float in orbiting spacecrafts because of
free fall.
• Two Motions Combine to Cause Orbiting An
object is orbiting when it is traveling around another
object in space.
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Chapter 6
Section 1 Gravity and Motion
Orbiting Objects Are in Free Fall, continued
• Orbiting and Centripetal Force The unbalanced
force that causes objects to move in a circular path is
called a centripetal force.
• Gravity provides the centripetal force that keeps
objects in orbit.
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Chapter 6
Section 1 Gravity and Motion
Projectile Motion and Gravity
• Projectile motion is the curved path an object
follows when it is thrown or propelled near the surface
of the Earth.
• Projectile motion has two components—horizontal
motion and vertical motion. These components are
independent, so they have no effect on each other.
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Chapter 6
Section 1 Gravity and Motion
Projectile Motion and Gravity, continued
• Horizontal Motion is a motion that is parallel to the
ground.
• When you throw a ball, your hand exerts a force on
the ball that makes the ball move forward. This force
gives the ball its horizontal motion.
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Chapter 6
Section 1 Gravity and Motion
Projectile Motion and Gravity, continued
• Vertical Motion is motion that is perpendicular to the
ground.
• A ball in your hand is prevented from falling by your
hand. After you throw the ball, gravity pulls it downward
and gives the ball vertical motion.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion
An object at rest remains at rest, and an object in
motion remains in motion at a constant speed and in a
straight line unless acted on by an unbalanced force.
• Newton’s first law of motion describes the motion of
an object that has a net force of 0 N acting on it.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
• Part 1: Objects at Rest Objects at rest will stay at
rest unless they are acted on by an unbalanced force.
• Part 2: Objects in Motion Objects will continue to
move with the same velocity unless an unbalanced
force acts on them.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
• Friction and Newton’s First Law Friction between
an object and the surface it is moving over is an
example of an unbalanced force that stops motion.
• Inertia and Newton’s First Law Newton’s first law
is sometimes called the law of inertia. Inertia is the
tendency of all objects to resist any change in motion.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
• Mass and Inertia Mass is a measure of inertia. An
object that has a small mass has less inertia than an
object that has a large mass.
• So, changing the motion of an object that has a small
mass is easier than changing the motion of an object
that has a large mass.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion
The acceleration of an object depends on the mass of
the object and the amount of force applied.
• Newton’s second law describes the motion of an
object when an unbalanced force acts on the object.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion, continued
• Part 1: Acceleration Depends on Mass The
acceleration of an object decreases as its mass
increases. Its acceleration increases as its mass
decreases.
• Part 2: Acceleration Depends on Force An object’s
acceleration increases as the force on the object
increases. The acceleration of an object is always in
the same direction as the force applied.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion, continued
• Expressing Newton’s Second Law Mathematically
The relationship of acceleration (a) to mass (m) and
force (F) can be expressed mathematically with the
following equation:
a 
F
, or F  m  a
m
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion
Whenever one object exerts a force on a second
object, the second object exerts an equal and opposite
force on the first.
• Newton’s third law of motion can be simply stated as
follows: All forces act in pairs.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion, continued
• Force Pairs Do Not Act on the Same Object A
force is always exerted by one object on another
object. This rule is true for all forces, including action
and reaction forces.
• Action and reaction forces in a pair do not act on the
same object. If they did, the net force would always be
0 N and nothing would ever move!
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion, continued
• All Forces Act in Pairs—Action and Reaction
Newton’s third law says that all forces act in pairs.
When a force is exerted, there is always a reaction
force.
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Chapter 6
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion, continued
• The Effect of a Reaction Can Be Difficult to See
When an object falls, gravity pulls the object toward
Earth and pulls Earth toward the object.
• You don’t notice Earth being pulled upward because
the mass of Earth is much larger than the mass of the
object. Thus, the acceleration of Earth is much smaller
than the acceleration of the object.
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Chapter 6
Section 3 Momentum
Momentum, Mass, and Velocity
• The momentum of an object depends on the object’s
mass and velocity.
• Calculating Momentum The relationship of
momentum (p), mass (m), and velocity (v) is shown in
the equation below:
pmxv
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Chapter 6
Section 3 Momentum
The Law of Conservation of Momentum
• The law of conservation of momentum states that any
time objects collide, the total amount of momentum
stays the same.
• Objects Sticking Together After two objects stick
together, they move as one object. The mass of the
combined objects is equal to the masses of the two
objects added together.
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Chapter 6
Section 3 Momentum
The Law of Conservation of Momentum,
continued
• The combined objects have a different velocity
because momentum is conserved and depends on
mass and velocity.
• So, when the mass changes, the velocity must
change, too.
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Chapter 6
Section 3 Momentum
The Law of Conservation of Momentum,
continued
• Objects Bouncing Off Each Other When two
objects bounce off each other, momentum is usually
transferred from one object to the other.
• The transfer of momentum causes the objects to
move in different directions at different speeds.
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Chapter 6
Section 3 Momentum
The Law of Conservation of Momentum,
continued
• Conservation of Momentum and Newton’s Third
Law Conservation of momentum can be explained
by Newton’s third law.
• Because action and reaction forces are equal and
opposite, momentum is neither gained or lost in a
collision.
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