Chapter Three: Newton's Laws of Motion

1. Chapter 3
Newton’s Laws of Motion
(‫)قوانين الحركة لنيوتن‬

2. Sir Isaac Newton
1642 – 1727
 Formulated basic laws of
mechanics
(‫صاغ قوانين الميكانيكا‬
‫)الساسية‬
 Discovered Law of
Universal Gravitation
(‫)اكتشف قانون الجذب العام‬
 Invented form of calculus
 Many observations dealing
with light and optics


3. Force
 Forces
are what cause any change in the
velocity of an object
(‫)القوة هي اي شيء يغير من سرعة الجسم‬
Newton’s definition
 A force is that which causes an acceleration
(‫) القوة هي التي تسبب التسارع‬


4. Classes of Forces
(‫)اشكال القوى‬

Contact forces(‫ )القوى الغير مجالية‬involve
physical contact between two objects


Examples a, b, c
Field forces(‫ )قوى المجال‬act through empty
space


No physical contact is required
Examples d, e, f

5. Fundamental Forces
(‫) القوى الساسية في الطبيعة‬
 Gravitational

force (‫)قوة الجاذبية‬
Between objects
 Electromagnetic

Between electric charges
 Nuclear

force (‫)القوة النووية‬
Between subatomic particles
 Weak

forces (‫)القوى الكهرمغناطيسية‬
forces (‫)القوى الضعيفة‬
Arise in certain radioactive decay processes
 Note:
These are all field forces

6. More About Forces



A spring can be used to calibrate (‫ )معايرة‬the
magnitude of a force
Doubling the force causes double the reading on the
spring
When both forces are applied, the reading is three
times the initial reading

7. Vector Nature of Forces
(‫) الطبيعة المتجهة للقوى‬



The forces are applied
perpendicularly to each
other
The resultant (or net)
force (‫)القوة المحصلة‬is
the hypotenuse (‫)الوتر‬
Forces are vectors, so
you must use the rules
for vector addition to
find the net force acting
on an object

8. Newton’s First Law
(‫)قانون نيوتن الول‬
 If
an object does not interact with other
objects, it is possible to identify a reference
frame in which the object has zero
acceleration


This is also called the law of inertia (‫قانون القصور‬
‫)الذاتي‬
It defines a special set of reference frames called
inertial frames
We call this an inertial frame of reference
(‫) اطار المرجع للقصور الذاتي‬


9. Inertial Frames (‫اطارات القصور‬
‫)الذاتي‬


Any reference frame that moves with constant
velocity relative to an inertial frame is itself an
inertial frame
A reference frame that moves with constant velocity
relative to the distant stars is the best approximation
of an inertial frame

We can consider the Earth to be such an inertial frame,
although it has a small centripetal acceleration associated
with its motion

10. Newton’s First Law –
Alternative Statement
(‫) صيغة اخرى لقانون نيوتن الول‬

In the absence of external forces, when viewed from
an inertial reference frame, an object at rest remains
at rest and an object in motion continues in motion
with a constant velocity


Newton’s First Law describes what happens in the absence
of a force
 Does not describe zero net force
Also tells us that when no force acts on an object, the
acceleration of the object is zero

11. More About Mass
is an inherent property (‫ )خاصية متاصلة‬of
an object
 Mass is independent of the object’s
surroundings
 Mass is independent of the method used to
measure it
 Mass is a scalar quantity
 The SI unit of mass is kg
 Mass

12. Mass vs. Weight
 Mass
and weight are two different quantities
 Weight is equal to the magnitude of the
gravitational force exerted on the object

Weight will vary with location
 Example:

wearth = 180 lb; wmoon ~ 30 lb

mearth = 2 kg; mmoon = 2 kg

13. Newton’s Second Law
(‫) قانون نيوتن الثاني‬

When viewed from an inertial reference frame, the
acceleration of an object is directly proportional to
the net force acting on it and inversely proportional
to its mass


Force is the cause of change in motion, as measured by
the acceleration
Algebraically,
r
aµ

r
∑F
m
r
r
→ ∑ F = ma
With a proportionality constant of 1 and speeds much lower
than the speed of light

14. More About Newton’s Second
Law
r
 ∑ F is the net force
 This is the vector sum of all the forces acting on
the object
 Newton’s
Second Law can be expressed in
terms of components:



ΣFx = m ax
ΣFy = m ay
ΣFz = m az

15. Units of Force (‫وحدات قياس‬
‫)القوة‬
 The

SI unit of force is the newton (N)
1 N = 1 kg·m / s2
 The
US Customary unit of force is a pound
(lb)

1
1 lb = 1 slug·ft / s2
N ~ ¼ lb

16. Gravitational Force (‫قوة‬
‫)الجاذبية‬
r
 The gravitational force, F , is the force that
g
the earth exerts on an object
 This force is directed toward the center of the
earth
 From Newton’s Second Law
r
r
 F = mg
g
 Its magnitude is called the weight of the
object

Weight = Fg= mg

17. More About Weight
 Because
it is dependent on g, the weight
varies with location


g, and therefore the weight, is less at higher
altitudes
This can be extended to other planets, but the
value of g varies from planet to planet, so the
object’s weight will vary from planet to planet
 Weight
object
is not an inherent property of the

18. Newton’s Third Law
(‫) قانون نيوتن الثالث‬
r
 If two objects interact, the force F exerted by
12
object 1 on object 2 is equal in magnitude
r
and opposite in direction to the force F21
exerted by object 2 on object 1
r
r
F12 = −F21


r
Note on notation: FAB is the force exerted by A on
B

19. Newton’s Third Law,
Alternative Statements
(‫)قانون نيوتن الثالث‬



Forces always occur in pairs(‫القوى عادتا تحدث بشكل‬
‫)ازواج‬
A single isolated force cannot exist
The action force is equal in magnitude to the
reaction force and opposite in direction



One of the forces is the action force, the other is the
reaction force
It doesn’t matter which is considered the action and which
the reaction
The action and reaction forces must act on different objects
and be of the same type

20. Action-Reaction Examples, 1
(‫)الفعل و رد الفعل‬

r
The force F12 exerted
by object 1 on object 2
is equal in magnitude
and opposite in
r
direction to F21 exerted
by object 2 on object 1

r
r
F12 = −F21

21. Action-Reaction Examples, 2

The normal force (table on
monitor) is the reaction of the
force the monitor exerts on
the table


Normal means perpendicular,
in this case
The action (Earth on monitor)
force is equal in magnitude
and opposite in direction to
the reaction force, the force
the monitor exerts on the
Earth

22. Free Body Diagram
(‫) المخطط البياني للجسم الحر‬

In a free body diagram,
you want the forces
acting on a particular
object


Model the object as a
particle
The normal force and
the force of gravity are
the forces that act on
the monitor

23. Free Body Diagram, cont.
 The
most important step in solving problems
involving Newton’s Laws is to draw the free
body diagram
 Be sure to include only the forces acting on
the object of interest
 Include any field forces acting on the object
 Do not assume the normal force equals the
weight

24. Applications of Newton’s Laws
(‫) تطبيقات على قوانين نيوتن‬

Assumptions (‫)الفتراضات‬
 Objects can be modeled as particles
 Interested only in the external forces acting on the object
 can neglect reaction forces
 Initially dealing with frictionless surfaces( ‫اسطح عديمة‬
‫)الحتكاك‬
 Masses of strings or ropes are negligible
 The force the rope exerts is away from the object and
parallel to the rope
 When a rope attached to an object is pulling it, the
magnitude of that force is the tension in the rope

25. Particles in Equilibrium
(‫) الجسيمات في حالة التزان‬
 If
the acceleration of an object that can be
modeled as a particle is zero, the object is
said to be in equilibrium

The model is the particle in equilibrium model
 Mathematically,
the net force acting on the
object is zero

∑F=0
∑F
x
= 0 and
∑F
y
=0

26. Equilibrium, Example 1a


A lamp is suspended from a
chain of negligible mass
The forces acting on the
lamp are



the downward force of gravity
the upward tension in the chain
Applying equilibrium gives
∑F
y
= 0 → T − Fg = 0 → T = Fg

27. Equilibrium, Example 1b



T and Fg

Not an action-reaction pair
Both on the lamp
act


 T and T '




T ' and T "



Action-reaction forces
Lamp on chain and chain on lamp

Action-reaction forces
Chain on ceiling and ceiling on
chain
Only the forces acting on the lamp
are included in the free body
diagram

28. Equilibrium, Example 2a


Example
Conceptualize the traffic
light



Assume cables don’t break
Nothing is moving
Categorize as an
equilibrium problem


No movement, so
acceleration is zero
Model as a particle in
equilibrium

29. Equilibrium, Example 2b

Analyze



Need two free-body
diagrams
Apply equilibrium
equation to the light
Apply equilibrium
equations to the knot

30. Equilibrium, Example 2 c
 Analyze,


cont.
Find T3 from applying equilibrium in the y-direction
to the light
Find T1 and T2 from applying equilibrium in the xand y-directions to the knot
 Finalize

Think about different situations and see if the
results are reasonable

31. Particles Under a Net Force
 If
an object that can be modeled as a particle
experiences an acceleration, there must be a
nonzero net force acting on it

Model is particle under a net force model
 Draw
a free-body diagram
 Apply Newton’s Second Law in component
form

32. Newton’s Second Law,
Example 1a

Forces acting on the
crate:



A tension, acting through
the rope, is the

magnitude of force T 
The gravitational force, Fg

The normal force, n ,
exerted by the floor

33. Newton’s Second Law,
Example 1b

Apply Newton’s Second Law in component form:
∑F
x
= T = max
∑F
y


= n − Fg = 0 → n = Fg
Solve for the unknown(s)
If the tension is constant, then a is constant and the
kinematic equations can be used to more fully
describe the motion of the crate

34. Note About the Normal Force


The normal force is not
always equal to the
gravitational force of the
object
For example, in this case
∑F
y

= n − Fg − F = 0
and n = Fg + F


n may also be less than Fg

35. Multiple Objects (‫اجسام‬
‫)متعددة‬
 When
two or more objects are connected or
in contact, Newton’s laws may be applied to
the system as a whole and/or to each
individual object
 Whichever you use to solve the problem, the
other approach can be used as a check

36. Multiple Objects,
Conceptualize




Observe the two
objects in contact
Note the force
Calculate the
acceleration
Reverse the direction of
the applied force and
repeat

37. Multiple Objects, Example 1




First treat the system as a
whole:
∑ Fx = msystemax
Apply Newton’s Laws to the
individual blocks
Solve for unknown(s)
Check: |P12| = |P21|

38. Forces of Friction (‫قوى‬
‫)الحتكاك‬
 When
an object is in motion on a surface
(‫ )سطح‬or through a viscous medium (‫وسط‬
‫ ,)لزج‬there will be a resistance to the motion

This is due to the interactions (‫ )تفاعلت‬between
the object and its environment (‫)البيئة‬
 This
resistance (‫ )المقاومة‬is called the force of
friction

39. Forces of Friction, cont.

Friction is proportional to the normal force

ƒs ≤ µs n and ƒk= µk n




μ is the coefficient of friction (‫)معامل التحتكاك‬
These equations relate the magnitudes of the forces,
they are not vector equations
For static friction, the equals sign is valid only at
impeding motion (‫ ,)اعاقة الحركة‬the surfaces are on the
verge of slipping (‫)وشك النزلق‬
Use the inequality if the surfaces are not on the verge
of slipping

40. Forces of Friction, final
 The
coefficient of friction depends on the
surfaces in contact
 The force of static friction is generally greater
than the force of kinetic friction
 The direction of the frictional force is opposite
the direction of motion and parallel to the
surfaces in contact
 The coefficients of friction are nearly
independent of the area of contact

41. Static Friction




Static friction acts to keep the
object from moving
r
r
If r increases, so does ƒ s
F
r
If F decreases, so does ƒ s
ƒs ≤ µs n

Remember, the equality holds
when the surfaces are on the
verge of slipping

42. Kinetic Friction



The force of kinetic
friction acts when the
object is in motion
Although µk can vary
with speed, we shall
neglect any such
variations
ƒk = µk n

43. Explore Forces of Friction


Vary the applied force
Note the value of the
frictional force


Compare the values
Note what happens
when the can starts to
move

44. Some Coefficients of Friction

45. Friction in Newton’s Laws
Problems
 Friction
r is a force, so it simply is included in
the ∑ F in Newton’s Laws
 The
rules of friction allow you to determine
the direction and magnitude of the force of
friction

46. Friction Example, 1



The block is sliding down the
plane, so friction acts up the
plane
This setup can be used to
experimentally determine the
coefficient of friction
µ = tan θ


For µs, use the angle where the
block just slips
For µk, use the angle where the
block slides down at a
constant speed