Chapter 13
Physical Science
Chapter 13
Forces and Motion
Preview
Section 1 Gravity: A Force of Attraction
Section 2 Gravity and Motion
Section 3 Newton's Laws of Motion
Concept Map
Chapter 13
Forces and Motion
Chapter
13
The Law
Section 1 Gravity: A Force of Attraction
of Universal Gravitation,
continued
• The force of gravity depends on the
distance between two objects.
• As the distance between two objects gets
larger, the force of gravity gets much
smaller.
Chapter 13
Forces and Motion
Chapter 13
Forces and Motion
Chapter 13
Forces and Motion
Chapter 13
Section 2 Gravity and Motion
Air Resistance and Falling Objects
• Air resistance is the force that opposes the
motion of objects through air. Air resistance
slows the acceleration of falling objects.
• The amount of air resistance acting on a
falling object depends on the size, shape,
and speed of the object.
Chapter 13
Section 2 Gravity and Motion
Air Resistance and Falling Objects,
continued
• An object falls at its terminal velocity when
the upward force of air resistance equals
the downward force of gravity.
• An object is in free fall if gravity is the only
force acting on it.
Chapter 13
Section 2 Gravity and Motion
Air Resistance and Falling Objects,
continued
• Because air resistance is a force, free fall
can happen only where there is no air.
• The term vacuum is used to describe a
place in which there is no matter. Vacuum
chambers are special containers from which
air can be removed to make a vacuum.
Chapter 13
Section 2 Gravity and Motion
Projectile Motion and Gravity
• Projectile motion is the curved path that an object
follows when thrown, launched, or otherwise
projected near the surface of Earth.
• Projectile motion is made of two different motions,
or movements: horizontal movement and vertical
movement. When these two movements are put
together, they form a curved path.
Section 2 Gravity and Motion
Chapter
13
Projectile Motion and Gravity,
continued
• Horizontal movement is movement parallel
to the ground.
• Gravity does not affect the horizontal
movement of projectile motion.
Section 2 Gravity and Motion
Chapter
13
Projectile Motion and Gravity,
continued
• Vertical movement is movement
perpendicular to the ground.
• Gravity affects the vertical movement of an
object in projectile motion by pulling the
object down at an acceleration of 9.8 m/s2
(if air resistance is ignored).
Chapter 13
Forces and Motion
Chapter 13
Section 2 Gravity and Motion
Orbiting and Gravity
• An object is orbiting when it is moving
around another object in space.
• The two movements that come together to
form an orbit are similar to the horizontal
and vertical movements in projectile
motion.
Chapter 13
Forces and Motion
Chapter 13
Section 2 Gravity and Motion
Orbiting and Gravity, continued
• The path of an orbiting object is not quite a
circle. Instead, the path is an ellipse.
• Centripetal force is the unbalanced force that
makes objects move in an elliptical path.
• Gravity provides the centripetal force that
keeps objects in orbit.
Chapter 13
Section 2 Gravity and Motion
Orbiting and Gravity, continued
• Gravity helps maintain the shape of the solar
system by keeping large objects such as the
planets in their orbit around the sun.
• Gravity also affects the movement of very
small objects in the solar system, such as the
tiny particles that make up the rings of Saturn.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s First Law
• Newton’s first law of motion states that the
motion of an object will not change if the
forces on it are balanced.
• Newton’s first law of motion describes the
motion of an object that has a net force of
0 N acting on it.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s First Law, continued
• An object that is not moving is said to be at
rest. Objects at rest will not move unless
acted upon by an unbalanced force.
• Objects in motion will continue to move at
a constant speed and in a straight line
unless acted upon by an unbalanced force.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s First Law, continued
• Friction is an unbalanced force that changes
the motion of objects.
• Because of friction, observing Newton’s first
law is often difficult.
• Newton’s first law of motion is sometimes
called the law of inertia.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s First Law, continued
• Inertia is the tendency of an object to resist
being moved or, if the object is moving, to
resist a change in speed or direction until an
outside force acts upon the object.
• Mass is a measure of inertia. An object that
has a small mass has less inertia than an
object that has a large mass.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s Second Law of Motion
• states that the acceleration of an object
depends on the mass of the object and the
amount of force applied.
• describes the motion of an object when an
unbalanced force acts on the object.
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s Second Law of Motion,
continued
• The greater the mass of an object is, the greater the
force needed to achieve the same acceleration.
• The acceleration of an object is always in the same
direction as the net force applied.
• An object’s acceleration increases as the force on
the object increases.
Chapter 13
Forces and Motion
Chapter 13
Section 3 Newton's Laws of Motion
Newton’s Third Law of Motion
• Newton’s third law of motion states that
whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object.
• All forces act in pairs. When a force is
exerted, there is always a reaction force.
Chapter
13
Newton’s
Section 3 Newton's Laws of Motion
Third Law of Motion,
continued
• Action and reaction force pairs are present
even when there is no movement.
• A force is always applied by one object on
another object. However, action and
reaction forces in a pair do not act on the
same object.
Vocabulary
• Gravity- the force of attraction between objects;
unbalanced force
• Mass- a measure that does not change when an
object’s location changes; a measure of the amount of
matter.
• Weight- the measure of gravitational force exerted on
an object; expressed in the SI unit of force, the newton
(N).
• Static- nonmoving, or, objects.
• law of universal gravitation- gravitational force is
related to mass and distance.
Vocabulary (continued)
• terminal velocity-This is the constant velocity of a
falling object when the force of gravity is
balanced by the force of air resistance.
• Free fall-motion of a body when only the force of
gravity is acting on the body
– (ONLY IN A VACUUM)
• Inertia- All objects tend to resist any change in
motion.
• Orbital motion- is a combination of forward
motion and free fall.
• gravitational pull
– is greater between two objects that have greater
masses.
• If a student has a weight of 420 N on Earth,
what is the student’s weight on the moon?
(Moon’s gravity = 1/6 of Earth’s gravity)
• 420 x 1/6= 70N
• WEIGHT X GRAVITY= N
• examples of projectile motion
– the path of a leaping frog
– the path of an arrow through the air
– the path of a pitched baseball
• If a tennis ball, a solid rubber ball, and a solid
steel ball were dropped at the same time from
the same height, which would hit the ground
first? (Assume there is no air resistance.)
– ALL DROP AT SAME TIME
• Which would hit the ground first?
– A crumpled piece of paper
– flat sheet of paper
• crumpled because there is more air resistance against the flat paper.
• A 5 kg object has less inertia than an object with a mass of 6 kg.
(TRUE)
• According to Newton’s first law of motion, a moving object that is
not acted on by an unbalanced force will remain in motion. (TRUE)
• action/reaction force pair: the forces between a bat and ball
• Why does a ball thrown horizontally follow a path that is curved
downward?
– accelerated by gravity in the vertical direction only.
Newton’s Laws of Motion
• Newton’s 1st law– 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 2nd
– The acceleration of an object depends on the mass of the
object and the amount of force applied.
• Newton’s 3rd law
– All forces act in pairs
– One object exerts a force on a second, the 2nd exerts an
equal/opposite force on the first
• KNOW EXAMPLES OF ALL OF THESE!!!
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Path
Orbit
Curve
Free fall