idea about the laws of motion in kinematics

• when a particle is moving along straight line, then the particle has two entities i.e., distance its moved and then time it completes the given distance. Here, the idea is the distance travelled by the particle is a function of time i.e., X(t). Can you analyse the graph between the distance and time?. If you take the graph between time and distance travelled by the particle, then your graph is like upside down hills because the particle may have different speeds at different points of its path
• One important condition to derive the laws of motion, the acceleration of the particle should be constant throughout its journey.
  
• It is the problem while taking the average velocity of the particle starting from a point along a straight line because the particle can be rest at any point its journey or it reaches to its destination point at rest position. So, it is better to take the change of velocity with time i.e., acceleration, like that acceleration concept comes out.
• By using calculus, we can easily derive the laws of motion. For derivation of the laws of motion, only acceleration remains constant is basic point and concept.