Problems based on energy in SHM
Q1

A particle of mass 0.5 kg is executing SHM of amplitude 0.03m. When the particle passes through the mean position its kinetic energy is 10 × 10 −3J. Obtain the equation of motion of this particle of the initial phase of oscillation in 45°?

Sol:

Equation of motion of a particle in simple harmonic motion is

Given, x = ACos(ωt + Φ)
Amplitude A = 0.3 m
Initial phase Φ = 45°
= rad
Mass of the particle m = 0.5 kg
Kinetic energy at the mean position Ek = 10 × 10−3J

= 0.67 s−1
∴ The equation of motion x = 0.3Cos(0.67t + )

Q2

A particle of mass 0.2kg executes a SHM of amplitude 2 cm and period 6 seconds. Find the total mechanical energy at any instant and the kinetic and potential energies when displacement is 1 cm ?

Sol:
Mass of the particle m = 0.2 kg
Amplitude of the motion A = 2 cm
= 0.02 m
Time period T = 6 sec
Angular frequency ω = rad/s

Total energy of the particle at any instant is

Kinetic energy at any instant, Ek = ½ mω2(A2− x2)
Given displacement x = 1 cm = 0.01mm

Potential energy at any instant, Ep = ½ mω2x2
At x = 1 cm = 0.01 m

Q3

An object of mass 3 kg is attached to a spring with spring constant k = 280N/m and is executing simple harmonic motion when the object is 0.02m from its equilibrium position it is moving with a speed of 0.55m/s. Calculate the amplitude of the motion and maximum velocity attained by the object?

Sol:

Given,

Mass of the object m = 3 kg
Spring constant k = 280 N/m
Displacement x = 0.02m
Speed of the object v = 0.055m/s
Amplitude of motion A = ?
Maximum Velocity Vmax = ?

Applying the law of conservation of energy at the point x = 0.02m, we have
A2 = 36.4 × 10−4
Amplitude, A = 6 × 10− 2m

Q4

At what displacement from equilibrium is the energy of a simple harmonic oscillator half P.E and half K.E ?

Sol:

Potential energy of a SHO at any instant, P.E = ½ mω2x2
Kinetic energy at any instant, K.E = ½ mω 2(A2 − x2)

Total energy E = ½ mω2 A2
where A is amplitude
Let x = pm be the displacement where the energy is half P.E and half K.E
i.e at x = P
P.E = ½ E
K.E = ½ E
We get two equations,
½ mω2P2 = ½ [½ mω2A2] and
½ mω2(A2− P2) = ½ [½ mω2A2]

Dividing two equation we get
i.e, At a distance of 0.707 times the amplitude on either side of the equilibrium position,
Kinetic energy and potential energy are equal.