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Newton’s Laws of Motion
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Newton’s Laws of Motion
In this chapter we will study Newton's laws of dynamic
motion.
These are some of the most fundamental and important
principles in physics.
Dr. Loai Afana
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What is the force
•
Intuitively, we know that force is a “push” or “pull”.
Idea: Force is the cause of motion in classical mechanics.
•
Forces come in different classes (types):
1- Contact Forces : involve physical contact between
objects
Example: friction, viscosity etc…
2- Field Forces :don't involve physical contact
between objects
Examples: Gravity, Electromagnetism
Dr. Loai Afana
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Force F is a vector quantity:
You push or pull in a specific direction
Dr. Loai Afana
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Fundamental force in nature
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:In 1686, Newton presented his
Three Laws of Motion
Newton’s First Law
An object at rest remains at rest, and an object in motion
continues in motion with constant velocity, unless it
experiences a net force.
F  0
Q: what dose mean: Velocity is constant ???
(i.e. acceleration = 0): there is no force (or if all forces add to zero).
Q: can an object has an eastward velocity while experiencing a
westward acceleration? (Example)
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Newton's first law sometimes called the law of inertia
Inertia Frames
The tendency of an object to resist a change in its velocity is called inertia.
The measure of inertia is mass.
– SI units measure mass as kilogram (kg=1000g)
Newton’s First Law: If F = 0, then a=0.
What if F  0?
Dr. Loai Afana
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Newton’s Second Law
The acceleration of an object is directly proportional to the resultant force
acting on it, and inversely proportional to its mass.
The direction of the acceleration is the direction of the resultant force.
a
F
or  F  ma
m
The SI unit of force is:
kg.(
m
)  Newtons ( N )
2
s
One Newton is the force required to accelerate one kg one meter per second.
Note that the first law is a special case of the second.
F = m.a

 F  0  a  0  v  0
Dr. Loai Afana

V is constant or zero
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Newton’s Third Law
If object 1 exerts a force F on object 2, then object 2 exerts a force – F on
object 1.
– Forces come in pairs.
– The force pairs act on different objects.
– The forces have the same magnitude but opposite direction.
F2on1  F1on2
Example:
I push on the wall with a force of 20 N. The wall pushes back on me with a force
of 20 N in the opposite direction.
Dr. Loai Afana
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The Vector Nature of Forces
In the formula F = ma, F is the total (net) force acting on the object.
We must consider the vector sum of all forces acting on an object. We can also
consider each dimension separately:
F  ma
F
F
F
x
 max
y
 ma y
z
 maz
Dr. Loai Afana
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Force & Mass
• Force is a vector:
– The net force is the vector sum of all forces acting on the object m.
• Mass is a scalar:
– The value of the mass of an object does not change with the direction
of the acceleration.
– The equation F=ma is also a definition of mass.
 Mass is constant:
If two objects are put together (or separated) , the mass of the combined
object is simply the arithmetic sum of the two masses :
m = m1+m2
Dr. Loai Afana
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Weight
The weight of any object on the Earth is the gravitational force exerted on it
by the Earth
:
W = mg
Note:
Weight is a force (and therefore a vector).
Weight is not equivalent to mass.
The weight of an object is different on the earth and on
the moon since the strength of the gravitational field is
different ( g e ≠ g m ).
Dr. Loai Afana
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