Ch. 4 Forces and Laws of
Motion
4-1 Changes in Motion
 Key
Terms pg. 150
 Force-push
or pull
 Force causes change in velocity
(acceleration)
 SI
unit of force is Newton (N)
 Weight is a measure of gravitational
force on an object
 Contact
forces-physical contact of
two objects
 Ex:
pull a string
pull a wagon
catch a football
 Field
Forces-force between two
objects without physical contact
 Ex:
gravity
electrical charges
Force Diagrams
 Magnitude
and direction of force
affects an object’s motion
 Force
is a vector
 Force diagrams show forces acting in
a situation
 Free-body diagrams show only forces
acting on an object
Force Diagram
 Figure
4-4 pg. 127
 4-1
Section Review Questions
4-2 Newton’s First Law
 An
object at rest remains at rest,
and an object in motion continues in
motion with constant velocity(that is,
constant speed in a straight line)
unless the object experiences a net
external force.
 Began by Galileo in 1630s, then
further developed by Newton in
1687.
 Inertia-
the tendency of an object
not to accelerate
 When
the net external force on an
object is zero, its acceleration (or
change in its velocity) is zero.
 Acceleration
is determined by net
external force
 Net
external force is the vector sum
of all the forces acting on an object.
 An
object’s acceleration is the same
as the net external force.
 Mass
is a measurement of inertia
 Ex: basketball vs golf ball
 Golf
ball will have larger acceleration
due to less inertia
 Objects
in motion tend to stay in
motion
 Ex: crash test dummy/seat belt
 Fig. 4-14
Equilibrium
 Objects
at rest or moving with
constant velocity are at equilibrium
 The
net external force acting on a
body in equilibrium must be equal to
zero.
 Resolve
all vectors into their x and y
components.
 When
all the x vectors equal 0 and
all the y vectors equal 0, then the
vector sum is zero and the body is in
equilibrium.
 Sample
Problem 4A in book pg. 132
 Another
Sample Problem 4A:Sample
Problem 4A.docx
 You
have to find the net Force
(hypotenuse force) and angle.
4-3 Newton’s 2nd and 3rd Laws
 Acceleration
of an object is directly
proportional to the net external force
acting on it.
 Less
force is needed to accelerate a
low mass object than a high mass
object at the same rate.
In other words…
A
low mass object accelerates faster
than a heavy object if the same force
is applied.
Newton’s Second Law
 The
acceleration of an object is
directly proportional to the net
external force acting on the object
and inversely proportional to the
object’s mass.
 ∑F
= ma
Sample 4B

Sample Problem 4B-Newton’s 2nd Law

Fnet = ΣF = ma

Roberto and Laura are studying across from
each other at a wide table. Laura slides a 2.2 kg
book toward Roberto. If the net external force
acting on the book is 2.6 N to the right, what is
the book’s acceleration?
4-3 continued…
 Forces
 Ex:
always exist in pairs
kicking a ball-the ball exerts a
force on you and you exert a force
on the ball
Newton’s 3rd Law
 If
two objects interact, the
magnitude of the force exerted on
object 1 by object 2 is equal to the
magnitude of the force
simultaneously exerted on object 2
by object 1, and these two forces are
opposite in direction.
 For
every action, there is an equal
and opposite reaction.
 Action-reaction
pair-describe the
forces between 2 objects at the same
time
 Action-reaction pairs do not result in
equilibrium fig. 4-18
 Field
forces also exist in actionreaction pairs
 Ex:
the force of Earth (gravity) on a
person and the force of a person on
Earth
4-4 Everyday Forces
 Fg
= Force of gravity is a vector
quantity toward the center of Earth
 Weight = a scalar quantity, the
magnitude of Force of gravity
 Fg = mg
 Weight depends on location (g)
 Objects weigh less at higher altitudes
because g decreases
Normal Force
 Normal
Force = Fn
 The force perpendicular to the
surface of contact
 Not always opposite of gravity
 In the absence of other forces, the
normal force is equal and opposite to
the force of gravity that is
perpendicular to the contact surface
 Fn
= mg(cosθ)
 The
angle is the angle between the normal
force and a vertical line and also the angle
between the contact surface and a
horizontal line. See fig. 4-20
Force of Friction
 Friction
opposes the applied force
 The resistive force that keeps an
object from moving is the static
friction (Fs)
 Fs = -Fapplied as long as the object
does not move
 Fs, max = when the force is as great as
it can be without moving the object.
Kinetic Friction
 Fapplied
> Fs, max =the object moves
but there is still friction
 Fk (kinetic friction) = the retarding
force on an object in motion
 The net external force is equal to the
difference between applied force and
kinetic friction force
 (Fnet = Fapplied – Fk)
 Friction
arises from interactions at
microscopic levels
 The force required to move a
stationary object is greater than the
force to keep an object moving.
 Fs, max > Fk
 Magnitude
of the force of friction is
proportional to the normal force
exerted on an object by a surface
(weight).
 The
force of friction depends on the
composition and quality of the
surfaces in contact. Ex: carpet vs.
tile
μk = Fk
Fn
μs = Fs, max
Fn

Ff = μFn
Coefficient of kinetic friction is always less than or
equal to the coefficient of static friction
 Air
resistance is a form of friction
 Air resistance = FR
 When
air resistance equals Fg
(gravity), an object has terminal
speed.

Sample Problem 4C-Coefficients of friction
A 24 kg crate initially at rest on a
horizontal floor requires a 75 N horizontal
force to set it in motion. Find the
coefficient of static friction between the
crate and the floor.
 μs = Fs, max = Fs, max
Fn
mg
