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Chapter-2
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FORCE AND MOTION
Ms. Eman Alqurashi
Lecture Outlines
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Objective 1: Explain the force and identify kind of forces.
Objective 2: Analyze and calculate the net force.
Objective 3: Explain the concept of Inertia.
Objective 4: Differentiate between mass and weight.
Objective 5: Differentiate between balanced and unbalanced forces.
Ms. Eman Alqurashi
2.1 Force
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Force is a push or pull on an object to change its position.
Ms. Eman Alqurashi
 The force has both magnitude and direction so it is a
vector quantity .
Like any vector quantity, force is represented by an arrow. The
length of the arrow gives the magnitude of the force and the
direction of the arrow gives the direction of the force.
Magnitude of the force
The force is measured in Newton (N)
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direction of the force.
Forces between objects which are not in physical contact
are called non-contact forces.
Example
The force between earth and the moon.
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2.2 Some Common Forces
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2.2.1 Earth’s Gravitational Force (weight)
Is a measure of the amount of
gravitational force acting on an
object having mass (m).
It is directed downward
toward the center of earth and
represents the weight of the
object w .
Fg = W= mg
Where ( g=9.8 m/s²)
Ms. Eman Alqurashi
Credit: Fundamental of physics,” By
Halliday, Resnick, and Walker,” 6th Ed. John
Willey &Sons, 2001.
2.2.2 Tension Force
Tension force is a contact force (pull) provided by a string,
rope, or wire on an object.
The direction of the tension T in the direction of the rope.
Credit: “Physics for scientists and engineers a strategic approach,” by
Randall D. Knight Pearson-Addison Wesley 2004.
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2.2.3 Normal Force
Normal force n is force provided by a surface (table or wall,
for example) against an object that is pressing on the
surface.
Normal force is always points vertically upward to the
surface.
Credit: “Physics for scientists and engineers a strategic approach,” by
Randall D. Knight, Pearson- Addison Wesley 2004.
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2.2.4 Friction Force
Friction force f is a force provided by a surface (table or wall,
for example) against any attempt to move an object along the
surface in contact with.
Credit: “Physics for scientists and engineers a strategic approach,”
by Randall D. Knight, Pearson-Addison Wesley 2004.
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Examples:
Identify the forces acting on the object
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 Friction
a) Always acts parallel to the surface of contact and
opposite to the direction of motion.
b) Acts in the direction of motion.
c) Is smaller when starting than moving.
d) In reality, there is no friction force.
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 When a person begin to move forward,
which force allows him to accelerate
a) Weight.
b) Normal force.
c) Air resistance.
d) Friction.
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2.2.5 Combining Forces: The Net (resultant) Force Fnet
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The net force F is a single force resulted from combing
all the forces such that it represents their net effect.
The direction of the net force is always in the direction
of the bigger force.
Find the net force?
Ms. Eman Alqurashi
Net force:
Force:
Is the combination of all
forces acting on that
object.
Is the push or a pull on
an object.
5N
5N
5N
5N
(5 + 5 = 10 N)
5N
(5 _ 5 = 0 N)
10N (10 _ 5 = 5 N)
10N
0N
5N
2.4 Mass and Inertia
What is inertia?
Is the tendency of an object to remain at rest if it’s already at
rest , or to keep moving if it’s already moving.
Or
Is the property of matter that resists change in motion.
An object with a large amount
of inertia requires a large
amount of force to start
or stop.
An object with a small amount
of inertia requires a small
amount of force to start
or stop.
Conclusion:
Mass is a measure of inertia.
Mass is measured in kilograms (kg)
Note:
The more mass an object has
have.
the more inertia it will
The less mass an object has
the less inertia it will have.
Mass
The mass of an object is the
amount of matter it contains.
m=F
a
Mass is measured in
grams(g), kilograms(kg).
Weight
Is a measure of the amount of
gravitational force acting on an
object having mass (m).(vector)
w = mg
Where ( g=10m/s²)
Weight is measured in pounds,
Newton's(N).
mass and weight are directly proportional to one another.
Example: A cube of butter has a mass of about 0.1 kg .
Find its weight?
W = mg = 0.1 × 10 =1 N
a
g
acceleration of the objet
unit(m/s²).
acceleration due to the gravity
or gravitational acceleration
unit(m/s²).
F = ma
Force of the object
unit (N).
W =mg
Gravitational force or weight
unit(N).
Remember:
Gravitational force (weight) is proportional to mass
double the mass and the gravitational force will be doubled
which means the ratio of weight to mass is always the same .
( w / m = mg /m = g)
Is your weight in the moon the same as
your weight in the earth?
what about your mass?
Example 2.3
How much mass is required to balance the
object on the left side, which weighs 600N?
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Equilibrium
Equilibrium for objects at rest
(static equilibrium)
An object is in
mechanical equilibrium
when the sum of all
forces (the net force)
acting on it is zero
∑F=0
Equilibrium for moving objects
(Dynamic equilibrium)
Moving objects also can
be in equilibrium when
the net force is zero
∑F=0
this is the case when an
object moves with
constant velocity
Identify the kind of equilibrium?
Credit: “Physics for scientists and engineers a strategic
approach” by Randall D. Knight Pearson-Addison Wesley 2004
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At rest
or
or
Net force is
zero
or
The object
can be in
equilibrium
if it is
Moving with
uniform
motion
or
Moving with
constant
velocity
Acceleration is
zero
or
If an object is moving with constant velocity
along a straight line, then
a) No forces acting on the object.
b) Single constant force acting on the object in the
direction of motion.
c) Single force acting on the object in the opposite
direction.
d) A net force of zero acting on the object.
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 If net force acting on an object is not zero,
then the object will accelerate.
a) True
b) False
 a=0 is the condition for mechanical
equilibrium.
a) True
b) False
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Balanced and unbalanced forces
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balanced forces have
Unbalanced forces have

Fnet  0

Fnet  0
F3
F1
F2
Example:
The sketch shows a painter’s staging in mechanical equilibrium .
The person in the middle weights 250N, and the tension in
both ropes are 200N. What is the weight of the staging?
∑F=0
∑F = 200 + 200 – 250 – W
=0
W = 200 + 200 – 250 =150
N
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