I was told that the motion of a simple pendulum is harmonic only when its amplitude is much less than the length of the pendulum (let's take this as first statement). Why it is so?  Also my teacher told me that a simple pendulum is SHM for small angular displacement and that's the reason for the above statement. But I felt that both the statements are implying the same idea. What could be the reason for the first statement?
Considering an ideal case(neglecting drag of air, damping etc.), a pendulum follows SHM if the angular displacement is small (upto 10 degrees). But, for large angular displacement(more than 10 degree), it does not follow SHM. Why? What force cause the pendulum not to follow SHM?
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