I'm just curious about the second law. I was taught in secondary school that the definition of the 2nd law was just when a resultant force acts on an object with a constant mass, the object will accelerate and move in the direction of the resultant force. The product of the mass and the acceleration of the object is the resultant force. However, the definition of 2nd law I've learnt is the rate of change of momentum is directly proportional to the net external force acting on the body and takes place in the same direction of the force. Which is correct??