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Copyright 2010 Pearson Education, Inc.Copyright 2010 Pearson
Education, Inc.
Megat Mohd Izhar Sapeli(B.Sc Physics (Hons), M.Sc
Microelectronics)
Bilik Pensyarah 21Level 7, FSK 1,5
UiTM Puncak Alam
[email protected]+60332584961
PHY094FOUNDATION PHYSICS 1
NEWTONS LAWS OF MOTIONcont
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Copyright 2010 Pearson Education, Inc.
Units of Chapter 5
5.1 Force and Mass
5.2 Newtons First Law of Motion
5.3 Newtons Second Law of Motion
5.4 Newtons Third Law of Motion
5.5 The Vector Nature of Forces: Forces in
Two Dimensions
5.6 Weight
5.7 Normal Forces
11.1 Torque
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5-6 Weight
The weight of an object on the Earths surface
is the gravitational force exerted on it by the
Earth.
F ma
W mg
W mg
W mg
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Copyright 2010 Pearson Education, Inc.
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Copyright 2010 Pearson Education, Inc.
Apparent weight:
When elevator moving downward comes to rest,
we feel heavier.
When elevator moving upward comes to rest,
we feel lighter.
( )
y
a
a
a
a
F ma
W W ma
W ma W
W ma mg
W m a g
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Copyright 2010 Pearson Education, Inc.
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Copyright 2010 Pearson Education, Inc.
5-7 Normal Forces
The normal force is
the force exerted by
a surface on an
object.
Normal force always
perpendicular to the
surface.
The normal force may be
equal to, greater than, or
less than the weight.
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Copyright 2010 Pearson Education, Inc.
12
45
20
?
m kg
F N
N
sin 20 0
sin 20
(12)(9.81) 45sin 20
103
y
y y y
F ma
W F N ma
mg F N
N mg F
N
N N
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Copyright 2010 Pearson Education, Inc.
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Copyright 2010 Pearson Education, Inc.
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Copyright 2010 Pearson Education, Inc.
The normal force is always perpendicular to
the surface.
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sin sin
cos cos
x
y
W W mg
W W mg
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Copyright 2010 Pearson Education, Inc.
11-1 Torque
From experience, we know that the same force
will be much more effective at rotating an
object such as a nut or a door if our hand is not
too close to the axis.
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We define a quantity called torque:
The SI unit for torque is N.m
The torque increases as the force increases,
and also as the distance increases.
Consider only F tangent to r
rF
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Only the tangential component of force causes
a torque:
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This leads to a more general definition of torque:
r F
( sin )r F
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If the torque causes a counterclockwise angular
acceleration, it is positive; if it causes a
clockwise angular acceleration, it is negative.
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Copyright 2010 Pearson Education, Inc.
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Copyright 2010 Pearson Education, Inc.
Summary of Chapter 5
Force: a push or pull
Mass: measures the difficulty in
accelerating an object
Newtons first law: if the net force on an
object is zero, its velocity is constant
Inertial frame of reference: one in which the
first law holds
Newtons second law:
Free-body diagram: a sketch showing all the
forces on an object
F ma
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Copyright 2010 Pearson Education, Inc.
Summary of Chapter 5
Newtons third law: If object 1 exerts a force
on object 2, then object 2 exerts a force on
object 1.
Contact forces: an action-reaction pair of
forces produced by two objects in physical
contact
Forces are vectors
Newtons laws can be applied to each
component of the forces independently
Weight: gravitational force exerted by the Earth
on an object
F
F
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Copyright 2010 Pearson Education, Inc.
Summary of Chapter 5
On the surface of the Earth, W = mg
Apparent weight: force felt from contact with a
floor or scale
Normal force: force exerted perpendicular to a
surface by that surface
Normal force may be equal to, lesser than, or
greater than the objects weight