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Chapter 12: Forces & Motion
12.1 (pg’s 356-362) Forces

Force: can cause a resting object to move, or it can accelerate a
moving object by changing the object’s speed or direction.

Force is measured in newtons (N). 1 N is the force that cause
a 1-kg mass to accelerate at a rate of 1 meter per second each
second (1N = 1kg x m/s2 )

A Newton (N) a unit of force, explains how force, mass, and
acceleration are related.

Representing Force: an arrow to represent the direction and
strength of force.

Combining Forces:
o Net Force: the overall force acting on an object after all
the forces are combined.

Forces in the same direction add together

Forces in opposite directions subtract from one
another.

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.

Unbalanced Forces: an unbalanced force acts on an object, the
object accelerates.

Friction: a force that opposes the motion of objects that touch
as they move past each other.
o Static Friction: force that acts on objects that are not
moving.
o Sliding Friction: force that opposes the direction of
motion of an object as it slides over a surface.
o Rolling Friction: force that act on rolling objects.
o Fluid Friction: force that opposes the motion of an
object through a fluid.

Gravity: is a force that acts between any 2 masses.
o Gravity is an attractive force that pulls object together.
o Earth’s gravitational force exerts a force of attraction on
every other object that is near Earth.
o The force of gravity does not require objects to be in
contact for it to act on them.
o Earth’s gravity act downward toward the center of
Earth.
o An upward force or supporting force balances the
fownward force of gravity.

Falling Objects: Gravity causes objects to accelerate
downward, where as air resistance acts in the direction opposite
to the motion and reduces acceleration.
12.2 (pg’s 363-371) Newton’s First & Second Laws of Motion:

1st Law: the state of motion of an object does not change as
long as the net force acting on the object is zero.
o Sometimes called the Law of Inertia: the tendency of
an object to resist a change in its motion.
o An object at rest tends to remain at rest, and an object
in motion tends to remain in motion with the same
direction and speed.

2nd Law: The acceleration of an object is equal to the force
acting on it divided by the object’s mass.
Acceleration =
Mass =
Force
Mass
Force
Acceleration
Force = Mass x Acceleration

Doubling the mass of an object cuts its acceleration in half.

The acceleraion of an object is always in the same direction as
the net force.

Mass & Weight:
o Mass is a measure of the inertia of an object, how much
“stuff” something is made up of.
o Weight is a measure of the force of gravity acting on an
object.
Weight = Mass x Acceleration due to gravity
12.3 (pgs. 372-377) Newton’s Third Law of Motion &
Momentum

Newton’s 3rd Law: whenever on object exerts a force on a
second object, the second object exerts an equal and opposite
force on the first object. (action and reaction forces)

Momentum: is the product of an object’s mass and its
velocity.

An object has a large momentum if the product of its mass and
velocity is large.
Momentum = Mass x Velocity

Conservation of Momentum: if not net force acts on a
system, then the total momentum of the system does not
change.

Closed system: other objects and forces cannot enter or leave a
system.

In a close system, the loss of momentum of one object equals
the gain in momentum of another object, momentum is
conserved.
12.4 (pgs. 378-382) Universal Forces:

Electromagnetic Forces: are associated with charged particles

Nuclear Forces: the strong nuclear force and the weak nuclear
force, act within the nucleus to hold it together

Gravitational Force: every object in the universe attracts
every other object.
o It takes a huge mass such as Earth’s to exert a large
gravitational force.
o Gravity is the weakest universal force, but it is the most
effective over long distances.