Work, Power and Energy

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Question : 4
Total: 30
The potential energy function for a particle executing linear simple harmonic motion is given by V(x)=
kx2
2
, where k is the force constant of the oscillator. For k=0.5Nm1, the graph of V(x) versus x is shown in figure. Show thata particle of total energy 1 J moving under this potential must ‘turn back’ when it reaches x=±2 m.
Solution:  
The total energy of an oscillator is the sum of kinetic energy and potential energy at any instant.
Total energy = Kinetic energy + Potential energy
E=
1
2
m
u2
+
1
2
k
x2

The particle turn back at the instant, when its velocity becomes zero, u=0,
Therefore E=
1
2
m
u2
+
1
2
k
x2
=0+
1
2
k
x2

1=
1
2
×
1
2
x2
(E=1J,k=
1
2
N
m
)

1=
1
4
x2

x2=4,x=±2
Thus, the particle of total energy 1 J moving under this potential, must turn back at x=±2 m.
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