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I.
Newton’s first law.
II.
Newton’s second law.
III. Particular forces:
Gravitational
 Weight
 Normal
 Friction
 Tension
IV.
Newton’s third law.
Chapter 4 – Force and Motion I
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View Full Document I. Newton’s first law:
If no
net force
acts on a body, then the body’s velocity
cannot change; the body cannot accelerate
b
v = constant in magnitude and direction.
 Principle of superposition:
when two or more forces act on a body, the net
force can be obtained by adding the individual forces vectorially.
Newton mechanics laws cannot be applied when:
1) The speed of the interacting bodies are a fraction of the speed of
light
b
Einstein’s special theory of relativity.
2) The interacting bodies are on the scale of the atomic structure
b
Quantum mechanics
 Inertial reference frame:
where Newton’s laws hold.
II. Newton’s second law:
The net force on a body is equal to the product of
the body’s mass and its acceleration.
)
1
.
5
(
a
m
F
net
a
a
=
)
2
.
5
(
,
,
,
,
,
z
z
net
y
y
net
x
x
net
ma
F
ma
F
ma
F
=
=
=
 The acceleration component along a given axis is caused only
by the sum
of the force components along the same axis, and not by force components
along any other axis.
 System:
collection of bodies.
 External force:
any force on the bodies inside the system.
III. Particular forces:
Gravitational:
pull directed towards a second body, normally the Earth
b
)
3
.
5
(
g
m
F
g
a
a
=
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View Full Document )
4
.
5
(
mg
W
=
 Weight:
magnitude of the upward force needed to balance the gravitational
force on the body due to an astronomical body
b
)
5
.
5
(
mg
N
=
 Normal force:
perpendicular force on a body from a surface against which
the body presses.
 Frictional force:
force on a body when the body
attempts to slide along a surface. It is parallel
to the surface and opposite to the motion.
Tension:
pull on a body directed away from the
body along a massless cord.
:
When two bodies interact, the forces on the bodies
from each other are always equal in magnitude and
opposite in direction.
)
6
.
5
(
CB
BC
F
F
a
a

=
QUESTIONS
Q2.
Two horizontal forces F
1
, F
2
pull a banana split across a frictionless counter.
Without using a calculator, determine which of the vectors in the free body diagram
below best represent: a) F
1
, b)F
2
. What is the net force component along (c) the
xaxis, (d) the yaxis? Into which quadrant do (e) the netforce vector and (f) the
split’s acceleration vector point?
j
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This note was uploaded on 07/30/2011 for the course PHY 2049 taught by Professor Saha during the Spring '08 term at University of Central Florida.
 Spring '08
 SAHA
 Physics, Force, Friction

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