# Forces

```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.
Forces are measured in Newtons.
One newton is equal to the amount of force needed to
accelerate a 1 kg mass 1 m/sec2.
In other words, a kilogram mass exerts about 9.8
newtons of force. To put that in perspective, about
half of one of Isaac Newton's apples would exert 1 N
of force.
Newtons are relatively small.
ewton-Definition.htm
When multiple forces act on the same object
at the same time, the forces are combined 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
» Friction is a force that stops objects from sliding
against each other.
» The amount of friction between two objects
depends on what the materials are made of.
» 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 acts
on stationary objects when trying to move
them.
» Sliding Friction – The frictional force that acts
on a moving object, causing it to slow down.
» Rolling Friction – The static frictional force
between the ground and the wheel as it rolls.
» Fluid Friction – The frictional force an object
experiences as it moves through a gas or
liquid.
» 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
increases air resistance and
decreases the speed of its fall.
It appears that a pieces of paper falls slower
than a book because it is smaller, but the real
reason is the piece of paper is effected by air
resistance.
Objects fall because gravity is pulling them down to the
ground. Because gravity accelerates objects, objects
will keep increasing in speed. The faster the objects
travel, the more they are effected by air resistance.
» When the upward air resistance force equals the
downward force of gravity, terminal velocity is
reached – the velocity becomes constant.
» Affects objects differently based on the objects size,
shape, and mass of the object
Terminal velocity is reached for the sky diver when air
resistance balances the downward force of gravity.
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