Forces and the Laws of
Motion
Chapter 4
Section 1
Changes in Motion : FORCE
 Force is the cause of an acceleration or the change in an
object’s velocity
 Force can cause objects to
 Start Moving
 Stop Moving
 Change Direction
 Notice that each of these cases involves a change in
velocity (this is acceleration).
Changes in Motion: FORCE
The SI unit of Force is the Newton named after Sir
Isaac Newton.
 The symbol for Newton is N.
 Force is measured in
Newton's.
 Force is a Vector Quantity.
 Force is equal to Mass times
Acceleration.
 1 N is equal to 1 kg x 1 m/s2
Forces can act through contact or at a
distance.
 Contact Force – force from the
physical contact of two objects
 Ex: catch a ball, its motion stops
Pull on a spring, it stretches
 Field Force – does not involve
physical contact between two
objects.
 Ex: the force of gravity
 Electrical charges
attraction/repulsion
Free-body Diagrams
 Free – body diagrams
isolate one object and the
forces acting on it.
 Shows only the forces
acting on the object of
interest.
 All forces are assumed to
act on a single point at the
center of the object.
Section Review pg. 128
 Work on problems 1-6
Newton’s First Law
Chapter 4 Section 4-2
Newton’s First Law of Motion
 Also referred to as the Law of
Inertia.
 Inertia is the tendency of an object to
maintain its state of motion.
 Mass is a measurement of inertia.
 States that an object at rest stays at rest
and an object in motion stays in motion
with constant velocity (constant speed
in a straight line) unless the object
experiences a net external force.
 When the net external force of an
object is zero, its acceleration is zero.
Acceleration is determined by net
external force.
 The Net External Force is the vector
sum of all the forces acting on an
object.
 We find X components and Y
components.
 Net External Force is the Resultant
Force.
 Objects that are at rest or moving
with constant velocity are said to be
in equilibrium.
 Equilibrium means there is no change
in a body’s motion.
Let’s Practice…
 Net External Force – Practice 4A
 Pg. 133
Chapter 4 Section 3
Newton’s Second and Third Laws
Newton’s 2nd Law relates force, mass,
and acceleration.
 Net External Force = Mass
 States : Acceleration is
produced when a Force
acts on a Mass. The greater
the mass of the object you
are trying to accelerate, the
greater the amount of force
needed to do so.
X Acceleration
 In equation form we
have :
∑F = ma
 ∑F represents the
vector sum of all
external forces acting on
an object.
Overview of Net External Force
Overview of Net External Force
Check for Understanding
 What acceleration will result when a 12-N net force is
applied to a 3-kg object?
 Answer:
 ∑F = ma → 12N = 3kg x a
 12N / 3kg = a
 4 m/s2 = a
Try this one on your own…
 A net force of 16 N causes a mass to accelerate at the rate of
5 m/s2. Determine the mass.
 Answer:
 ∑F = ma → 16N = m x 5 m/s2
 16N / 5m/s2 = m
 3.2 kg = m
Sample 4B
 Pg. 137
Newton’s Third Law
 Forces always exist in pairs.
 For every action, there is
an equal and opposite
reaction.
 Action =Ghost’s hand’s
force exerted on wall.
 Reaction = Wall’s force
exerted on Ghost’s hand.
Check for Understanding…
 If a book is on a table what
are the action and reaction
forces involved?
 Answer:
 Action : Force the book
exerts on table.
 Reaction : Force the table
exerts on the book.
Newton’s 3rd Law Examples