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25/03/31
Chapter (3)
Newton’s laws of motion
3.1 Force, weight and gravitational mass
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Force: any push or pull on an object
unit: Newton (N)
* The net force acting on an object is defined as
the vector sum of all forces acting on the object.
* If the net force exerted on an object is zero, the
acceleration of the object is zero and its velocity remains
constant. The object is said to be in equilibrium.
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Type fo Forces
Contact force
Field force
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m
W
g
Man who weight 1000N on the earth has a gravitational
mass of:
m
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W 1000

 102 Kg
g
9.8
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Chapter (3)
Newton’s laws of motion
 3.1 Force, weight and gravitational mass
3.2 Density
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Density:

mass
m kg
  3
Volume V m
Relative density:
The ratio of the density of a substance to that of water at 0 C0
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Chapter (3)
Newton’s laws of motion
 3.1 Force, weight and gravitational mass
 3.2 Density
3.3 Newton’s First Law
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“Every object continues in a state of rest, or of
uniform motion in a straight line, unless it is
compelled to change that state by force action
upon it”
F  0
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Suppose a force acting on an object of mass m1 produces an
acceleration a1, and the same force acting on an object of
mass m2 produces an acceleration a2. The ratio of the two
masses is defined as the inverse ratio of the magnitudes of
the accelerations produced by the force:
For example, if a given force acting on a 3-kg object
produces an acceleration of 4 m/s2, the same force applied
to a 6-kg object produces an acceleration of 2 m/s2. If one
object has a known mass, the mass of the other object can
be obtained from acceleration measurements.
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1st Law of Motion
(Law of Inertia)
Because of inertia, objects (including you) resist changes in their
motion. When the car going 80 km/hour is stopped by the brick
wall, your body keeps moving at 80 m/hour.
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“If one object exerts a force F on a second, then the second object
exerts an equal but opposite force –F the first”
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“The force F needed to produce an acceleration a is:
F=ma
Where m is the mass of the object”
the acceleration of an object is directly proportional to the force acting on it.
the magnitude of the acceleration of an object is inversely proportional to
its mass.
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A hockey puck having a mass of 0.30 kg slides on the horizontal, frictionless
surface of an ice rink. Two hockey sticks strike the puck simultaneously, exerting
the forces on the puck .The force F1 has a magnitude of 5.0 N, and the force F2
has a magnitude of 8.0 N. Determine both the magnitude and the direction of the
puck’s acceleration.
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A hockey puck having a mass of 0.30 kg slides on the horizontal, frictionless
surface of an ice rink. Two hockey sticks strike the puck simultaneously, exerting
the forces on the puck .The force F1 has a magnitude of 5.0 N, and the force F2
has a magnitude of 8.0 N. Determine both the magnitude and the direction of the
puck’s acceleration.
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‫‪Fg  mg‬‬
‫‪Fg  5  9.8  49 N‬‬
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‫‪Fg  9  9.8  88.2 N‬‬
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Two blocks of masses m1 and m2, with m1 + m2, are placed in contact with
each other on a frictionless, horizontal surface. A constant horizontal
force F is applied to m1 as shown. Find the magnitude of the acceleration
of the system
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(3-47)
An elevator of mass 900 kg accelerates upward at 3ms-2.
What is the tension in the cable where it is attached to the
elevator?
T
T-W=ma or
T-mg=ma
T=m(a+g)
=900(3+9.8)
=11520N
W=mg
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Chapter (3)
Newton’s laws of motion
 3.1 Force, weight and gravitational mass
 3.2 Density
 3.3 Newton’s First Law
 3.5 Newton’s Third Law
 3.6 Newton’s Second Law
3.8 Some Examples of Newton’s laws
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