» 1. What is a force?
» 2. What does a force do?
» 3. What kinds of forces are
present in our everyday life?
Guided Discussion
Student notes are shown in blue.
» Any influence that can cause a change in an object’s
motion.
A bat strikes
the ball with a
force that
causes the ball
to stop and
then move in
the opposite
direction.
» A push or pull that one object exerts on another
» Has a size and direction
» Measured in newtons (N)
You have to apply a force of about 3 N
to lift a full can of soda.
When two or more forces act on an object at
the same time, the forces combine to form the
net force.
» The combination of all forces on an object
Balanced forces: no acceleration
In a tug of war, each side
exerts a force on the ground.
If the opposing forces on the
ground are equal, they are
balanced and the people do
not move.
Unbalanced forces: acceleration
If one of the forces on the
ground is greater than the
other, the forces are
unbalanced and the people
move in the direction of the
greater force.
» Do not change motion
» Cancel each other
» Combine to produce a net force of zero
» Do not have the same strength (size)
» Cause acceleration
» A force that opposes the sliding motion of two
surfaces
» Depends upon the materials the surfaces are
made from and the roughness of the surfaces
» Rule of Friction – Friction always acts in a direction
to oppose motion.
Examples:
If you push a solid block along the floor to the right, the force of
friction on the block will be to the left.
A boat propelled to the east by its motor experiencing water
friction to the west.
When an object falls downward through the air, the force of
friction (air drag or air resistance) acts upward.
PUSH
FRICTION
AIR RESISTANCE
WEIGHT
Unbalanced forces: acceleration
Balanced forces: constant speed
Balanced forces: no motion
A When a car is
accelerating, the forces
are unbalanced. The
force moving the car
forward is greater than
the opposing force of
motion.
B When a car is cruising
at constant speed, the
forces moving the car
forward is balanced by
the force of friction.
C This car does not
roll because the
force of friction
between the brakes
and the wheels
balances the force of
gravity.
» Static Friction – The frictional force that
prevents two surfaces in contact from
sliding past each other.
» Sliding Friction – The force that acts in
the opposite direction to the motion of a
surface sliding on another surface.
» A type of frictional force that opposes the motion of
objects that move through the air
» Causes objects to fall with different accelerations and
different speeds
» Acts in the direction opposite to the velocity of an
object moving in air
» Size of the force depends on the size and shape of
the object.
The flying squirrel increases its
area by spreading out. This
increases air resistance and
decreases the speed of its fall.
Air resistance, not an object’s mass, is why
feathers, leaves, and sheets of paper fall more
slowly than pennies, acorns, and apples.
As an object falls, the downward force of gravity causes
the object to accelerate. As the speed of the object
increases, the upward force of air resistance also
increases.
» When the upward air resistance force equals the
downward force of gravity, terminal velocity is
reached – the velocity becomes constant.
» Depends on the size, shape, and mass of the object
Terminal velocity is reached for the sky divers when air
resistance balances the downward force of gravity.