Summary
Whether it's a pendulum, a mass on a spring or an electrical oscillator, an oscillating system can be described in terms of energy changing from one form to another.
Get the basics
Energy changes in a pendulum
When a pendulum is released, EP is converted to EK as it swings to the lowest point. On the way back up the other side EK is converted back to EP. EP is again converted to EK and back to EP on the swing back.
Energy is constantly changing from EP to EK and back but the total is always the same.
Energy changes in a mass on a spring
It requires a force to both stretch and compress a spring. As a mass oscillates on a spring, elastic potential energy is stored when the spring is both stretched and compressed. In between, the mass has kinetic energy.
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Energy changes in a mass on a spring - this time with extra gravity!
A mass oscillating on a spring in a gravitational field is complicated by the fact that there are two components of potential energy, gravitational and elastic.
NB: The overall oscillating energy graph looks identical as the equilibrium position shifts, but oscillations still occur symmetrically around this.
Test yourself
Use quizzes to practise application of theory.
START QUIZ!
Which graph represents the KE vs time for the pendulum starting fom the position shown
KE starts from zero and increases.
Which graph represents the PE vs time for the pendulum starting fom the position shown
KE starts from max and decreases.
The graph below shows the KE vs time for an oscillating body
What is the time period of the oscillation?
KE is maximum twice per cycle
A 100g mass is hung on a spring causing it to oscillate as shown in the animation below.
Taking the lowest point to be at an extension of 20 cm and g = 10 ms-2 calculate the following
The loss of gravitational PE as the mass moves from A to B J
Given that the spring constant is 10 Nm-1 and elastic PE = 1/2 kx2
The elastic PE at B J
The KE at B J
The loss of gravitational PE as the mass reaches C J
The KE at C J
The elastic PE at C J
A 100 g mass is hung on a sping causing it to extend 10 cm. The mass is the pulled down a further 20 cm and released causing it to oscillate as shown in the animation.
Taking the gravitational PE at the lowest point to be 0 J calculate the following energies.
Elastic PE = 1/2kx2
k = 10 Nm-1
Elastic PE at A J
Gravitational PE at A J
Elastic PE at B J
Work done stretching spring J
KE passing through A J
Total energy at A J
KE at C J
Gravitational PE at C J
Elastic PE at C J
Total energy at C J
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