Chapter 13: Oscillatory Motions
Simple
harmonic motion
Spring
and Hooke’s law
• When a mass hanging from a spring and
in equilibrium, the Newton’s 2
nd
law says:
This means the force due to the spring is
equal to the force by gravity and opposite
in direction when the spring is stretched.
∑
=
→
=
−
→
=
g
s
g
s
y
F
F
F
F
ma
F
0
• Hooke’s law states that increasing the
weight by equal amounts increases the
stretch of the spring by equal amount.
Therefore, the force due to the spring must be proportional to the
stretch of the spring.
constant.
spring
the
is
where
k
x
k
F
y
s
r
r
−
=
x is deviation from the spring w/o weight
This is also true when the spring shrinks

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