Do you agree or disagree?
1. Objects must be in contact with one
another to exert a force.
2. Gravity is a force that depends on the
masses of the two objects and the
distance between them.
Forces
• How do different types of forces affect
objects?
• What factors affect the way gravity
acts on objects?
• How do balanced and unbalanced
forces differ?
Forces
• force
• gravity
• contact force
• balanced forces
• noncontact force
• unbalanced
forces
• friction
What are forces?
• A push or pull on an object is a force.
• A force has both size and direction.
• Arrows are used to show the size and
direction of the force.
• There are two ways a force can affect an
object:
– A force can change an object’s speed and
– the direction in which the object is moving.
Types of Forces
• A contact force
is a push or a pull
applied by one
object to another
object that is
touching it.
• Contact forces are
also called
mechanical forces.
Steve Cole/Getty Images
Types of Forces (cont.)
• A force that pushes or pulls an object
without touching it is a noncontact
force.
• Gravity is an
example of a
noncontact force.
Types of Forces (cont.)
What is the difference between
the way contact and noncontact
forces affect objects?
Friction
• Friction is a contact force that resists
the sliding motion between two objects
that are touching.
• The force of friction acts in the opposite
direction of the motion.
• Rougher surfaces produce greater
friction than smooth surfaces.
Gravity
• Gravity is a noncontact attractive force
that exists between all objects that have
mass.
• Gravitational force depends on the mass
of the objects and the distance between
them.
Gravity (cont.)
• Weight is a
measure of the
force of gravity
acting on an
object.
• As two objects get
farther apart, the
gravitational force
between the
objects decreases.
• As the mass
of one or both
objects increases,
the gravitational
force between
them increases.
• Doubling the
mass of one of
the objects doubles
the force of attraction.
Gravity (cont.)
What factors affect the way
gravity acts on objects?
Combining Forces
• When several forces act on an object,
the forces combine to act as a single
force.
• The sum of the forces acting on an
object is called the net force.
Combining Forces (cont.)
When different forces act on an object in
the same direction, you can find the net
force by adding the forces together.
Combining Forces (cont.)
When forces act in opposite directions,
you must include the direction of the force
when you add them.
Combining Forces (cont.)
When the net force on an object is 0 N, the
forces acting on it are balanced forces.
Combining Forces (cont.)
If the forces acting on an object are
balanced, the object’s motion does not
change.
• When the net force acting on an object is
not 0, the forces acting on the object are
unbalanced forces.
• Unbalanced forces cause objects to
change their motion, or accelerate.
Combining Forces (cont.)
How do balances and unbalanced
forces differ?
• Forces are pushes and pulls exerted
by objects on each other. Contact
forces occur when objects are
touching. Noncontact forces act from
a distance.
• Gravity is a force of
attraction between
two objects. The
amount of
gravitational force
depends on the
mass of the objects
and the distance
between them.
• Balanced forces do not affect motion.
Unbalanced forces change motion.
Which is a measure of the force
of gravity acting on an object?
A. friction
B. mass
C. unbalanced force
D. weight
Which term refers to the sum of
the forces acting on an object?
A. friction
B. net force
C. balanced forces
D. unbalanced forces
Which force pushes or pulls an
object without touching it?
A. balanced
B. contact
C. net
D. noncontact
Do you agree or disagree?
1. Objects must be in contact with one
another to exert a force.
2. Gravity is a force that depends on the
masses of the two objects and the
distance between them.