Both can mean to weaken, but often are often used in subtly different contexts.
The first one can mean when something or someone weakens or declines in health.
弱る (or 弱った) can also be used in Japanese when you are talking about "feeling weak" when you are in a bind due to something unfavorable happening to you that you cannot do anything about.
The second one is when something weakens in strength or intensity, or when something weakens gradually.
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Both can mean to weaken, but often are often used in subtly different contexts.
The first one can mean when something or someone weakens or declines in health.
弱る (or 弱った) can also be used in Japanese when you are talking about "feeling weak" when you are in a bind due to something unfavorable happening to you that you cannot do anything about.
The second one is when something weakens in strength or intensity, or when something weakens gradually.
The first is transitive and the second is intransitive.
'to become weak' and 'to weaken'/'to make weak'