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Chapter 4
•
Force and Newton’s Laws of Motion
o
A force is a push or a pull.
It is a vector quantity and has units of Newton: 1 N – 1
kg m/s
2
.
o
The
net force
on a system is the vector sum of all the forces
acting on it.
o
Newton’s 1
st
Law: If zero net force
acts on an object, the object’s velocity does
NOT change.
If there is a net force, there is acceleration, so the velocity (speed or
direction) must change.
o
Newton’s 2
nd
Law: The net force acting on an object is related to the objects
acceleration through its mass:
F
a
m
=
∑
r
r
or
F
ma
=
∑
r
r
o
Many problems can be solved by treating the x and y components separately.
o
Drawing a free body diagram involves drawing vector arrows representing every
force acting on a chosen object.
o
The magnitude of the gravitational force between two objects is
1
2
2
Gm m
F
r
=
where
r is the distance between the objects’ centers.
Each object is always pulled toward
the other’s center
o
Weight
is defined to be the magnitude of the gravitational force acting on an object:
W
mg
=
.
Weight is proportional to mass.
o
A
normal
force (usually denoted by
N
) is a contact force perpendicular to a surface
that pushes each object away from the other.
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 Fall '08
 REHSE
 Energy, Force, Mass, Potential Energy

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