Chapter 12 Forces and Motion
Section 1: Forces
What is a Force?
•  A force is a push or pull on an object
•  A force can cause a resting object to move, or it
can accelerate a moving object by changing the
object’s speed or direction
Measuring Force
•  Force is measured in newtons, abbreviated N.
•  One newton is the force that causes a 1-kg mass
to accelerate at a rate of 1 meter per second each
second.
•  1 N = 1 kg m/s2
Representing Force
•  You can use arrows to represent the strength and
direction of a force.
Combining Forces
•  You can combine forces.
•  Forces in the same direction add together, and
forces in opposite directions subtract from one
another.
•  The net force is the overall force acting on an
object after all the forces are combined.
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. Ex. Tug of War
Unbalanced Forces
•  An unbalanced force results when the net force
acting on an object is not equal to zero.
•  When an unbalanced force acts on an object,
that object accelerates.
Friction
•  Friction is a force that opposes the motion of
objects that touch as they move past one
another.
•  Without friction, our world would be crazy!
•  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.
•  Acts in the direction opposite to the applied
force.
Sliding Friction
•  A force that opposes the direction of motion of
an object as it slides over a surface.
•  This force is less than static friction.
Rolling Friction
•  Friction force that acts on rolling objects
•  Ex. Ball bearings
Fluid Friction
•  The force of fluid friction opposes the motion of
objects through a fluid.
•  Ex. Submarines
•  Air resistance- fluid friction acting on an
object moving through the air.
Gravity
•  Gravity is an attractive force that acts between
any two masses.
•  Earth’s gravity acts downward toward the center
of the Earth.
•  Gravity causes objects to accelerate downward,
whereas air resistance reduces acceleration.
Projectile Motion
•  When an object takes a curved path after it is
initially given a forward velocity.
Newton’s First Law of Motion
•  The state of the object does not change as long as
the net force acting on it is zero.
•  Also called “Law of Inertia”
•  Inertia- the tendency of an object to resist
change
•  An object at rest remains at rest & an object in
motion remains in motion.
Newton’s Second Law of Motion
•  Objects will accelerate when the net force is
unbalanced
•  Object will accelerate in the direction of the net
force
•  F = ma
•  Force- N
•  Mass- kg
•  Acceleration- m/s/s
Practice
•  A car has a mass of 1,000 kg and accelerates at 4
m/s/s. What is the force acting on the car?
Are Weight and Mass the Same Thing?
•  They are NOT the same thing!
•  Weight- force of gravity acting on an object
•  Weight = mass x acceleration due to gravity
•  W=mg
•  Gravity on Earth= 9.8 m/s2
•  Mass is a measure of the inertia of an object
•  Weight is measure of the force acting on that
object (in N)
▫  We measure in pounds (lbs)
Practice
•  If an astronaut has a mass of 112 kg, what is his
weight on Earth?
•  What if the same astronaut were on the moon
where gravity is 1.6 m/s/s?
Newton’s Third Law of Motion