In the form F=ma, Newton's second law tells us the force, F, is needed to implement an acceleration on a body of mass, m. Newton's first law of mechanics says that unless acted upon by forces, a body at rest will remain at rest or a body in motion will remain in motion moving with the same speed and direction. This first law actually follows from the second: if F=0, then a=F/m=0 also, and if an object doesn't accelerate then it doesn't change its motion.