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Chapter 12
Forces and Motion
Section 1
Forces
Key Concepts
• How do forces affect the motion of an
object?
• What are the four main types of friction?
• How do gravity and air resistance affect a
falling object?
• In what direction does Earth’s gravity act?
• Why does a projectile follow a curved
path?
What Is a Force?
• A force can cause a resting object to
move, or it can accelerate a moving
object by changing the object’s speed
or direction.
• A push or a pull
Measuring Force
• using a spring scale
The downward force arrows
represent the weight (a type of force)
on the scales. The dial indicator
gives a visual measure of the weight.
You can use an arrow to represent
the direction and strength of a force.
The direction of the arrow
represents the direction of the force.
The length of the arrow represents
the strength, or magnitude, of
the force.
Units of Force
• Force is measured in newtons (N)
• One newton is the force that causes a 1kilogram mass to accelerate at a rate of 1
meter per second each second (1 m/s2).
Combining Forces
• Forces in the same direction add together.
• Forces in opposite directions subtract from
one another.
Adding forces
+
=
Subtracting forces
-
=
Balanced Forces
• When the forces on an object are
balanced, the net force is zero and
there is no change in the object’s
motion.
3 In this tug of war, the
two groups pull with
equal forces in
opposite directions.
The forces combine by
subtracting from each
other
+
= 0
Unbalanced Forces
• When an unbalanced force acts on an
object, the object accelerates.
• Often, the forces on an object are
unbalanced.
• An unbalanced force is a force that results
when the net force acting on an object is
not equal to zero.
Adding forces
+
=
Subtracting forces
-
=
Friction
• Friction, a force that opposes the motion
of objects that touch as they move past
each other
• There are four main types of friction:
static friction, sliding friction, rolling
friction, and fluid friction.
Static Friction
• The friction force that acts on objects that
are not moving.
• Static friction always acts in the direction
opposite to that of the applied force.
• You experience static friction every time
you take a step. As you push off with each
step, static friction between the ground
and your shoe keeps your shoe from
sliding.
Static friction acts opposite the
Static
friction
direction of the force you apply to
move the
Push
plant.
Sliding Friction
• A force that opposes the direction of
motion of an object as it slides over a
surface.
• Because sliding friction is less than static
friction, less force is needed to keep an
object moving than to start it moving.
Potted tree
accelerates.
Sliding
friction
Push
When you push with more force, the
potted tree begins to slide.
Rolling Friction
• The friction force that acts on rolling
objects.
• For a given set of materials, the force of
rolling friction is about 100 to 1000 times
less than the force of static or sliding
friction.
Ball bearings
in these wheels greatly
reduce friction by
replacing sliding friction
with rolling friction.
Fluid Friction
• Force that opposes the motion of an object
through a fluid.
• Water and a mixture of gases such as air are
known as fluids.
• You feel fluid friction when stirring thick cake
batter.
• Fluid friction acting on an object moving through
the air is known as air resistance.
• At higher speeds, air resistance can become a
significant force.
Gravity
• A force that acts between any two masses.
• Gravity is an attractive force, that is, it pulls
objects together.
• gravity does not require objects to be in contact
for it to act on them
• Earth’s gravity acts downward toward the
center of Earth.
• an upward force usually balances the downward
force of gravity
Gravity
Supporting force
Falling Objects
• Gravity causes objects
to accelerate
downward, whereas air
resistance acts in the
direction opposite to
the motion and reduces
acceleration.
• Terminal velocity is the
constant velocity of a
falling object when the
force of air resistance
equals the force of
gravity.
Gravity
Air resistance
This flying
squirrel takes
advantage of air
resistance to slow its
fall and increase the
distance covered in
the jump.
Projectile Motion
• When you throw a ball forward,
you’ll notice that it actually
follows a curved path.
• projectile motion, the motion
of a falling object (projectile)
after it is given an initial
forward velocity.
• Air resistance and gravity are
the only forces acting on a
projectile.
• The combination of an initial Although their
forward velocity and the
masses
downward vertical force of are different, the
gravity causes the ball to
blue and green
follow a curved path.
balls fall at the
same rate.
The yellow ball is a
projectile, following a
curved path. The two
balls fall with the
same acceleration and
strike the ground at
the same time.
Reviewing Concepts
• 1. How is the motion of an object affected
when a force acts on it?
• 2. List the four types of friction.
• 3. How does air resistance affect the
acceleration of a falling object?
• 4. Earth’s gravitational force acts in what
direction?
• 5. Describe why a projectile follows a
curved path.