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## Physics - Simple Harmonic Motion

Simple harmonic motion is like a wave. It has period, frequency and amplitude. Simple harmonic motion is always caused by some force. If you know the displacement and something called the spring constant, you can figure out how much force is involved in the motion at any moment.

Rule Number 42: Hooke’s Law

Force equals negative spring constant times displacement, or F = -kx.

The negative sign in this equation indicates that in simple harmonic motion, force is opposite to the direction of displacement.

If you were asked anything about a pendulum, remember that the spring constant for a pendulum is approximately equal to the weight divided by the length.

At every other point in the cycle, the pendulum’s energy is a mixture of potential energy and kinetic energy.

Rule Number 43: Simple harmonic motion and energy

For any object in simple harmonic motion, kinetic energy = 1/2mv2, where m is the mass and v is the velocity.

Potential energy = 1/2kx2, where k is the spring constant and x is the length of the spring.

Total energy = kinetic energy + potential energy.

When the object’s velocity is zero, it is at maximum displacement and all of its energy is potential energy. It has no kinetic energy. When the object is at the zero force point, it is at maximum velocity and all of its energy is kinetic energy. It has no potential energy.