So I was wondering how did Newton himself state his second law. One account I've read says that he first expressed it in the form that we refer to as impulse and momentum i.e. FΔt=mΔv. Today I was told that Newton never even wrote F=ma, and that the expression a=F/m is a much "Better" way of stating the law. How is this "Better", if different at all? So how did Newton himself state this law mathematically. Also any links to derivations and other helpful related content would be appreciated.