Chapter 4
Dynamics: Newton’s Laws of Motion
1
Units of Chapter 4
9
Force
9
Newton’s First Law of Motion
9
Mass
9
Newton’s Second Law of Motion
9
Newton’s Third Law of Motion
9
Weight—the Force of Gravity
9
Free-Body Diagrams
9
Problem Solving—A General Approach
2
4-1 Force
A
force
is a
push
or
pull
. An object at rest needs a force to get
it
moving
; a moving object needs a force to change its
velocity
.
3
4-1 Force
Force
is a
vector
, having both
magnitude
and
direction
.
The magnitude of a force can be measured using a spring
scale.
4

4-1 Force
Identifying forces…
(a)
Non-contact forces
… arise between two objects even
though they do not touch each other.
–
Gravity
.
Weight
–
Electromagnetic forces.
–
Nuclear interaction forces.
(b)
Contact forces
… arise when two objects touch each
other.
–
Push
or
pull
.
–
Tension
…
act along the length of a rope.
–
Normal forces
… are perpendicular to a surface.
–
Frictional forces
… are parallel to a surface.
5
6
Isaac Newton
(
1642 – 1727)
7
4-2 Newton’s First Law of Motion
This is
Newton’s first law
, which is often called the law of
inertia
:
Every object continues in its state of rest, or of uniform velocity
in a straight line, as long as no net force acts on it.
F
represents the force applied by the person and
F
fr
represents the
force of friction.
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