A grandfather clock demonstrates simple harmonic motion when the pendulum swings. Say the pendulum swings one full cycle in 10 seconds. Let's assume the pendulum moves four inches in the ten seconds.
The equation would be y=4sin(π/5)x. You get this equation by finding the period first. The period would be 2π / π/5 which would equal 10. The key point would be found by quartering the period and you would get 2.5. The amplitude is just the absolute value of a |4| = 4.