Chapter 4
The Laws of Motion
Classical Mechanics
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Describes the relationship between the
motion of objects in our everyday world
and the forces acting on them
Conditions when Classical Mechanics
does not apply
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very tiny objects (< atomic sizes)
objects moving near the speed of light
Forces
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Usually think of a force as a push or
pull
Vector quantity
May be a contact force or a field
force
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Contact forces result from physical contact
between two objects
Field forces act between disconnected
objects
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Also called “action at a distance”
Contact and Field Forces
Fundamental Forces
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Types
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Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravity
Characteristics
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All field forces
Listed in order of decreasing strength
Only gravity and electromagnetic in
mechanics
External and Internal
Forces
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External force
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Any force that results from the
interaction between the object and its
environment
Internal forces
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Forces that originate within the object
itself
They cannot change the object’s
velocity
Sir Isaac Newton
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1642 – 1727
Formulated basic
concepts and laws
of mechanics
Universal
Gravitation
Calculus
Light and optics
Newton’s First Law

An object moves with a velocity
that is constant in magnitude and
direction, unless acted on by a
nonzero net force

The net force is defined as the vector
sum of all the external forces exerted
on the object
Galileo
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Used ramps to investigate motion.
Described an objects motion in
terms of inertia.
Inertia
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Is the tendency of an object to
continue in its original motion
Mass
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A measure of the resistance of an
object to changes in its motion due
to a force
Scalar quantity
SI units are kg
Newton’s Second Law
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The acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to its mass.
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F and a are both vectors
Can also be applied three-dimensionally
Units of Force

SI unit of force is a Newton (N)
1N  1
kg m
s

2
US Customary unit of force is a
pound (lb)

1 N = 0.225 lb
Gravitational Force
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Mutual force of attraction between
any two objects
Expressed by Newton’s Law of
Universal Gravitation:
Fg  G
m1 m2
r
2
Weight
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The magnitude of the gravitational
force acting on an object of mass
m near the Earth’s surface is called
the weight w of the object
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w = m g is a special case of Newton’s
Second Law
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g is the acceleration due to gravity
g can also be found from the Law
of Universal Gravitation
More about weight
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Weight is not an inherent property
of an object
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mass is an inherent property
Weight depends upon location
Newton’s Third Law
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If object 1 and object 2 interact,
the force exerted by object 1 on
object 2 is equal in magnitude but
opposite in direction to the force
exerted by object 2 on object 1.
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
F1 2   F2 1
Equivalent to saying a single isolated
force cannot exist
Newton’s Third Law cont.
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F12 may be called the
action force and F21
the reaction force
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Actually, either force
can be the action or
the reaction force
The action and
reaction forces act
on different objects
Some Action-Reaction
Pairs
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n and n '
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is the normal force,
the force the table
exerts on the TV
n is always
perpendicular to the
surface
n ' is the reaction – the
TV on the table
n
n  n '
More Action-Reaction pairs
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'
Fg an d Fg
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Fg is the force the
Earth exerts on
the object
'
F
 g is the force the
object exerts on
the earth
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'
Fg   Fg
Forces Acting on an Object
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Newton’s Law
uses the forces
acting on an
object
n an d Fg are
acting on the
object
'
n ' an d Fgare
acting on other
objects
Applications of Newton’s
Laws
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Assumptions
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Objects behave as particles
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can ignore rotational motion (for now)
Masses of strings or ropes are
negligible
Interested only in the forces acting
on the object
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can neglect reaction forces
Free Body Diagram
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Must identify all the forces acting
on the object of interest
Choose an appropriate coordinate
system
If the free body diagram is
incorrect, the solution will likely be
incorrect
Free Body Diagram,
Example
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The force is the
tension acting on the
box
The tension is the same
at all points along the
rope
n an d Fg are the
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forces exerted by the
earth and the ground
Free Body Diagram, final
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Only forces acting directly on the
object are included in the free
body diagram
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Reaction forces act on other objects
and so are not included
The reaction forces do not directly
influence the object’s motion
Solving Newton’s Second
Law Problems
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Read the problem at least once
Draw a picture of the system
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Identify the object of primary interest
Indicate forces with arrows
Label each force
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Use labels that bring to mind the
physical quantity involved
Solving Newton’s Second
Law Problems
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Draw a free body diagram
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Apply Newton’s Second Law
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If additional objects are involved, draw
separate free body diagrams for each object
Choose a convenient coordinate system for
each object
The x- and y-components should be taken
from the vector equation and written
separately
Solve for the unknown(s)
Example 1
A 10.0-kg object undergoes an acceleration
of 2.5 m/s2. (a) What is the magnitude of
the resultant force acting on it? (b) If this
same force is applied to a 5.0-kg object,
what acceleration is produced?
Example 2
A football punter accelerates a football
from rest to a speed of 10 m/s during
the time in which his toe is in contact
with the ball (about 0.20 s). If the
football has a mass of 0.50 kg, what
average force does the punter exert on
the ball?
Example 3
Find the weight of a 20Kg object at
the surface of the Earth. What is
the weight of the object on the
Moon where the acceleration due to
gravity is 1/6th what it is on Earth.
Example 4
A freight train has a mass of 1.5 × 107
kg. If the locomotive can exert a
constant pull of 7.5 × 105 N, how long
does it take to increase the speed of
the train from rest to 80 km/h?
Example 5
A 5.0-g bullet leaves the muzzle of a
rifle with a speed of 320 m/s. What
force (assumed constant) is exerted
on the bullet while it is traveling
down the 0.82-m-long barrel of the
rifle?
Example 6
After falling from rest from a height
of 30 m, a 0.50-kg ball rebounds
upward, reaching a height of 20 m.
If the contact between ball and
ground lasted 2.0 ms, what average
force was exerted on the ball?
Equilibrium
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An object either at rest or moving
with a constant velocity is said to
be in equilibrium
The net force acting on the object
is zero (since the acceleration is
zero)
F  0
Equilibrium cont.
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Easier to work with the equation in
terms of its components:
F
x

 0 and
F
y
0
This could be extended to three
dimensions
Equilibrium Example –
Free Body Diagrams
Example 7
A 150-N bird feeder is supported by
three cables as shown in Figure P4.17.
Find the tension in each cable.
Figure P4.17
Example 8
The leg and cast in Figure P4.18 weigh 220 N (w1).
Determine the weight w2 and the angle α needed so that no
force is exerted on the hip joint by the leg plus the cast.
Figure P4.18
Example 9
Two people are pulling a boat through the water as in
Figure P4.20. Each exerts a force of 600 N directed at a
30.0° angle relative to the forward motion of the boat. If
the boat moves with constant velocity, find the resistive
force exerted by the water on the boat.
Example 10
The distance between two telephone
poles is 50.0 m. When a 1.00-kg bird
lands on the telephone wire midway
between the poles, the wire sags 0.200
m. Draw a free-body diagram of the
bird. How much tension does the bird
produce in the wire? Ignore the weight
of the wire.
Multiple Objects –
Example
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When you have more than one
object, the problem-solving
strategy is applied to each object
Draw free body diagrams for each
object
Apply Newton’s Laws to each
object
Solve the equations
Multiple Objects –
Example, cont.
Example 11
(a) An elevator of mass m moving upward has two forces acting on
it: the upward force of tension in the cable and the downward force
due to gravity. When the elevator is accelerating upward, which is
greater, T or w? (b) When the elevator is moving at a constant
velocity upward, which is greater, T or w? (c) When the elevator is
moving upward, but the acceleration is downward, which is greater,
T or w? (d) Let the elevator have a mass of 1 500 kg and an upward
acceleration of 2.5 m/s2. Find T. Is your answer consistent with the
answer to part (a)? (e) The elevator of part (d) now moves with a
constant upward velocity of 10 m/s. Find T. Is your answer
consistent with your answer to part (b)? (f) Having initially moved
upward with a constant velocity, the elevator begins to accelerate
downward at 1.50 m/s2. Find T. Is your answer consistent with your
answer to part (c)?
Example 12
A 1000-kg car is pulling a 300-kg trailer.
Together, the car and trailer have an
acceleration of 2.15 m/s2 in the forward
direction. Neglecting frictional forces on the
trailer, determine (a) the net force on the car,
(b) the net force on the trailer, (c) the force
exerted by the trailer on the car, and (d) the
resultant force exerted by the car on the road.
Forces of Friction
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When an object is in motion on a
surface or through a viscous
medium, there will be a resistance
to the motion
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This is due to the interactions
between the object and its
environment
This is resistance is called friction
More About Friction
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Friction is proportional to the normal
force
The force of static friction is generally
greater than the force of kinetic friction
The coefficient of friction (µ) depends
on the surfaces in contact
The direction of the frictional force is
opposite the direction of motion
The coefficients of friction are nearly
independent of the area of contact
Static Friction, ƒs
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Static friction acts
to keep the object
from moving
If F increases, so
does ƒs
If F decreases, so
does ƒs
ƒs  µ n
Kinetic Friction, ƒk
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The force of
kinetic friction
acts when the
object is in
motion
ƒk = µ n

Variations of the
coefficient with
speed will be
ignored
Example 13
A dockworker loading crates on a ship finds that a
20-kg crate, initially at rest on a horizontal surface,
requires a 75-N horizontal force to set it in motion.
However, after the crate is in motion, a horizontal
force of 60 N is required to keep it moving with a
constant speed. Find the coefficients of static and
kinetic friction between crate and floor.
Example 14
A hockey puck is hit on a frozen lake and starts
moving with a speed of 12.0 m/s. Five seconds
later, its speed is 6.00 m/s. (a) What is its average
acceleration? (b) What is the average value of the
coefficient of kinetic friction between puck and ice?
(c) How far does the puck travel during the 5.00-s
interval?
Example 15
Consider a large truck carrying a heavy load, such as steel
beams. A significant hazard for the driver is that the load may
slide forward, crushing the cab, if the truck stops suddenly in
an accident or even in braking. Assume, for example, that a
10 000-kg load sits on the flat bed of a 20 000-kg truck
moving at 12.0 m/s. Assume the load is not tied down to the
truck and has a coefficient of static friction of 0.500 with the
truck bed. (a) Calculate the minimum stopping distance for
which the load will not slide forward relative to the truck. (b)
Is any piece of data unnecessary for the solution?
Example 16
A car is traveling at 50.0 km/h on a flat
highway. (a) If the coefficient of friction
between road and tires on a rainy day is
0.100, what is the minimum distance in
which the car will stop? (b) What is the
stopping distance when the surface is dry
and the coefficient of friction is 0.600?
Inclined Planes
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Choose the
coordinate
system with x
along the incline
and y
perpendicular to
the incline
Replace the force
of gravity with its
components
Block on a Ramp, Example
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Axes are rotated as
usual on an incline
The direction of
impending motion
would be down the
plane
Friction acts up the
plane
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Opposes the motion
Apply Newton’s Laws
and solve equations
Example 17
A 3.00-kg block starts from rest at the top of a
30.0° incline and slides 2.00 m down the incline in
1.50 s. Find (a) the acceleration of the block, (b)
the coefficient of kinetic friction between the block
and the incline, (c) the frictional force acting on the
block, and (d) the speed of the block after it has
slid 2.00 m.
Example 18
A crate with mass m slides at a constant rate
down an incline, inclined at 30o with respect
to the horizontal what is the coefficient of
friction between the crate and the incline.
Connected
Objects
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Apply Newton’s Laws
separately to each
object
The magnitude of the
acceleration of both
objects will be the
same
The tension is the
same in each diagram
Solve the simultaneous
equations
More About Connected
Objects

Treating the system as one object
allows an alternative method or a
check

Use only external forces

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Not the tension – it’s internal
The mass is the mass of the system
Doesn’t tell you anything about
any internal forces
Example 19
(a) What is the minimum force of friction required to hold
the system of Figure P4.58 in equilibrium? (b) What
coefficient of static friction between the 100-N block and
the table ensures equilibrium? (c) If the coefficient of
kinetic friction between the 100-N block and the table is
0.250, what hanging weight should replace the 50.0-N
weight to allow the system to move at a constant speed
once it is set in motion?
Example 20
In Figure P4.30, m1 = 10 kg and m2 = 4.0 kg. The
coefficient of static friction between m1 and the horizontal
surface is 0.50, and the coefficient of kinetic friction is
0.30. (a) If the system is released from rest, what will its
acceleration be? (b) If the system is set in motion with m2
moving downward, what will be the acceleration of the
system?